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Records of Nutrient-Enhanced Coastal Ocean Productivity in Sediments from the Louisiana Continental Shelf
Abstract
Shelf sediments from near the mouth of the Mississippi River were collected and analyzed to examine whether records of the
consequences of anthropogenic nutrient loading are preserved. Cores representing approximately 100 yr of accumulation have
increasing concentrations of organic matter over this period, indicating increased accumulation of organic carbon, rapid early
diagenesis, or a combination of these processes. Stable carbon isotopes and organic tracers show that virtually all of this
increase is of marine origin. Evidence from two cores near the river mouth, one within the region of chronic seasonal hypoxia
and one nearby but outside the hypoxic region, indicate that changes consistent with increased productivity began by approximately
the mid-1950s when the inorganic carbon in benthic forams rapidly became isotopically lighter at both stations. Beginning
in the mid-1960s, the accumulation of organic matter, organic δ13C, and δ15N all show large changes in a direction consistent with increased productivity. This last period coincides with a doubling
of the load of nutrients from the Mississippi River, which levelled off in the mid-1980s. These data support the hypothesis
that anthropogenic nutrient loading has had a significant impact on the Louisiana shelf.
... See Brush (2001), Cronin and Vann (2003), Kemp et al. (2000) and Rabalais et al. (2007) for reviews of Chesapeake Bay and Gulf of Mexico records. Benthic 1) Adelson et al. (2001); 2) Alvarez Zarikian et al. (2000); 3) Alve (2000); 4) Alve (1991); 5) Andrén (1999); 6) Andrén et al. (1999); 7) Barmawidjaja et al. (1995); 8) Blackwelder et al. (1996); 9) Bratton et al. (2003) ; 10) Brunner (2006); 11) Cearreta et al. (2000); 12) Chen et al. (2001); 13) Chmura et al. (2004); 14) Clarke et al. (2003); 15) Clarke et al. (2006); 16) Colman and Bratton (2003); 17) Cooper (1995); 18) Brush (1991, 1993); 19) Cornwell et al. (1996); 20) Cronin and Vann (2003); 21) Dale (2000); 22) Dale (2000); 23) Eadie et al. (1994); 24) Ellegaard et al. (2006) ; 25) Ikeya (1995); 26) Itoh et al. (2003); 27) Karlsen et al. (2000); 28) Kauppila et al. (2005); 29) Matsuoka (1999); 30) Nelsen et al. (1994); 31) Osterman (2003) Rabalais et al. (1996); 43) ; 44) Rabalais et al. (2002); 45) Sangiorgi and Donders (2004); 46) Sato (1995); 47) Schaffner et al. (1992); 48) Sen ; 49) Smittenberg et al. (2004); 50) Smittenberg et al. (2005); 51) Struck et al. (2000); 52) Swarzenski et al. (2008); 53) Thibodeau et al. (2006); 54) Thomas et al. (2000Thomas et al. ( , 2004; 55) Toyoda and Kitazato (1995); 56) Tsujimoto et al. (2006a); 57) Tsujimoto et al. (2008); 58) Turner and Rabalais (1994); 59) Turner et al. (2004); 60) Turner et al. (2006); 61) Voss and Struck (1997); 62) Willard et al. (2003); 63) Yasuhara et al. (2003); 64) Yasuhara and Yamazaki (2005); 65) Yasuhara et al. (2007); 66) Zheng et al. (2003); 67) Zimmerman and Canuel (2000); 68) Zimmerman and Canuel (2001); 69) Zimmerman and Canuel ( Indicate photic zone anoxia Derived from strictly anaerobic organisms that require light and sulphide. B-chl in sediments is hypoxia-specific proxy. ...
... In two cores from the Mississippi Delta Bight, δ 13 C values obtained from multispecies benthic foraminiferal samples, and from Buliminella morgani, were about 0.6-1.0% heavier before the mid-1960s compared to those derived from later sediments (Eadie et al., 1994). A similar trend was reported by Thomas et al. (2000) in Long Island Sound where calcareous tests collected in 1961 were isotopically slightly heavier than those collected in 1996/1997. ...
... Ellegaard et al. (2006) reached a similar conclusion based on a core from Mariager Fjord (Denmark). C-isotopebased approaches have also been applied in the Gulf of Mexico (Eadie et al., 1994;Turner et al., 2006) and New Bedford Harbor, Massachussetts (Chmura et al., 2004) with increases of marine-source carbon in more recent years consistent with eutrophication (Fig. 14). However, over the western Indian shelf, where the organic matter is largely of marine origin, no major change in δ 13 C org appears to have occurred during the last 1300 years (Agnihotri et al., 2008). ...
Under certain conditions, sediment cores from coastal settings subject to hypoxia can yield records of environmental changes over time scales ranging from decades to millennia, sometimes with a resolution of as little as a few years. A variety of biological and geochemical proxies derived from such cores have been used to reconstruct the development of eutrophication and hypoxic conditions over time. Proxies based on 1) the preserved remains of benthic organisms (mainly foraminiferans and ostracods), 2) sedimentary features (e.g. laminations) and 3) sediment chemistry and mineralogy (e.g. presence of sulphides and redox-sensitive trace elements) reflect conditions at or close to the seafloor. Those based on 4) the preserved remains of planktonic organisms (mainly diatoms and dinoflagellates), 5) pigments and lipid biomarkers derived from prokaryotes and eukaryotes and 6) organic C, N and their isotope values reflect conditions in the water column. However, the interpretation of these proxies is not straightforward. A central difficulty concerns the fact that hypoxia is strongly correlated with, and often induced by, organic enrichment (eutrophication), making it difficult to separate the effects of these phenomena in sediment records. The problem is compounded by the enhanced preservation in anoxic and hypoxic sediments of organic microfossils and biomarkers indicating eutrophication. The use of hypoxia-specific indicators, such as the trace metals molybdenum and rhenium and the bacterial biomarker isorenieratene, which have not been used often in historical studies, may provide a way forward. All proxies of bottom-water hypoxia are basically qualitative; their quantification presents a major challenge to which there is currently no satisfactory solution. Finally, it is important to separate the effects of natural ecosystem variability from anthropogenic effects. Despite these problems, in the absence of historical data for dissolved oxygen concentrations, the analysis of sediment cores can provide plausible reconstructions of the temporal development of human-induced hypoxia, and associated eutrophication, in vulnerable coastal environments.
... Although the Mississippi River discharges organic matter, whose decomposition could consume oxygen in the coastal ecosystem, the principal source of organic matter reaching the bottom waters of the northern Gulf of Mexico influenced by the Mississippi River effluent and characterized by hypoxia is from in situ phytoplankton production Turner and Rabalais 1994b;Eadie et al. 1994;Justic´ et al. 1996. These results are verified by data from studies of the sedimentary environment, retrospective sediment analysis, empirical relationships of nutrient flux to primary production and net production, carbon flux studies, linkages of surface-water net production with oxygen uptake in the lower water column, and modeling experiments. ...
... The amount of organic loading in the Mississippi River is not large enough to account for the observed decline in oxygen over such a large area and volume (Turner and Allen 1982b). Also, the δ 13 C signature of the particulate organic matter found in the sediments of the Mississippi River Bight indicates that terrestrial carbon sources are localized near the delta and nearshore sediments, and the bulk of carbon in the sediments of the hypoxic zone is from a marine origin Turner and Rabalais 1994b;Eadie et al. 1994; see Section 7.2). ...
... The nutrient most relevant to the overall production of phytoplankton production over the broad region fueling hypoxia is nitrogen. It follows, and is supported with evidence from long-term data sets (Turner and Rabalais 1994a) and the sedimentary record (Turner and Rabalais 1994b;Eadie et al. 1994), that increases in riverine dissolved inorganic nitrogen loads are highly correlated with indicators of increased productivity in the overlying water column, i.e. eutrophication of the continental shelf waters. Phosphorus and silicon limitation have been identified as well, along with multiple limitations of combinations of N, P, and Si. ...
... One might expect a propensity for the northern Gulf of Mexico ecosystem to develop hypoxia naturally because of the high volume of fresh water and nutrients delivered by the Mississippi River and the nature of the stratified coastal system. However, sediment core indicators clearly document recent eutrophication and increased organic sedimentation in bottom waters, with the changes more apparent in areas of chronic hypoxia and coincident with the increasing nitrogen loads from the Mississippi River system in the 1950s (Eadie et al., 1994;Turner and Rabalais, 1994;Rabalais et al., 2007aRabalais et al., , 2014. This evidence comes as an increased accumulation of diatom remains (biologically bound silica) and marine-origin carbon (stable carbon isotope analysis) accumulation in the sediments. ...
... The diffusion of oxygen from surface to bottom is prevented by the presence of the pycnocline that forms a density barrier below which decomposition of organic matter leads to reduced oxygen levels in the bottom waters. The source of the organic matter settling to the seabed in the northern Gulf of Mexico is mostly from phytoplankton growth stimulated by riverine-delivered nutrients, although some river-borne organic matter is also delivered by the Mississippi River (Eadie et al., 1994;Turner and Rabalais, 1994). The concentrations and total loads of nitrogen, phosphorus, and silica to the coastal ocean influence the productivity of the phytoplankton community, the types of phytoplankton that are most likely to grow, and ultimately the downward flux of phytoplankton-derived organic matter (Lohrenz et al., 1997;Turner et al., 1998;Dortch et al., 2001). ...
The epoch of the Anthropocene, a period during which human activity has been the dominant influence on climate and the environment, has witnessed a decline in oxygen concentrations and an expansion of oxygen depleted environments in both coastal and open ocean systems since the middle of the 20th century. This paper provides a review of system-specific drivers of low oxygen in a range of case studies representing marine systems in the open ocean, on continental shelves, in enclosed seas and in the coastal environment. Identification of similar and contrasting responses within and across system types and corresponding oxygen regimes is shown to be informative both in understanding and isolating key controlling processes and provides a sound basis for predicting change under anticipated future conditions. Case studies were selected to achieve a balance in system diversity and global coverage. Each case study describes system attributes, including the present-day oxygen
environment and known trends in oxygen concentrations over time. Central to each case study is the identification of the physical and biogeochemical processes that determine oxygen concentrations through the tradeoff between ventilation and respiration. Spatial distributions of oxygen and time series of oxygen data provide the
opportunity to identify trends in oxygen availability and have allowed various drivers of low oxygen to be distinguished through correlative and causative relationships. Deoxygenation results from a complex interplay of hydrographic and biogeochemical processes and the superposition of these processes, some additive and others
subtractive, makes attribution to any particular driver challenging. System-specific models are therefore required to achieve a quantitative understanding of these processes and of the feedbacks between processes at varying scales.
... One might expect a propensity for the northern Gulf of Mexico ecosystem to develop hypoxia naturally because of the high volume of fresh water and nutrients delivered by the Mississippi River and the nature of the stratified coastal system. However, sediment core indicators clearly document recent eutrophication and increased organic sedimentation in bottom waters, with the changes more apparent in areas of chronic hypoxia and coincident with the increasing nitrogen loads from the Mississippi River system in the 1950s (Eadie et al., 1994;Turner and Rabalais, 1994;Rabalais et al., 2007aRabalais et al., , 2014. This evidence comes as an increased accumulation of diatom remains (biologically bound silica) and marine-origin carbon (stable carbon isotope analysis) accumulation in the sediments. ...
... The diffusion of oxygen from surface to bottom is prevented by the presence of the pycnocline that forms a density barrier below which decomposition of organic matter leads to reduced oxygen levels in the bottom waters. The source of the organic matter settling to the seabed in the northern Gulf of Mexico is mostly from phytoplankton growth stimulated by riverine-delivered nutrients, although some river-borne organic matter is also delivered by the Mississippi River (Eadie et al., 1994;Turner and Rabalais, 1994). The concentrations and total loads of nitrogen, phosphorus, and silica to the coastal ocean influence the productivity of the phytoplankton community, the types of phytoplankton that are most likely to grow, and ultimately the downward flux of phytoplankton-derived organic matter (Lohrenz et al., 1997;Turner et al., 1998;Dortch et al., 2001). ...
The epoch of the Anthropocene, a period during which human activity has been the dominant influence on climate and the environment, has witnessed a decline in oxygen concentrations and an expansion of oxygen-depleted environments in both coastal and open ocean systems since the middle of the 20th century. This paper provides a review of system-specific drivers of low oxygen in a range of case studies representing marine systems in the open ocean, on continental shelves, in enclosed seas and in the coastal environment. Identification of similar and contrasting responses within and across system types and corresponding oxygen regimes is shown to be informative both in understanding and isolating key controlling processes and provides a sound basis for predicting change under anticipated future conditions. Case studies were selected to achieve a balance in system diversity and global coverage. Each case study describes system attributes, including the present-day oxygen environment and known trends in oxygen concentrations over time. Central to each case study is the identification of the physical and biogeochemical processes that determine oxygen concentrations through the tradeoff between ventilation and respiration. Spatial distributions of oxygen and time series of oxygen data provide the opportunity to identify trends in oxygen availability and have allowed various drivers of low oxygen to be distinguished through correlative and causative relationships. Deoxygenation results from a complex interplay of hydrographic and biogeochemical processes and the superposition of these processes, some additive and others subtractive, makes attribution to any particular driver challenging. System-specific models are therefore required to achieve a quantitative understanding of these processes and of the feedbacks between processes at varying scales.
... The northern Gulf of Mexico is strongly in¯uenced by the Mississippi/Atchafalaya rivers. Freshwater discharge from these rivers forms a highly strati®ed coastal current¯owing, on average, westward along the Louisiana coast (Eadie et al., 1994). The low salinity surface waters form a tongue-shaped belt o the Louisiana coast extending from the Mississippi delta to as far west as south Texas. ...
... Lower C/N ratios in upper sediments indicated that the contribution of marine source organic carbon increased at that time, presumably caused by enhanced primary production in surface waters. Previous studies at a site near this study site also showed that large changes in the accumulation of organic matter derived from phytoplankton (based on d 13 C, and d 15 N) occurred at the beginning of the 1950s (Eadie et al., 1994). ...
... The rivers bring nutrients (e.g., nitrogen, phosphorus, and silica) that facilitate the growth of phytoplankton that eventually sink to the seabed and provide carbon for decomposition (Turner, 2002). This decomposition reduces dissolved oxygen in the bottom water (Rabalais et al., 1991;Eadie et al., 1994;Turner & Rabalais, 1994). ...
... These dates also agree well with the evidence of increasing PEB index in the Louisiana Bight in the mid-20 th century (Osterman et al., 2008(Osterman et al., , 2009. Other studies of foraminiferal assemblages in this region document an increasing influence from hypoxia from the late 1940s to the early 1960s (Sen Platon & Sen Gupta, 2001;Platon et al., 2005;Osterman et al., 2009) as do other indicators of hypoxia such as total marine organic matter, percent biologicallybound silica, phytoplankton assemblages, and pigment concentrations from anoxygenic bacteria Eadie et al., 1994;Rabalais et al., 1996Rabalais et al., , 2000Rabalais et al., , 2004Rabalais et al., , 2007bChen et al., 2001). ...
The frequency of anthropogenic seasonal hypoxia on the continental shelf west of the Mississippi Delta (the Louisiana Bight)
has increased since the middle of the 20th century. This study applies the PEB index, a proxy for hypoxia, to four ~2 m kasten cores taken southwest of Southwest Pass
of the Mississippi Delta. The PEB index is defined as the cumulative percent of Protononion atlanticum (= Nonionella atlantica of this study), Nonionella opima, Epistominella vitrea and Buliminella morgani. The PEB index varies little in the shallowest core, KC4 (59 m water depth). Assemblages were strongly dominated by PEB taxa
(average 95%), due mainly to the dominance of E. vitrea (average 76%). In core KC3 (75 m), an average PEB value of 84% is also due mainly to E. vitrea (61%). The PEB index of KC3 was consistently higher above 90 cm, a level dated at 1955–1960 (210Pb age estimate), and could reflect an increased influence of hypoxia or an increased rate of sediment accumulation associated
with delta progradation. A similar trend characterizes core KC2 (87 m). The PEB index is 18% from 240 cm to 140 cm, increasing
up core as the abundance of E. vitrea increases, likely due to delta progradation. Other PEB taxa, B. morgani and N. opima, increase in the top 30 cm of the core, probably due to increased hypoxia between 1952 and 1968 (210Pb age estimate). Core KC1 (473 m) differed from shallower cores in its higher species diversity. Bolivina lowmani, Cassidulina neocarinata, and Bolivina ordinaria each comprise ~15% of assemblages and PEB taxa are rare (average 5%) but increase to 19% in a 30 cm interval attributed to
off-shelf transport. We conclude that the PEB index is a useful indicator of anthropogenic hypoxia but dominance of E. vitrea may also be attributable to sedimentological factors related to delta progradation.
... The relative importance of terrestrial-sourced carbon delivered by the Mississippi is considered minimal (Turner and Rabalais, 1994;SAB, 2007), as is the potential for degrading marsh materials exported offshore SAB, 2007;Das et al., 2010). Evidence from stable carbon isotopes (Eadie et al., 1994;Turner and Rabalais, 1994) indicated that the marsh signal is very close to shore and that the terrestrial river carbon signature remains close to the Mississippi River Delta. The overwhelming signature on the continental shelf affected by hypoxia is from in situ produced marine carbon (Eadie et al., 1994;Turner and Rabalais, 1994). ...
... Evidence from stable carbon isotopes (Eadie et al., 1994;Turner and Rabalais, 1994) indicated that the marsh signal is very close to shore and that the terrestrial river carbon signature remains close to the Mississippi River Delta. The overwhelming signature on the continental shelf affected by hypoxia is from in situ produced marine carbon (Eadie et al., 1994;Turner and Rabalais, 1994). Continued increase in knowledge of the bio logical importance of constituents, such as dissolved organic phosphorus and urea (Dyhrman and Ruttenberg, 2006;Glibert et al., 2006), particularly within the lower salinity, higher suspended sediment portion of the river plumes, dictate increased need to monitor these constituents. ...
Excess nitrogen and phosphorus loads from the Mississippi River Basin have led to degraded water quality in the watershed and in the coastal ecosystem of the Gulf of Mexico where vast, perennial ‘dead zones’ are a common feature now that did not exist historically. Efforts to mitigate and remediate the excess nutrients, although acknowledged by many state and federal institutions as essential, are minimal, ineffective, and stymied by the inertia of a complex, multi-jurisdictional quagmire.
... Until now, historical changes in ecosystem health resulting from eutrophication and hypoxia in the Changjiang Estuary have not been thoroughly evaluated due to the absence of long-term environmental monitoring data. Fortunately, the elemental, molecular, and isotopic signatures of organic matter preserved in sediments can be used to reconstruct historical changes in transport processes of carbon and nutrients in estuarine regions, and the interactions between terrestrial activities and coastal environment (Chen et al., 2001;Eadie et al., 1994;Rabalais et al., 2007;Swarzenski et al., 2008). Many techniques used to trace environmental change employ biogenic elements (e.g., C, N and their isotopes) to explain material sources (Struck et al., 2000;Wu et al., 2007;Yang et al., 2009). ...
... While in core CJ56, TOC was 0.59% on average (0.35% to 0.89%), with no significant trends down-core but instead large oscillations in values (Fig. 2B). TOC in core CJ43 was significantly lower than other eutrophic estuaries (Eadie et al., 1994;Jia et al., 2002;Struck et al., 2000) probably due to high TOC remineralization rates in the gigantic sedimentary incinerator of mobile muds in the Changjiang Estuary (Aller, 1998;Aller and Blair, 2006;Aller et al., 1985;Liu et al., 2007). TOC in core CJ56 was also low, likely due to its small specific surface area (i.e., larger particle size). ...
... Additionally, MAR discharge and nutrient flux are tied to the spatial and temporal variability of the summer hypoxic area, or "dead zone" in the NGoM (characterized by dissolved oxygen content of < 2 mg L −1 ) 48 . The increase in MAR DIN loading from the 1950s to 1990s coincided with increased NGoM primary production, sediment C accumulation, and hypoxia-hypoxia did not appear as widespread or recurrent prior to the 1950s increase in DIN 35,49,50 . The areal extent of the NGoM dead zone is also correlated with MAR DIN loading and with primary production only in the MAR plume, not the full shelf area of the GoM 35,44,51 . ...
Dissolved inorganic nutrient concentrations in the surface waters (0 to 5 m) of the Northern Gulf of Mexico (NGoM) were analyzed from 1985 to 2019 (> 10,000 observations) to determine spatiotemporal trends and their connection to nutrients supplied from the Mississippi/Atchafalaya River (MAR). In the NGoM, annual mean dissolved inorganic P (DIP) concentrations increased significantly over time, while dissolved inorganic N (DIN) concentrations showed no temporal trend. With greater salinity, mean DIN:DIP decreased from above the Redfield ratio of 16 to below it, reflecting DIN losses and the more conservative behavior of DIP with salinity. Over the same time period, annual mean P (total dissolved P, DIP, dissolved organic P) loading from the MAR to the NGoM significantly increased, annual mean DIN and total dissolved N loading showed no temporal trend, and dissolved organic N loading significantly decreased. Though DIP increased in the MAR, MAR DIP alone was insufficient to explain the surface distribution of DIP with salinity. Therefore, increases in surface DIP in the NGoM are not simply a reflection of increasing MAR DIP, pointing to temporal changes in other DIP sources. The increase in NGoM DIP suggests greater N limitation for phytoplankton, with implications for N fixation and nutrient management.
... In addition, storms and intense precipitation bring about sedimentation. Coastal ocean productivity is enhanced from nutrient-rich sediments (Eadie et al. 1994). This could either lead to increase in fish productivity or a potentially harmful algal bloom (HAB) event. ...
Commercial, municipal, and aquaculture fisheries, and the marine ecosystem as a whole, are expected to experience climate change impacts in the coming decades. This is alarming, as marine resources contribute a significant portion (19-36%) to the food supply of the Philippines. Projections reveal that the Philippines shall experience increases in sea surface temperature, more intense storms, locally prolonged droughts, and intense episodic rainfalls. The country is also likely to experience effects of ocean acidification, and sea level rise is projected to be higher than the global estimates for the Philippines. These impacts are additional pressure on top of the many, and mostly anthropogenic pressures which the marine ecosystem is already experiencing. Although the Philippines' high biodiversity can help reduce overall vulnerability, urgent actions are needed to build marine food resiliency.
... Bulk stable isotopes (ẟ 13 C) show an initial depleted value in the surface of TMBA (− 22.9‰) and a fairly stable average of − 21.3 ± 0.2‰ below 2-cm depth with little variation (light green circles in Fig. 4). Freshwater signatures in the region range from approximately − 24‰ to − 27‰, based on the relative proportions of C3 (~ − 27‰) and C4 (~ − 13‰) plant material (Gordon and Goñi, 2003), while the published local marine phytoplankton signature is − 19.5‰ (Eadie et al., 1994). Carbon to Nitrogen molar ratios (C/Ndark green circles in Fig. 4) are for the most part anticorrelated with ẟ 13 C data, showing a decrease in the upper 3 cm from ~8 to <7, then steadily increasing downcore to >8.5 at the base of the core. ...
Extreme episodic events have the capacity to transport large amounts of terrestrial material to the coastal ocean. While estuaries and deltas are typically thought to trap most of this material, some escapes these coastal features and is transported to distal depocenters along the continental shelf. Distal shelf depocenters can act as the ultimate sink for event deposits, yet event sedimentation and the impact of terrestrial carbon burial within these shelf deposits remains understudied. Therefore, this study investigated event sedimentation within the Texas Mud Blanket (TMB), a distal shelf depocenter of the Brazos River located in the northwestern Gulf of Mexico. Using a core that was collected from the TMB during a months-long elevated discharge event in late 2015 to early 2016 we applied a multi-proxy approach to delineate the event deposit. We utilized physical sediment properties (fabric, texture, and bulk density), radioisotopes (⁷Be and ²¹⁰Pb), and organic geochemical properties (ẟ¹³C, C/N, and lignin-phenols) to identify a 10 cm event layer at the surface of the core. Within this 10 cm layer, we also distinguished two sub-layers associated with separate extreme discharge periods in the winter and spring, respectively. Using these event layer characteristics, we identified three other earlier potential event deposits in the core that we have attributed to events in 2010, 2007, and 2000–2001. From these results, this study has demonstrated that the TMB may be an archive for event deposits over multi-decadal time periods, in particular the transition from periods of prolonged drought to extreme wetness associated with the Brazos River. This event-dominated sedimentation regime can enhance long-term carbon burial, potentially increasing current estimates of carbon burial significantly in the TMB and other similar distal shelf depocenters. Ultimately this study highlights the potential for distal shelf depocenters as organic carbon burial “hotspots,” and postulates that this role may expand in the future as climate change increases the intensity and frequency of extreme events.
... Moreover, the warmer temperatures of coastal waters support greater activity at a lower metabolic cost (Clarke and Gaston 2006;Currie et al. 2004;Tittensor et al. 2010) and the waters are nutrient (and prey) rich due to the input of terrigenous nutrients and upwelling of deep-ocean water up the slope and onto the shelf (Seitzinger and Giblin 1996). The episodic nature of upwelling events and river flows can make the environment highly variable, more than doubling productivity in short pulses (Yoder et al. 1983;Eadie et al. 1994), meaning species like some cephalopods that can respond quickly see maximum benefit. The productivity of shelf waters has a broader footprint, however, as excess production and nutrients (rapidly remineralised in shelf sediments) are exported to adjacent coastal sea waters (Thomas and Borges 2012). ...
Cephalopods, especially squids, are believed to have a structuring role in marine ecosystems as a link between different trophic levels, primarily due to their voracious prey consumption and high production rate. Cephalopod ecology, however, is still poorly understood as observational studies often give highly uncertain and variable results due to the peculiarities of cephalopod behaviour and biology, and their responsiveness to external drivers. This review evaluates our representation of cephalopods in ecosystem models and the insights given by these models on the role of cephalopods in our oceans. We examined ecosystem models from 13 regions to analyse the representation of cephalopods and compared their results to local trophic studies. Our analysis indicated that most ecosystem models inadequately include cephalopods in terms of model structure and parametrization; although some models still have the capacity to draw valuable conclusions regarding the impact and role of cephalopods within the system. Oceanic squid species have a major role linking trophic levels and food webs from different habitats. The importance of neritic species varies locally, but generally cephalopods have a substantial impact via their consumer role. To better understand the ecological role of cephalopods, improved representation of these species in ecosystem models is a critical requirement and could be achieved relatively easily to more accurately articulate the mechanisms regulating the ecological role of cephalopods.
... Therefore, only δ 13 C was used for further exploring the organic matter provenance. We used a two end-member mixing model using δ 13 C as source markers to estimate the proportion of terrestrial and marine OM in the surface sediments of the ECSS ( Sweeney et al., 1978;Westerhausen et al., 1993;Eadie et al., 1994). The estimated values were gained based on the following equation: ...
... We used a two end-member mixing model using δ 13 C as source markers to estimate the proportion of terrestrial and marine OM in the surface sediments of the ECSS (Sweeney et al., 1978;Westerhausen et al., 1993;Eadie et al., 1994). The estimated values were gained based on the following equation: ...
... The MAR decline was also associated with a decrease in the burial rate of particulate organic carbon (POC). Although other cores across the subaqueous front were analyzed, the pattern of MAR decline was not observed due to bioturbation and sediment focusing, and earlier work also did not identify the MAR decline at~1950 (Eadie et al., 1994;Corbett et al., 2006;Allison et al., 2007). The primary objective of this study is to assess trends in sedimentation patterns of the delta front over a larger spatial scale during a time period where the Mississippi River system has undergone major anthropogenic modifications. ...
... The physiographic characters that separate the east from west coast of India may have an important influence on marine biodiversity. Rivers are important factors in controlling productivity; the presence of larger rivers in a region leads to higher productivity, which in turn may affect marine species diversity and composition (Eadie et al., 1994;Gallmetzer et al., 2017). Suspension-feeding molluscs depend on primary productivity in the water column for food, which is also affected by nutrients carried in freshwater inflow of the rivers. ...
Tropical coastal areas are amongst the most diverse ecosystems in the world. However, there are quite a few coasts that have rarely been studied for their macro-benthic diversity. The Indian coastline presents one such gap area. Two sub-parallel coastlines of India have a wide latitudinal span (8–23°N) and strikingly different physiographic environments. While the east coast receives a high siliciclastic input from large river systems flowing to the Bay of Bengal with fluctuating salinity, the west coast has a large shelf area and high productivity of the Arabian Sea. Such difference enables us to evaluate the effect of regional environmental parameters on marine molluscan diversity and distribution in an intra-tropical setting. Because of the wide latitudinal range, it is also possible to assess if spatial difference in species richness in such a regional scale follows the large-scale biodiversity pattern such as Latitudinal Biodiversity Gradient (LBG) despite inherent environmental variation. We used species distribution of marine bivalves, compiled using bioSearch and the Ocean Productivity database, to address this question. Our results show that intra-tropical species richness of marine bivalves is guided primarily by regional environmental parameters. Even with identical latitudinal extent, higher nutrient availability and larger shelf area, the west coast has significantly lower richness than the east coast; among environmental variables, productivity, salinity and coastline length emerged as significant predictors of species diversity. Moreover, a positive influence of a South Asian biodiversity hotspot on east coast fauna and a negative impact of the oxygen-depleted condition of Arabian Sea on west coast fauna, may have a significant contribution in developing such coastal variation in species richness. The latitudinal variation in species richness did not follow LBG. In contrast to the coast-specific diversity difference, species composition is not found to be dictated by coastal affiliation. The composition corresponds primarily to physiographic conditions. We identified three distinct eco-regions (north-western, southern, north-eastern) with characteristic species composition corresponding to unique physiography and productivity mechanism. The NW region has low siliciclastic input and high productivity associated with upwelling during winter cooling. The NE region has a distinctly high riverine input and salinity fluctuation. The southern region, in contrast, has well developed reefal system with moderate variation in salinity. Such correspondence underscores the importance of the regional environment in dictating the species diversity and distribution in the shallow marine realm.
... The enrichment of surface water nutrients by cold water transported from subsurface to surface through upwelling process or by horizontally advection forms the thermal boundaries. The variability of the air-sea interaction process such as latent heat, sensible heat and wind intensity, riverine runoff, tidal mixing and coastal upwelling plays significant role in formation of frontal features in the coastal and open oceans (Eadie et al., 1994;Davis et al., 2014). These mesoscale processes are associated with enriched nutrients and Chla and considered to be high productive regions. ...
In this paper we demonstrate the persistence of sea surface thermal fronts based on the frequency of Sea Surface Temperature gradient satellite data, at seasonal and non-seasonal cycles. Prominent thermal fronts were observed in the northeast Arabian Sea between the 50 m bathymetry contour and continental self break (∼200 metre contour). A total of 45–75% of thermal fronts occurred at the annual time-scales whereas 20–30% occurred at the semi-annual time-scales. Maximum frequency occurrence was observed in the coastal waters south and southwest of Saurashtra peninsula during February and March. Primary productivity co-varies with the frequency of the thermal fronts with a time-lag of 2–4 weeks. This outcome can help on timely and reliable prediction of biologically productive zones.
... However, actions taken to manage the distribution of river flow through the Mississippi-Atchafalaya deltaic plain in the future, especially as a mechanism for coastal restoration, could be of major consequence to the development and distribution of hypoxia on the continental shelf and eutrophication of ambient receiving waters(Ren et al., 2009). Further, the inputs of terrestrial carbon from the watershed or loss of carbon from deteriorating wetlands along the coast have been ruled out as contributors to the carbon loading that leads to hypoxia(Eadie et al., 1994;Turner and Rabalais, 1994;Turner et al., 2007;Das et al., 2010). Nutrient stimulation viafigure 5. Distribution of bottom-water dissolved oxygen content on the louisiana-Texas continental shelf in July 2006. ...
The gusher has ended, but before it did, an estimated 206 million gallons of crude oil and methane gas escaped from the Macondo well in lease block Mississippi Canyon 252. We know it better as the BP Deepwater Horizon oil spill that resulted from a series of mechanical and safety failures leading to an explosion, the deaths of 11 workers, and the largest accidental oil spill in history. The well was in the northern Gulf of Mexico in 1500 m of water, not the deepest in this petroleum production fron-tier, but in an otherwise blue-water, pristine ocean home to deep-water corals and pods of sperm whales, and one of two spawning areas for Atlantic bluefin tuna. Satellite images of black oil at the surface marred this picture as the oil continued to spew from the ocean bottom and spread into the northern Gulf of Mexico. Innumerable lives were affected—from microbes to humans—and the world was transfixed by the continuous images of oil and gas blowing from the Gulf bottom while technology raced to catch up with Mother Nature.
... However, not all flooding events in the Changjiang drainage basin were recorded by organic proxies in Core D2 (Figure 3). Lignin has been shown to be more reactive than previously thought, and diagenetic processes in sediments may obscure the terrestrial OC signals of ancient flooding events (Eadie et al., 1994;Ward et al., 2013). Nevertheless, we did not find significant decrease of TOC/SSA ratios and terrestrial OC contents during ancient flooding events, which probably indicated limited influence of digenetic processes on the terrestrial OC signal of flooding events. ...
A gravity core collected from the East China Sea (ECS) inner shelf was analyzed for elemental and stable isotopic composition, lignin-phenols, and sedimentary pigments to investigate changes of organic carbon (OC) inputs during the past two centuries. In particular, we examined the linkages between terrestrial and marine OC inputs with climate variability and anthropogenic activities. The decrease of terrestrial OC contribution (from 41% to 28%) and increasing diagenetic indices of lignin-phenols (P/(S+V): from 0.12 to 0.22; 3,5-Bd/V: from 0.03 to 0.09) after the 1970s were possibly attributed to intensified deforestation, dam construction, and channel erosion. Lignin content (Λ8) ranged from 0.35 mg/100 mg OC to 6.92 mg/100 mg OC, with lower values corresponding to the worst flooding events in the Changjiang watershed and weaker East Asian Winter Monsoon (EAWM), while higher Λ8 was more correlated to the strengthening of EAWM. This indicates that terrestrial inputs to Zhe-Min Coast are different from those in Changjiang Estuary during flooding events and strongly linked with regional climate variability. The total contents of sedimentary chloropigments (i.e. pheophorbide-a, pheophytin-a, pyropheophytin-a, sterol chlorin esters, and carotenol chlorin esters) ranged from 663.4 to 74.9 nmol g−1 OC, and decreased exponentially downwards. Sedimentary chloropigments that were used to document historical change of phytoplankton biomass were decoupled with historical changes of Changjiang riverine nutrient inputs but corresponded well to the fluctuation of regional climate variability. Higher phytoplankton biomasses usually were observed during positive phases of Pacific Decadal Oscillation (PDO) and/or warm El Niño-Southern Oscillation (ENSO) events, and lower algal biomass usually corresponded to the negative phase of PDO and/or cold ENSO events.
... Researchers have noted increases in the concentration of TOC preserved in Ž sediments of CB Cooper andBrush, 1991, 1993; . Cornwell and Sampou, 1995;Cornwell et al., 1996 and estuarine and coastal environments in general Ž e.g., Eadie et al., 1994;Gong and Hollander, 1997; . Louchouarn et al., 1997 during the 20th century; but it is difficult to distinguish the effects of OM decomposition from those due to an increase in OM delivery to the sediments over time because both processes may yield the same downcore OM profile. ...
Organic and inorganic geochemical indicators were examined in a 3-m core collected from the mesohaline region of Chesapeake Bay (CB) to determine how sources of organic matter (OM) have changed during the preceding three centuries of increasing anthropogenic influence in this region. This study also establishes the history of eutrophication and anoxia/hypoxia and relates these processes to changes in OM deposition and preservation and to historical events within the Bay's watershed. The sediment record shows that a marked increase in organic carbon (35%–50%), biogenic silica (18%) and total sulfur (42%) occurs between 1934 and 1948. This transition is likely due to increasing anoxic/hypoxic bottom water conditions as indicated by an abrupt change in sulfur speciation. Lipid biomarker distributions indicate that a substantial change in the sources of OM deposited since 1934 has also occurred. Biomarker compounds derived from phytoplankton and microbial sources show a 2- to 4-fold increase in their abundance relative to total organic carbon (TOC) between 1948 and 1975. Using both diagenetic models and information on lipid reactivity, an effort is made to distinguish compositional changes due to changes in OM delivery (both quantity and quality) from changes that may be due to OM degradation. It appears that enhanced OM production in the mesohaline region of CB has contributed to the observed changes in quantity and character of OM preserved in sediments from this site. Increased inorganic fertilizer application and human population growth in the watershed are coincident with the onset of eutrophic and hypoxic conditions in CB, suggesting that anthropogenic activities within estuarine watersheds may exert a substantial influence on carbon cycling processes in estuaries and potentially the coastal ocean.
... However, actions taken to manage the distribution of river flow through the Mississippi-Atchafalaya deltaic plain in the future, especially as a mechanism for coastal restoration, could be of major consequence to the development and distribution of hypoxia on the continental shelf and eutrophication of ambient receiving waters(Ren et al., 2009). Further, the inputs of terrestrial carbon from the watershed or loss of carbon from deteriorating wetlands along the coast have been ruled out as contributors to the carbon loading that leads to hypoxia(Eadie et al., 1994;Turner and Rabalais, 1994;Turner et al., 2007;Das et al., 2010). Nutrient stimulation viafigure 5. Distribution of bottom-water dissolved oxygen content on the louisiana-Texas continental shelf in July 2006. ...
... However, both C/N molar ratios and δ 13 C failed to track the more ancient floods in the CRB. This is not surprising since both TOC and δ 13 C signatures can be altered during mineralization and diagenetic processes in sediments (Eadie et al., 1994). Changes in phytoplankton abundance and community structure have also been Fig. 3. Variations of phosphorus species of the sediment core D2in the CRB for the past 230 years. ...
In this study, sediment grain size (MGS), specific surface area (SSA), total organic carbon (TOC) contents, C/N molar ratios, stable carbon isotope, and P species in a sediment core, collected from the East China Sea (ECS) inner-shelf were measured to explore the applicability of detrital phosphorus (De-P) as a potential indicator of past flooding events in the Changjiang River Basin (CRB). In particular, we examined the linkages between the evolution of floods with regional climate changes and anthropogenic activities in the CRB. Peaks of De-P concentrations in sediments corresponded well with the worst flooding events of the CRB over the past two centuries (e.g., 1850s, 1860s, 1900s, 1920s, 1950s, 1980s, and 2000s). Moreover, De-P also corresponded well with the extreme hypoxic events in 1981 and 1998 in the Changjiang Estuary as indicated by Mo/Al ratios, indicating potential linkages between De-P as a flooding proxy to flood-induced hypoxia events in this region. In addition, a robust relationship was found among De-P, the floods in 1950s, 1980s, 2000s of the CRB, the intensive El Niño-Southern Oscillation (ENSO), the abnormally weak East Asian Summer Monsoon (EASM) and the warm phase of Pacific Decadal Oscillation (PDO), suggesting that De-P also provided insights to linkages between regional climate change and flooding events in this region.
... However, both C/N molar ratios and δ 13 C failed to track the more ancient floods in the CRB. This is not surprising since both TOC and δ 13 C signatures can be altered during mineralization and diagenetic processes in sediments (Eadie et al., 1994). Changes in phytoplankton abundance and community structure have also been Fig. 3. Variations of phosphorus species of the sediment core D2in the CRB for the past 230 years. ...
In this study, sediment grain size (MGS), specific surface area (SSA), total organic carbon (TOC) contents, C/N molar ratios, stable carbon isotope, and P species in a sediment core, collected from the East China Sea (ECS) inner-shelf were measured to explore the applicability of detrital phosphorus (De-P) as a potential indicator of past flooding events in the Changjiang River Basin (CRB). In particular, we examined the linkages between the evolution of floods with regional climate changes and anthropogenic activities in the CRB. Peaks of De-P concentrations in sediments corresponded well with the worst flooding events of the CRB over the past two centuries (e.g., 1850s, 1860s, 1900s, 1920s, 1950s, 1980s, and 2000s). Moreover, De-P also corresponded well with the extreme hypoxic events in 1981 and 1998 in the Changjiang Estuary as indicated by Mo/Al ratios, indicating potential linkages between De-P as a flooding proxy to flood-induced hypoxia events in this region. In addition, a robust relationship was found among De-P, the floods in 1950s, 1980s, 2000s of the CRB, the intensive El Niño-Southern Oscillation (ENSO), the abnormally weak East Asian Summer Monsoon (EASM) and the warm phase of Pacific Decadal Oscillation (PDO), suggesting that De-P also provided insights to linkages between regional climate change and flooding events in this region.
... A number of sediment core studies have interpreted increases in δ 13 C to represent greater inputs of marine organic carbon associated with eutrophication (e.g. Eadie et al., 1994, Turner et al., 2006, Chmura et al., 2004. Bratton et al. (2003) identifies several other sources and processes that influence δ 13 C in coastal marine sediments: phytoplankton uptake of bicarbonate due to CO 2 limitation caused by high productivity (causes enrichment), increasing influence of C 4 plants (causes depletion), and proximity to salt-marsh vegetation and seagrass. ...
... In the lower Mississippi River and its tributaries, average δ 15 N ranges from $3 to 9‰ for nitrate and $ 5 to 7‰ for POM, providing a relatively high δ 15 N source to the Gulf of Mexico and possibly to the northwestern portion of our study area (Battaglin et al., 2001;Kendall et al., 2001;Dorado et al., 2012). δ 15 N values from sedimentary organic matter near the mouth of the Mississippi River have increased by approximately 1.5-2‰ over the past century (Eadie et al., 1994). If current trends in eutrophication continue, this source of high δ 15 N may become even more prominent in the future. ...
... The Mississippi River, draining 41% of the land area of the continental USA (Eadie et al., 1994;Goolsby et al., 2001), delivers approximately 953,000 Mg nitrate-nitrogen each year to the Louisiana coast (Goolsby et al., 2001). About 174,600 Mg of this input is discharged from Mississippi River's largest distributary, the Atchafalaya River (Xu, 2006a). ...
Periodic summer hypoxia occurring in the Northern Gulf of Mexico has been attributed to large nutrient inputs, especially nitrate-nitrogen, from the Mississippi-Atchafalaya River system. The 2008 Gulf Hypoxia Action Plan calls for river corridor wetland restoration to reduce nitrate loads, but it is largely unknown how effective riverine wetland systems in the lower Mississippi River are for nitrate removal. We carried out an intensive isotope study to address this question by comparing nitrate isotopic signatures of the well-channelized Mississippi River with those of the Atchafalaya River, which has extensive floodplains and backwater swamps. We investigated changes in δ15NNO3 and δ18ONO3 for water samples collected biweekly to monthly over a 2-year period at the Atchafalaya River outlets (Morgan City and Wax Lake) and on the Mississippi River at Baton Rouge. In addition, in-situ water quality parameters including temperature, dissolved oxygen and pH were recorded for each sampling date. Waters from both rivers showed moderately high nitrate concentration (>1 mg L-1) and undetectable (< 0.01 mg L-1) nitrite throughout the study period. The Mississippi River had significantly higher mean nitrate concentrations (1.5 mg L-1) and higher δ15NNO3 (7.7o/oo) than the Atchafalaya (1.1 mg L-1, 7.0o/oo); while no difference in δ18ONO3 (4.6o/oo) was found between the rivers. Flux-weighted mean isotope values were overall lower than mean values for both the Mississippi and Atchafalaya Rivers, with a greater difference between the two rivers (7.4o/oo versus 6.5o/oo, respectively). River flooding and hurricane storm surge also appeared to affect nitrate isotopic values. The lack of large difference in isotopic values between the Atchafalaya and Mississippi Rivers suggests that the majority of nitrate is transported through the Atchafalaya River with relatively little processing, and that riverine floodplains and wetlands are not effective sinks for nitrate, as previously assumed, because of insufficient residence time and well-oxygenated river waters.
The thermal front in the oceanic system is believed to have a significant effect on biological activity. During an era of climate change, changes in heat regulation between the atmosphere and oceanic interior can alter the characteristics of this important feature. Using the simulation results of the 3D Regional Ocean Modelling System (ROMS), we identified the location of thermal fronts and determined their dynamic variability in the area between the southern Andaman Sea and northern Malacca Strait. The Single Image Edge Detection (SIED) algorithm was used to detect the thermal front from model-derived temperature. Results show that a thermal front occurred every year from 2002 to 2012 with the temperature gradient at the location of the front was 0.3 °C/km. Compared to the years affected by El Niño and negative Indian Ocean Dipole (IOD), the normal years (e.g., May 2003) show the presence of the thermal front at every selected depth (10, 25, 50, and 75 m), whereas El Niño and negative IOD during 2010 show the presence of the thermal front only at depth of 75 m due to greater warming, leading to the thermocline deepening and enhanced stratification. During May 2003, the thermal front was separated by cooler SST in the southern Andaman Sea and warmer SST in the northern Malacca Strait. The higher SST in the northern Malacca Strait was believed due to the besieged Malacca Strait, which trapped the heat and make it difficult to release while higher chlorophyll a in Malacca Strait is due to the freshwater conduit from nearby rivers (Klang, Langat, Perak, and Selangor). Furthermore, compared to the southern Andaman Sea, the chlorophyll a in the northern Malacca Strait is easier to reach the surface area due to the shallower thermocline, which allows nutrients in the area to reach the surface faster.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Decomposition of particulate organic matter (POM) plays a key role in the formation of hypoxia in subsurface waters of coastal ocean, yet little is known about the lability and transformation of POM in the hypoxic zone. Suspended particles were collected from surface waters to overlying waters (~30 cm above the sediment‐water interface) along the shelf of northern Gulf of Mexico (nGOM) in late spring/early summer of 2010–2013. Total hydrolyzable amino acids (THAA) and pigments were measured in these particulate samples to trace organic matter lability. The degradation indices, derived from the THAA and chloropigments, were positively correlated with dissolved oxygen (DO) concentrations in the shelf region, suggesting that decomposition of POM contributed greatly to DO utilization. Bacterial degradation appears to be the major pathway for POM decomposition on both inner and mid shelves, while zooplankton grazing played a minor role. POM samples in the overlying water on the inner shelf were the most degraded from the THAA and pigment results, and they also had high C/N ratios (9–14) and depleted δ ¹³C values (−29‰ to −24‰), pointing to a source of terrestrial C3 plant material. This distinct terrestrial signal of POM in the overlying water suggests strong selective degradation of marine‐sourced organic matter, but how the terrestrial organic matter is settled to this layer and its ultimate fate remain unclear. Taken together, these data offer new angles looking into the lability and degradation pathways of POM, and mechanisms of hypoxia formation in coastal waters.
Distribution of total organic carbon (TOC), total nitrogen (TN), stable isotope ratio of organic carbon and nitrogen (δ13C and δ15N) in 31 surface sediments were examined to quantitatively discriminate the source and burial of sedimentary organic matter (SOM) in the East China Sea (ECS). Results show that high content of TOC and TN occurred in the southern inner shelf and the northeast area due to the transport of organic matter (OM) from the Changjiang (Yangtze) River southwards by the Zhejiang Fujian Coastal Current (ZFCC) and the delivery from the old Huanghe (Yellow) River estuary by the Subei Coastal Current (SbCC), respectively. The significant relationship of TOC to clay provided evidence that the fine-grained sediment was the primary carrier for SOM from the inner shelf to open sea. The TN varied proportionally to the TOC with the neglected intercept, suggesting that most of nitrogen measured was related to the SOM and the influence of the sorption of inorganic nitrogen was insignificant. The seaward enrichment of δ13C and seaward depletion of C/N and δ15N illustrated the decrease of terrestrial OM (TOM) and the increase of marine OM (MOM). The close of average C/N ratio to the Redfield ratio indicated the minor role of terrestrial plant debris in the SOM. Distribution of bulk OM properties was ascribed to the terrestrial and marine input by the shelf circulation and phytoplankton with the contributions of 76.1% MOM and 23.9% TOM to SOM, respectively. The burial flux of OC ranged within 0.34–7.56 mg/(cm2·a) (averaged 2.80 mg/(cm2·a)) and enriched along the shore and in the fine-grained area, manifesting the significant input of the land-based sources and the effect of shelf mud depositional process on the fate of SOM. The sharp seaward decrease of burial fluxes of terrestrial and marine OC gave evidences that the majority of organic carbon were settled in the inner shelf with the small fraction of them further transported eastwards.
To evaluate the sources and budgets of sedimentary organic matter in the East China Sea (ECS), bulk organic matter properties (total organic carbon (TOC), C/N ratio, carbon and nitrogen isotopic composition (δ¹³C and δ¹⁵N)) and sterols in surface sediments were analyzed. The distributions of bulk organic matter properties and sterols showed a strong regionality with higher contents in the southern inner shelf as well as the offshore upwelling area. It was consistent with the increasing mud and chlorophyll a (Chl a) contents southwards. These distribution patterns were attributed to the terrestrial inputs by shelf circulations and phytoplankton contribution. Both of bulk organic matter properties and sterols indicated a mixture of terrestrial and marine derived organic matter inputs with marine source as a dominant input. Budgets suggested that organic carbon (OC) and sterol influxes from terrestrial source were 5.15 × 10⁶ and 3.32 × 10³ t/yr, with the Changjiang River input as the primary contributor; and while their influxes from marine source were 26.3 × 10⁶ and 2.19 × 10³ t/yr, respectively. The burial fluxes of OC and sterol were 3.99 × 10⁶ t C/yr and 0.21 × 10³ t/yr, with the inner shelf as main accumulation area. Although there was a huge amount of OC influx from terrestrial and marine sources, but only 24.6% and 10% of them could be preserved in sediments. Consequently, the combined effect of riverine input, marine primary productivity and deposition decided the organic matter behaviors in the ECS.
The sediments that are preserved in estuarine environments (saltmarsh, riverine estuaries, mangrove habitats, lagoons, isolation basins and fjords) contain organic matter that allows investigation of the provenance of that material. These data can then be used specifically to investigate past sea level/land level changes and changes in freshwater flux. Where microfossils are poorly preserved or absent, C/N and δ13C analyses offer an alternative method to deduce environmental histories, but they are especially useful when used in conjunction with a range of other proxies, and when local modern end-member organic variables can be measured to ‘calibrate’ the sedimentary C/N and δ13C. There are a wide range of C/N-δ13C based carbon studies, here we describe examples of studies in a variety of estuarine environments.
The upwelling area in the Changjiang Estuary was selected to collect the core, where the red tide occurred frequently and hypoxic existed. The total organic carbon (TOC), total nitrogen (TN), biogenic silica (BSi) and stable organic carbon isotopic ratios(delta(13) C(org)) were determined on the (210)Pb-dated sediment core. The concentrations of TOC, TN, BSi as well as their sedimentation fluxes have increased to some extent since the 1970s. TOC and TN fluxes increased about 45%, 36% respectively. The average delta(13) C(org) value in the core was - 23.67 x 10(-3) which remained nearly constant before the 20 century. The delta(13) C(org) values increased after the 1900s, two marked increases were observed from the 1950s and the 1970s. A simple delta(13) C(org) model was used to estimate the contribution of terrigenous and marine organic matter inputs for the sediment, which indicated the increase in accumulation since the 1970s has been almost exclusively marine. The increasing of marine organic matter accumulation (TOC, TN and BSi) was corresponding with the increasing of fertilizer consumption and the NO(3)-N budgets from tire Changjiang River. The riverine runoff of fertilizers and nutrients stimulated the algae blooming. Enhanced primary production resulted in air enrichment of organic matter in the sediment. These data support the hypothesis that anthropogenic nutrient loading has been a significant factor on tire eutrophication in the Changjiang Estuary.
The Mississippi River currently delivers approximately 1.82 Tg N year(-1) (1.3 x 10(11) mol N year(-1)) to the northern Gulf of Mexico. This large input dominates the biological processes of the region. The "new" nitrogen from the river stimulates high levels of phytoplankton production which in turn support high rates of bacterial production, protozoan and metazoan grazing, and fisheries production. A portion of the particulate organic matter produced in the pelagic food web sinks out of the euphoric zone where it contributes to high rates of oxygen consumption in the bottom waters of the inner shelf, resulting in the development of an extensive zone of hypoxia each summer. In spite of the significance of this river system to the coastal ocean of the northern gulf, we do not have an adequate understanding of the inputs, processing and ultimate fates of river nitrogen. Here we review available literature on this important system and propose a conceptual model showing how biological processes evolve in the river plume between the point of discharge and the point where plume waters are fully diluted by mixing with oceanic water.
Nutrient over-enrichment in many areas around the world is having pervasive ecological effects on coastal ecosystems. These effects include reduced dissolved oxygen in aquatic systems and subsequent impacts on living resources. The largest zone of oxygen-depleted coastal waters in the United States, and the entire western Atlantic Ocean, is found in the northern Gulf of Mexico on the Louisiana/Texas continental shelf influenced by the freshwater discharge and nutrient load of the Mississippi River system. The mid-summer bottom areal extent of hypoxic waters (< 2 mg l -1 O2) in 1985-1992 averaged 8,000 to 9,000 km 2 but increased to up to 16,000 to 20,000 km 2 in 1993-2000. Hypoxic waters are most prevalent from late spring through late summer, and hypoxia is more widespread and persistent in some years than in others. Hypoxic waters are distributed from shallow depths near shore (4 to 5 m) to as deep as 60 m water depth but more typically between 5 and 30 m. Hypoxia occurs mostly in the lower water column but encompasses as much as the lower half to two -thirds of the water column. The Mississippi River system is the dominant source of fresh water and nutrients to the northern Gulf of Mexico. Mississippi River nutrient concentrations and loading to the adjacent continental shelf have changed in the last half of the 20 th century. The average annual nitrate concentration doubled, and the mean silicate concentration was reduced by 50%. There is no doubt that the average concentration and flux of nitrogen (per unit volume discharge) increased from the 1950s to 1980s, especially in the spring. There is considerable evidence that nutrient enhanced primary production in the northern Gulf of Mexico is causally related to the oxygen depletion in the lower water column. Evidence from long-term data sets and the sedimentary record demonstrate that historic increases in riverine dissolved inorganic nitrogen concentration and loads over the last 50 years are highly correlated with indicators of increased productivity in the overlying water column, i.e., eutrophication of the continental shelf waters, and subsequent worsening of oxygen stress in the bottom waters. Evidence associates increased coastal ocean productivity and worsening oxygen depletion with changes in landscape use and nutrient management that resulted in nutrient enrichment of receiving waters. Thus, nutrient flux to coastal systems has increased over time due to anthropogenic activities and has led to broad -scale degradation of the marine environment.
Major rivers transfer high loads of continental particulate organic matter to deltaic environments, where the impact on the biogeochemistry and productivity of coastal sediments depends on the sources and lability of these inputs. Our aims are to provide new insights into the reactivity of riverine inputs in coastal environments and to delineate the parameters controlling their fate in these dynamic systems. Sediment cores were collected from a Mediterranean deltaic system (the Rhône prodelta and its adjacent shelf) during a period of moderate river discharge (Spring 2007) and analyzed for their fatty acid composition. Sediment properties were also described using biochemical bulk analyses (organic carbon and lipids), granulometry and profiles of redox potential. Based on the bulk compositional changes and hierarchical clustering of the fatty acid biomarkers, we determined the principal sources of organic matter and their spatial distribution. Fatty acids were primarily of continental origin in the prodelta, shifting to a higher contribution from marine sources in the shelf area. Fatty acids derived from both continental and marine sources were efficiently degraded in the sediments by microbial decay processes in the upper oxic layer. Degradation rate constants calculated from the down-core decreases in concentrations indicate that fatty acid degradation was enhanced in sediments influenced by the Rhône River. The most important parameters affecting fatty acid preservation were the inherent stability of individual molecular components and their physical association with mineral matrices, the source and freshness of the inputs, and the depositional environment (redox condition, accumulation rates).
The increasing input of nutrients to lakes and estuaries by the anthropogenic activities has resulted in the aquatic eco-environment changes, such as the change of food chain, eutrophication and frequently seasonal hypoxia in bottom waters, which can be recorded in sediments. The biogenic elements and their stable isotopes in sediments are effective proxies to reflect the paleoproductivity and nutrient level in waters. The indicative significance of biogenic elements (C, N, P, Si) to the change of primary production, material sources and nutrient levels was reviewed in this paper. TOC, TN, δ 13 C, CaCO 3 and BSi in sediments reflecting the content of organic matter and the growth of phytoplankton, can be used as effective proxies to indicate the primary production level. Source identification of organic matter by the differences of δ13 C, δ 15 N and C/ N values between different types of plants has important value to study the influence of anthropogenic activities on the aquatic environment and eutrophication. The increasing trend of TN, δ 15 N, TP and non-apatite phosphorus (NAIP) in sediments reflects the increasing input of terrestrial nitrogen and phosphorus to coastal waters. BSi can reflect the phytoplankton growth, and further indicate the Si concentration variations and eutrophication in waters. Biogenic elements are influenced by many factors in their deposition and burial, such as diagenesis and hydrodynamic conditions, which can disturb the sedimentary records of biogenic elements and then hinder the proper indication of environmental changes by these proxies. Consequently, in order to exactly understand the aquatic environmental changes, it is essential to comprehensively analyze the information of environmental changes provided by multiple biogenic element proxies and to fully consider the potential interference factors for the paleoenvironmental reconstructions.
This textbook provides a unique and thorough look at the application of chemical biomarkers to aquatic ecosystems. Defining a chemical biomarker as a compound that can be linked to particular sources of organic matter identified in the sediment record, the book indicates that the application of these biomarkers for an understanding of aquatic ecosystems consists of a biogeochemical approach that has been quite successful but underused. This book offers a wide-ranging guide to the broad diversity of these chemical biomarkers, is the first to be structured around the compounds themselves, and examines them in a connected and comprehensive way.This timely book is appropriate for advanced undergraduate and graduate students seeking training in this area; researchers in biochemistry, organic geochemistry, and biogeochemistry; researchers working on aspects of organic cycling in aquatic ecosystems; and paleoceanographers, petroleum geologists, and ecologists.Provides a guide to the broad diversity of chemical biomarkers in aquatic environmentsThe first textbook to be structured around the compounds themselvesDescribes the structure, biochemical synthesis, analysis, and reactivity of each class of biomarkersOffers a selection of relevant applications to aquatic systems, including lakes, rivers, estuaries, oceans, and paleoenvironmentsDemonstrates the utility of using organic molecules as tracers of processes occurring in aquatic ecosystems, both modern and ancient.
The oft-cited general correlation between net sediment accumulation and preservation of organic matter, while revealing in many ways, can be a misleading indicator of general elemental cycling processes and controls on storage of biogenic material at the continental-ocean boundary. Deltaic environments are characterized by the highest rates of net sedimentation and are the single most important class of depocenters on Earth. Available data indicate that sedimentary organic C (Corg) of both terrestrial and marine origin is efficiently decomposed in deltaic areas, with decomposition percentages reaching ≥70% and ≥90%, respectively, the latter percentage (marine) being quite comparable to deep-sea, low sedimentation environments. Despite high primary productivity associated with most deltas and evidence of substantial deposition of fresh planktonic debris, patterns of SO4= reduction indicate that the reactivity of organic material being buried is low, and that a larger proportion of Corg is often degraded compared to other marine deposits of similar net accumulation rate. As indicated by properties of the surficial Amazon delta and downdrift coastal region of northeast South America (∼1600-km extent), the primary reasons for efficient remineralization are related to intense and massive physical reworking of sediment associated with estuarine fronts, upwelling, tidal oscillation, and wind-driven waves. Fluid muds and mobile surface material cause the seafloor and continental boundary to act as a massive, suboxic, fluidized bed reactor dominated in some cases by bacterial rather than macrofaunal biomass. Reoxidation, repetitive redox successions, metabolite exchange, and continual mixing-in of fresh planktonic debris with refractory terrestrial components, result in an efficient decomposition system largely decoupled from net accumulation. Similar processes occur on smaller scales in most estuarine-shelf systems, but appear to be most dramatically expressed off the major rivers forming deltas.
The seasonal formation of a bottom-water layer severely depleted in dissolved oxygen has become a perennial occurrence on the Louisiana continental shelf adjacent to the Mississippi River system. Dramatic changes have occurred in this coastal ecosystem in the last half of the 20th century as the loads of dissolved inorganic nitrogen tripled. There are increases in primary production, shifts in phytoplankton community composition, changes in trophic interactions, and worsening severity of hypoxia. The river-influenced continental shelf is representative of similar ecosystems in which increased nutrient flux to the coastal ocean has resulted in eutrophication and subsequently hypoxia. The hypoxic conditions (dissolved oxygen less than 2 mg L-1) cover up to 22,000 km² of the seabed in mid-summer. Dissolved oxygen concentrations seldom decrease to anoxia, but are often below 1 mg L-1 and down to 0.5 mg L-1. The biogeochemical processes of oxic versus suboxic conditions in the water column and sediments of the Louisiana shelf are similar to other areas of oxygen deficiency. However, the suboxic conditions in the Gulf of Mexico are less persistent in time and space, and anoxia at the seabed is not common or long-lasting.
Patterns of temporal and spatial variability in hypoxia (<2 mg O2 l−1) on the inner Louisiana–upper Texas (LaTex) shelf were examined using FVCOM LaTex, an unstructured grid, three-dimensional, hydrodynamic-water quality model. Dynamics of dissolved oxygen were modeled using an expanded and revised version of the Water Analysis Simulation Program (WASP) that was fully coupled to a Finite Volume Coastal Ocean Model (FVCOM). The coupled model was driven by surface wind forcing, tidal forcing, offshore remote forcing, heat fluxes, oxygen exchanges at the air-sea interface, solar radiation, and freshwater and nutrient (nitrogen and phosphorus) fluxes from the Mississippi and Atchafalaya Rivers. The model simulations were carried out over a 9-month period, from January 1 to October 4, 2002, and the model skill was assessed using multiple sets of observational data that included time series of dissolved oxygen concentrations from a station within the core of the Gulf hypoxic zone (C6), dissolved oxygen measurements collected during the mid-summer shelfwide cruise, and vertical dissolved oxygen profiles through the year. The model results indicate that hypoxia originates in bottom waters on the mid-continental shelf, where isolated pockets of hypoxic water develop during early spring and later join into a larger continuous hypoxic zone. The model accurately described the seasonal cycle of hypoxia at station C6, including the episodes of intermittent hypoxia during May and June, persistent hypoxia during July and August, and dissipation of hypoxia during September. The onset of hypoxia coincided with high stability of the water column (i.e., Richardson number values>1) and the initial transition from normoxia (i.e., 6 mg O2 l−1) to hypoxia lasted about three weeks. The model results point to a significant short-term variability in the extent of hypoxic bottom waters, indicating that the size of the mid-summer hypoxic zone cannot be adequately captured by a single shelfwide cruise. The dynamics of bottom-water hypoxia is clearly influenced by the bathymetric features of the LaTex shelf, namely the presence of three shallow shoals (<5 m) in the Atchafalaya Bay region and several deeper shoals (<10 m) in the northwestern section of the study area. Lastly, the model results support the view that dynamics of hypoxia on the LaTex shelf is strongly modulated by the frequency and intensity of cold fronts and tropical storms. High winds associated with these events disturb stratification, causing partial or complete breakdown of hypoxia. However, cold fronts and tropical storms also cause significant sediment resuspension that fuels respiration in the lower water column, and in this manner promote redevelopment of hypoxia.
We studied the potential for using stable carbon and nitrogen isotope ratios in sediment profiles to trace external nutrient sources and eutrophication at four coastal sites in the Baltic Sea. The sites are characterized by various present and past activities in their catchments, including residential development, sugar processing, agriculture and fish farming. Radiometrically dated sediment cores were analysed for nutrient isotope ratios, organic carbon and total nitrogen. Background information was collected from historical sources, literature and water monitoring data. Despite the multiple organic enrichment sources, it was possible to identify individual sources and processes in the sediment profiles using stable isotope analysis of bulk sediment. The largest changes in δ15N values were seen at sites receiving urban wastewaters. The site that received effluents from a sugar cane (C4-plant) refinery in the past showed a clear effect on δ13C values compared to the site that received wastewater from a sugar beet (C3-plant) factory. Fish farming produced detectable, albeit minor changes in the sediment profile. Slightly lower δ13C values reflected the influence of fish feed and fish metabolism, and higher δ15N values likely indicated the influence of increased sediment denitrification. The land-sea connection via river discharge was observable in the overall δ13C levels of the sediment cores. Our results suggest that temporal changes in sources of organic matter enrichment can be detected in well-dated coastal sediment cores using nutrient stable isotope analyses, even at sites subjected to multiple impacts. There is not, however, a simple relationship between sediment stable isotope profiles and the eutrophication history of our study sites.
We examined a suite of geochemical and microfaunal indicators in 210Pb-dated sediment cores from the Pearl River Estuary, South China, to determine the historical course of oxygen conditions. The results revealed a substantial increase in nutrient elements and primary production in the sediment over the last 3 decades, which could be interpreted as a recent increase in the extent of eutrophic condition in the Pearl River Estuary. Furthermore, total foraminifers decreased abruptly in abundance while the species Ammonia beccarii, which is tolerant of low-oxygen conditions and high organic fluxes, showed a marked increase in relative abundance after the 1970s. These faunal changes implied that significant deterioration had occurred in the bottom water oxygen conditions since the late 1970s. This degradation could be caused by the increased nutrient and organic loading to the estuarine system, which is likely related to the rapid urbanization and industrial development in the Pearl River Delta during this period.
Increasing eutrophication and seasonal anoxia in bottom water in the Changjiang Estuary and its adjacent waters has progressed in recent decades, caused by elevated anthropogenic N and P input. Sedimentary biogenic elements were investigated to determine whether the biogenic proxies could be used in paleoenvironmental studies in an energetic estuary, as well as to reconstruct the histories of environmental changes in the East China Sea (ECS). Two 210Pb-dated cores from the coastal and offshore waters were analyzed for organic C (TOC) and its stable isotope (δ13C), total N (TN), biogenic Si (BSi), total P (TP) and P species. In coastal sediment, the variations of P species, especially Fe-P, Al-P and detrital apatite P (Det-P), reflected the dry–wet oscillations in the Changjiang River for the past century, which has influenced the sediment grain size and terrestrial material input. Much lower BSi content (0.756%) at 16–22 cm likely recorded the pronounced decrease in silicate flux in the Changjiang River and its lower flow in the late 1980s. In offshore sediment, higher concentrations of TOC, TN, BSi, Ex-P, Fe-P and Lea-OP indicated higher primary productivity in response to the strong winter monsoons during the 1960s–1980s, and their 20-a fluctuations were in agreement with the decadal variations of the winter monsoon. Low contents and little variations of Al-P and Det-P indicated the slight influence of the terrestrial sediment input in offshore waters. The influence of human activities on the environment in recent decades has also been recorded in coastal sediment. Grain-size normalized concentrations of TOC, TN, TP, Ex-P, Fe-P and Lea-OP increased by 24%, 23%, 15%, 13% and 51% in the upper 16 cm of coastal sediment, indicating elevated P and N load and primary productivity since the 1990s. Elevated TN/TP ratios and decreased BSi/TOC recorded the changed nutrient structure and the decrease in the proportion of the diatom to phytoplankton community. However, the sediment record indicated that the eutrophication might actually have started from the end of the 20th century rather than the reported middle of 20th century. In contrast, biogenic elements in offshore sediment did not reflect disturbance by human activities. This study revealed that multi-nutrient proxies in sediment in the ECS could indicate natural environmental changes including runoff and the winter monsoon over the past century, as well as the influence of human activities in recent decades. Phosphorus species with distinct origins and biogeochemical behaviors could effectively reflect different aspects of past environmental conditions.
Oxygen-deficient conditions occur from April-October on the inner to middle continental shelf of the N Gulf of Mexico and may cover up to 9500 km2 during mid-summer off the Louisiana coast. Hypoxic bottom waters are found in 5-60 m water depth, 5-60 km offshore and extend up to 20 m above the bottom. Stratification is directly correlated with hypoxia in time and space, suggesting that reaeration of bottom waters is controlled by physical processes that are influenced by regional wind fields, river discharge and continental shelf scale currents. Phytoplankton biomass reaches the bottom waters in the hypoxic zones in large amounts (>5 μg/l), fueling water column and benthic respiration rates. Increased nutrient loadings in the two major rivers and changes in the proportion of those nutrients essential to phytoplankton growth have probably changed both the phytoplankton community species composition and community production. -from Authors
Analyses of stable carbon isotope ratios were conducted on components of the planktonic food web in the northern Gulf of Mexico to evaluate the importance of terrestrial organic matter as a source of carbon to this food web. These analyses were made on samples collected in areas of high (Southwest Pass, Louisiana) and low (Cape San Bias, Florida) riverine input. Dissolved organic carbon at 7 and 26 km from Southwest Pass and the 0.45-20 μm particulate organic carbon size fraction at 7, 26, and 43 km from the Pass were the only components that displayed isotope ratios approaching terrestrial carbon values, means of -24.0‰ and -24.60/0‰ respectively. Phytoplankton had a mean δ13C of -22.7‰ for both northern Gulf areas, whereas three copepod genera and total zooplankton had mean values of -20.5‰ and -21.9‰, respectively. Four species of larval fish had similar δ13C values in both areas (-21.1‰), suggesting their tissue carbon was derived ultimately from phytoplankton. Gut analyses indicated an intermediate zooplankton link for three species. Gulf menhaden appeared to derive their carbon directly from phytoplankton as well as through the phytoplankton-zooplankton pathway. Isotopic fractionation values between trophic levels did not exceed ±1.8‰, a range similar to that reported for other ecological systems.
Hypotheses related to variability in seasonal hypoxic conditions, coastal nutrient enhancement, and off-shelf transport of
carbon on the Louisiana continental shelf were tested by characterization of biogenic, lithogenic, and authigenic components
from two shelf and one Mississippi Canyon sediment cores. The authigenic-phase glauconite occurs above detection limits only
in the core from the hypoxic area. A major increase in glauconite concentration was coincident with the onset (≈1940) of the
increased use of commercial fertilizers in the United States. In the same hypoxic-area core, benthic foraminifera species
diversity decreases upcore from approximately the turn of the century to the present in a manner concurrent with glauconite
and fertilizer increases. A subset of opportunistic benthic foraminifera species, known to become more prominent in stressed
environments (i.e., hypoxic), increased upcore from ∼52% of the total population at core bottom to ≈90% at core top. These
benthic foraminifera population and diversity changes were not apparent in a “control” core outside the area of documented
hypoxia. Seaward of the shelf, in the Mississippi Canyon, coincident increases in sediment accumulation rate, percentages
of coarse fraction and of organic carbon at core top indicate increased offshelf transport of carbon and other components.
Quartz percentages indicate that episodic down-canyon transport has been active to core bottom (prior to the mid 1800s).
Seasonal remineralization rates of organic carbon are calculated in the top 20–30 cm of biologically irrigated, organic-rich sediments of Buzzards Bay, MA. Six cores were collected over a period of two years, and the pore water concentrations of the following species were measured: dissolved inorganic carbon (ΣCO2), PO3−4, ΣH2S, Alk, and Ca2+. Overall, these constituents showed large gradients with depth, which are larger in summer than in winter.Remineralization rates in the sediments were estimated by applying a non-local exchange, vertical molecular diffusion, reaction model to the ΣCO2 depth profiles. The major processes affecting the pore water concentration of ΣCO2 described in the model are diffusion, irrigation, and the oxidation of organic carbon. The calculated remineralization rates varied seasonally with the high of 7.5 × 10−9 mol/L-sec observed in August 84 and the low (0.6 × 10−9) in December 1983. The remineralization rates were dependent on the amount of irrigation in the sediments. It was possible to calculate remineralization rates between 0 and 20 cm because the amount of irrigation was well-characterized at this site. We calculated that 69 gC/m2 are oxidized annually and 5–33 gC/m2-yr are buried. It appears that there is a highly reactive portion of organic matter which is oxidized at the sediment water interface. Examination of the Alk and dissolved Ca2+ profiles indicates that there was significant production of acid which dissolved CaCO3 in the spring and early summer.
Inputs of pollutant lead to the Gulf of Mexico from the Mississippi River have declined by about 40 percent within the past decade. This decrease has been determined from annual lead loads of the Mississippi River and from the lead record in Mississippi Delta sediments. The observed trend is consistent with reduced consumption of lead in gasoline in the United States. More than 90 percent of the riverborne lead is associated with suspended sediments. Most of this particle-bound lead is deposited within 50 kilometers of the river mouth and is not easily leached at pH values above 3.
Nutrients (N and P) are important in controlling primary production in aquatic ecosystems. The availability of N or P within an ecosystem is partly a function of the rates of external inputs to the system and permanent removal within the system by biological, chemical and/or physical processes. N is a limiting nutrient in many estuaries, coastal and continental shelf marine systems (Ryther and Dunstan, 1971), some lakes (Keeney, 1973; Gerhart and Likens, 1975), and some streams and rivers (Grimm and Fisher, 1986). Denitrification in the sediments or anoxic water removes N from aquatic systems as the gaseous nitrogen produced diffuses into the atmosphere. Denitrification in aquatic systems, therefore, decreases the amount of nitrogen available for primary production, as well as decreases the transport of N to downstream or offshore waters. In aquatic systems that receive N from pollution sources, the removal of a portion of that N via denitrification may help control the degree of eutrophication of the system.
Stable isotope ratios (δ13C) of total organic carbon were measured in surface sediments from the continental margins of the northern and western Gulf of Mexico, the north coast of Alaska and the Niger Delta. Gulf of Mexico outer-shelf isotope ratios were in the same range as has been reported for Atlantic coastal shelf sediments, −21.5 to −20‰. Off large rivers including the Mississippi, Niger and Atchafalaya (Louisiana), δ13C values increased from terrigenous-influenced (around −24‰) to typically marine (∼−20‰) within a few tens of kilometers from shore. This change was accompanied by a decrease in the amount of woody terrigenous plant remains in the sediment. Alaskan continental margin samples from the cold Beaufort Sea had isotopically more negative carbon (−25.5 to −22.6‰) than did warmer-water sediments. The data indicate that the bulk of organic carbon in Recent sediments from nearshore to outer continental shelves is marine derived.
Seventy-nine δ13C analyses of oceanic particulate matter (> 0·μ) from semi-tropical (Gulf of Mexico, Caribbean and Atlantic) and polar (South Indian Ocean) waters showed that the carbon isotope composition of the particulate matter from the cold polar surface waters was lighter () than that from the surface in the semi-tropical regions (), reflecting the temperature effect on the photosynthetic fixation of carbon. δ13C for deep samples (> 330 m) were generally more negative than the surface samples, except in some well-mixed polar areas.A difference both in organic carbon isotopic composition and percentage organic carbon in the POM and the tops of sediment cores was also apparent; a loss of approximately 95 % of incoming carbon and an increase in 13C of several per mille being observed during deposition of particulate matter. This indicates that after settling on the bottom there is extensive diagenesis of the POM by organisms, indicating the non-refractory nature of the organic matter.
Sediments deposited near and impacted by a major wastewater outfall system in southern California were examined for their organic content and composition. A combination of elemental (% organic carbon (OC), % organic nitrogen (ON)), stable isotopic (σ13C, σ15N) and molecular analyses were used to reconstruct the depositional history of these Recent sediments. In the immediate vicinity of the outfalls, the variations in the bulk properties (% OC; % ON; ; σ15N) of the sedimentary organic matter were closely correlated and appeared to reflect changing inputs of sewage particulates. This allowed the successful application of a two-source mixing model, whereby the fraction of waste-derived organic matter in the sediments was estimated. With increasing distance from the point of waste discharge, however, the validity of the model became tenuous owing to inputs from other sources, changing depositional environments and other factors. Source-specific molecular markers were used to corroborate predictions based on the mixing model and to establish sediment chronology. Key marker compounds used in this study include: (i) the high molecular weight normal alkanes derived from terrigenous plant debris; (ii) 17α (H), 18α (H), 21β (H) -28,30-bisnorhopane, a pentacyclic triterpane characteristic of California oil seeps and shales; (iii) DDT + metabolites; (iv) the long-chain alkylbenzenes present in munipical wastes as a result of detergent use.
Pollution of the hydrosphere and atmosphere by compounds of nitrogen is a serious problem. This paper reviews the manner in which studies of natural abundance 15N/14N ratios may be employed in investigating the sources and mechanisms of pollution.Cultivation-induced mineralization of soil nitrogen, fertilizer, and animal or sewage wastes are the three main sources of nitrate pollution in the hydrosphere. In many cases these sources produce nitrate with distinguishable 15N/14N ratios, and on this basic isotopic data for nitrate have been successfully used for identifying the source of pollution in a wide variety of ground- and surface water environments. Distinction between continentally- and marine-derived organic nitrogen in ecologically sensitive coastal waters also appears possible. These differences in 15N/14N ratios, however, are largely the result of kinetic isotope fractionation associated with bacterially-mediated reactions. The unpredictable magnitude of this type of fractionation tends to restrict the use of nitrogen isotope data in the hydrosphere to semi-quantitative interpretations.Observations of the isotopic fractionation between nitrogen compounds in the atmosphere may provide valuable information on whether their physico-chemical reactions are controlled by kinetic or by equilibrium processes. The possibility of using 15N/14N data for distinguishing between anthropogenic and natural sources of NOx gases, potentially a very important application, is as yet unproven.
In the past, marine sedimentologists have been unable to quantitatively investigate modern sediment accumulation on continental shelves, but recent development of Pb-210 geochronology provides a tool to overcome this limitation. It is used in this study to examine the accumulation of modern Columbia River sediments on the Washington continental shelf.Pb-210 profiles in Washington shelf sediments reveal three characteristic regions: a homogeneous surface layer (about 10 cm thick) where sediments are actively mixed by physical and biological processes, a region where Pb-210 activities decrease logarithmically with depth in the sediment, and a lower region of background activities. The surface mixed layer generally reflects erosion by wave and current activity in inner shelf sediments (shallower than about 60 m), and biological mixing in mid and outer shelf sediments. The region of logarithmic activities (i.e., radioactive decay) in the Pb-210 profiles provides the rate of sediment accumulation, which is on the order of mm/yr. The predominant depositional feature is a mid-shelf silt deposit in which accumulation rates progressively decrease north-northwestward away from the Columbia River.These results are consistent with other sedimentological observations, and indicate that Pb-210 geochronology not only allows determination of modern sediment accumulation rates, but also provides additional insight into processes affecting accumulation. Pb-210 geochronology can be an important sedimentological tool for future studies of continental shelf sedimentation in areas undergoing modern sediment accumulation.
Hypoxic bottom water occurred off the upper Texas coast in May-July 1979 after heavy spring runoff and a diatom bloom. Benthic assemblages off Freeport, Texas decreased from spring densities of 3000-4000 ind./m2 to 300-600 ind./m2 by late July. Species diversity also decreased and perturbations occurred in Shannon-Weiner diversity indices. Of the dominant taxa, polychaetes were least affected by hypoxia, amphipods and echinoderms most affected. Storm-caused waves in late July initiated the breakup of hypoxic conditions, which had completely abated by late September. A short-lived irruption occurred immediately afterward as depopulated bottoms were reinvaded.-from Authors
We used stable isotope measurements to investigate the possible nutritional importance of diatoms for consumers in planktonic food webs. Several lines of evidence indicated that rapidly growing diatoms had C-13-rich isotopic compositions in the Georges Bank (USA) ecosystem. Diatoms in spring blooms and in well-mixed summer waters were relatively rich in C-13, with delta-C-13 values in the -15 to -19 parts per thousand range, while other phytoplankton and most particulate organic matter collected over a 13 mo period had C-13-depleted values of -21 to -25 parts per thousand. Culture experiments with nutrient-enriched seawater performed on Georges Bank and in Woods Hole Harbor (MA, USA) also showed a C-13 distinction between fast-growing diatoms with C-13-rich contents and other algae that were depleted in C-13. Zooplankton from central Georges Bank where diatoms are abundant had relatively high delta-C-13 values, consitent with an important nutritional role for C-13 diatoms. We estimate that a minimum of 40% of the carbon present in zooplankton consumers of central Georges Bank is derived from diatoms. Bloom diatoms from the North Atlantic, northeastern Pacific, and the nearshore Gulf of Mexico also had C-13-rich compositions, indicating that diatoms can be a source of C-13-rich carbon in many marine food webs.
Laboratory study of the bacterial decomposition of Long Island Sound plankton in oxygenated seawater over a period of 2 years shows that the organic material undergoes decomposition via first‐order kinetics and can be divided into two decomposable fractions, of considerably different reactivity, and a nonmetabolizable fraction. This planktonic material, after undergoing varying degrees of oxic degradation, was added in the laboratory to anoxic sediment taken from a depth of 1 m at the NWC site of Long Island Sound and the rate of bacterial sulfate reduction in the sediment measured by the ³⁵ S radiotracer technique. The stimulated rate of sulfate reduction was in direct proportion to the amount of planktonic carbon added. This provides direct confirmation of the first‐order decomposition, or G model, for marine sediments and proves that the in situ rate of sulfate reduction is organic‐matter limited. Slower sulfate reduction rates resulted when oxically degraded plankton rather than fresh plankton was added, and the results confirm the presence of the same two fractions of organic matter deduced from the oxic degradation studies.
Near‐surface Long Island Sound sediment, which already contains abundant readily decomposable organic matter, was also subjected to anoxic decomposition by bacterial sulfate reduction. The decrease in sulfate reduction rate with time parallels decreases in the amount of organic matter, and these results also indicate the presence of two fractions of organic carbon of distinctly different reactivity. From plots of the log of reduction rate vs. time two first‐order rate constants were obtained that agree well with those derived from the plankton addition experiment. Together, the two experiments confirm the use of a simple multi‐first‐order rate law for organic matter decomposition in marine sediments.
CHRONIC pollution of the open ocean has occurred since 1986 through disposal of municipal sewage sludge at a deep-water (~2,500 m) dumpsite off the coast of New Jersey. Dispersal and dilution of sewage particulates in surface waters were presumed to be sufficient to prevent or minimize accumulation of detectable amounts of sewage-derived material on the sea floor. Using stable isotope ratios of carbon, nitrogen and sulphur as tracers of sewage-derived organic material, we show here that this material reaches the sea floor and enters the benthic food web, specifically through surface-deposit feeding activities of the urchin, Echinus affinus and the sea cucumber, Benthodytes sanguinolenta.
A sensitive, reproducible method is described for the characterization of lignin in untreated plant and geochemical samples containing 10 mg or less organic matter. The whole sample is treated with alkaline cupric oxide at 170 degrees to produce simple lignin- derived phenols that are extracted with ethyl oxide and analyzed by gas capillary chromatography on fused silica columns. A pattern of 11 or less phenols is produced that reflects the relative concentration and plant tissue sources of lignins present in the sample.
Lignin oxidation products and stable carbon isotope distributions are used to investigate the sources, transport, and chemical stability of land-derived organic matter in dated cores of modern sediment from the southern Washington State continental shelf and slope. There is no evidence for significant chemical alteration of lignin compounds in these sediments for time periods of up to 400 yr. Gymnosperm woods and nonwoody angiosperm tissues account for most of the land-derived organic matter in the deposits. These land plant remains have an average 13 C of approximately -25.5% and are concentrated in a narrow band of silty sediment which extends northward from the Columbia River mouth along the mid-shelf. Marine organic matter having an approximate 13 C of -21.5%, strongly predominates in most other shelf and slope environments. Net fluxes of land-derived organic matter into the surface 5 cm of the cores vary directly with sediment accumulation rates. Net fluxes of marine organic material into the surface sediments are highest in environments which favor the preservation of organic matter, but correspond to less than 1% of the primary productivity in the overlying waters.
In two distal carbonate turbidites (labelled a and e) from the Madeira Abyssal Plain oxidation of organic matter (OM) has been accompanied by a reduction in Hydrogen Index (HI) and change in the isotopic composition of carbon. Since the emplacement of turbidite α 0.5 ka ago 35% of the OM in the upper part has been oxidised by oxygen and nitrate diffusing from sea water into the sediment. There is an accompanying decrease of ≤ 0.3‰ in δ13C-OM, and a decrease of 25% in HI. Turbidite e was emplaced about 73 ka and was exposed to bottom water for 12.3 ka before being buried beneath later turbidites. During exposure oxygen and nitrate diffused into its upper part and oxidised 50% of the TOC without an accompanying change in isotopic composition; further oxidation of the more refactory organic components decreased TOC by a further 25% and decreased δ13C by 2.5‰. Oxidation also reduced HI by 50% from originally low values. Unless corrected for, the isotopic changes accompanying extended oxic and suboxic degradation of organic matter in sediments will give rise to erroneous estimates of marine-terrestrial OM ratios.
Bottom sediments around the outfall of the Humber Sewage Treatment Plant (STP), which, with the Humber River, comprises the major contaminant point source emptying into Humber Bay, Lake Ontario, were analyzed for a number of cultural and industrial organic contaminants, including sewage-related compounds such as the faecal sterol coprostanol, -tocopheryl acetate (-TA), linear-chain n-alkane hydrocarbons, and carbon and nitrogen isotope ratios of refractory sediment organic matter. The goal was to test these chemical markers as tracers of contaminated fine sediment transport. Although these chemical species are not all conservative (some decompose at fairly rapid rates), they are believed to be good natural tags for fine sediment and should nonetheless exhibit clear dispersal plumes from a presumed source. Their plumes can thus provide an effective way to obtain qualitative, intermediate- and long-term transport pathway indicators for contaminated sediments discharged on a continuing basis. The contoured plots for coprostanol, alpha-tocopheryl acetate, and n-alkane distribution were characterized by high values near the STP outfall, and these tracers were detectable at distances of 1 km or more. Interpretation of the net transport patterns indicates a primary transport toward the south and southwest, with a secondary trend northward, curving eastward. The carbon and nitrogen ratios, while showing a slightly different pattern, served to differentiate STP-source materials from those coming from the river and the open lake. Thus, transport from these sources, sometimes in the opposite direction to that from the main source, were resolved. Systematic variations in the interpreted transport patterns are explained by the presence of different pathways taken by the sediments, i.e. whether bedload or suspended load in a stratified water column.
Net remineralization rates of organic matter and bacterial growth rates were observed in dark-bottle incubation experiments
conducted in July–August and February with water samples collected from sites in the Mississippi River plume of the Gulf of
Mexico. Our objectives were to measure site-specific degradation rates of labile dissolved and particulate organic matter,
quantify the potential importance of bacteria in these processes, and examine the kinetics of degradation over time. Unfiltered
samples, and samples treated to remove (or dilute out) particles larger than bacteria, were enclosed in 9-1 bottles and incubated
in the dark for 3–5 d. Respiration rates and inorganic compound accumulation rates were higher in summer than in winter and
were highest in unfiltered surface samples at sites of intermediate salinities where phytoplankton were most abundant. The
ratio of ammonium accumulation to oxygen removal in summer experiments suggested that the mineralized organic material resembled
“Redfield” stoichiometry. Chemical fluxes were greater in bottles containing large (>1–3 μm) particles than in the bottles
with these particles removed, but bacterial activities were generally similar in both treatments. These results suggest that
particle consumers were an important component of total organic matter degradation. However, these experiments may have underestimated
natural bacterial degradation rates because the absence of light could affect the production of labile organic substrates
by phytoplankton. In agreement, with this hypothesis, bacterial growth rates tended to decrease over time in summer in surface
plume waters where phytoplankton were abundant. In conjunction with other data, our results indicate that heterotrophic processes
in the water column are spatially and temporally dependent on phytoplankton production.
Early diagenesis is the combination of biological, chemical, and physical processes which change the quantity and composition of the organic matter in the upper several hundred meters of marine sediments. During early diagenesis, from 30% to more than 99% of the organic matter deposited on the sediment surface is remineralized by sediment organisms. Rates of organic matter oxidation or oxidant consumption have now been measured in a variety of sediments and range over at least three orders of magnitude, from 20 mmol C m⁻² year⁻¹ to 60 mol C m⁻² year⁻¹. High rates of organic matter oxidation are associated with high rates of sediment accumulation, organic matter influx to the sediment surface, and organic matter burial.
Hydrocarbon results from gas chromatography of 60 recent sediment and 10 benthic algae samples delineate two distinct shelf environments in the northeastern Gulf of Mexico.Sediments off Florida (shell hashes and sands) have moderate amounts of lipids/total sediment (average 113ppm ± 80%) but low hydrocarbon levels (average 3.06 ppm ± 41%). Aliphatic hydrocarbons are dominated by a series of branched or cyclic, unsaturated C25 isomers. The major n-alkane is n-C17. The n-alkane and isoprenoid patterns are consistent with a marine hydrocarbon source.Sediments closer to the Mississippi River (silts and clays) contain large amounts of lipids (average 232 ppm ± 53%) and hydrocarbons (average 11.7 ppm ± 55%) to total sediment. Aliphatic hydrocarbons are mainly odd carbon number high molecular weight n-alkanes, indicating a terrigenous hydrocarbon source. Isoprenoids are present in greater abundance than in sediments off Florida (n-C17/ pristane and n-C18/phytane ratios ~2to 3). Relatively large amounts of n-C16, together with an even distribution of n-alkanes in the range C14–C20 and a substantial unresolved envelope all point to a fossil fuel input to the Mississippi samples.Samples off the Alabama coast show intermediate characteristics.
The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition.
The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet.
The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources.
The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.
Study of an algal, sapropelic sediment from Mangrove Lake, Bermuda shows that the mass balance of carbon and stable carbon isotopes in the major organic constituents is accounted for by a relatively straightforward model of selective preservation during diagenesis. The loss of 13C-enriched carbohydrates is the principal factor controlling the intermolecular mass balance of 13C in the sapropel. Results indicate that labile components are decomposed leaving as a residual concentrate in the sediment an insoluble humic substance that may be an original biochemical component of algae and associated bacteria. An overall decrease of up to about 4‰ in the δ 13C values of the organic matter is observed as a result of early diagenesis.
δ13CPDB compositions for 39 samples of dissolved organic carbon (DOC) from the Gulf of Mexico-Caribbean Sea-Atlantic Ocean system, the South Pacific and Ross Sea are reported. Deep water values are similar with a mean of −21.8%. attesting to the homogeneity of the oceanic DOC pool. In Antarctic waters, a 5%. difference between DOC and particulate organic carbon (POC), with POC having values similar to modern plankton (δ13CPDB approx −27%.) supports the idea of the transient nature of POC as compared to DOC.Total, lipid, acid hydrolyzed, amino acid and residue fractions of POC are about 5, 3, 7, 5 and 3%. respectively, more negative in 2000 m water as compared to surface water samples from the Gulf of Mexico.
Lignin oxidation products and 13C/12C ratios were compared as indicators of land-derived organic matter in surface sediments from the western Gulf of Mexico. Whole sediments were reacted with cupric oxide to yield phenolic oxidation products that indicated the types and relative amounts of the lignins that were present.Measurements of lignin concentration and carbon isotope abundances both indicated a sharp offshore decrease of land-derived organic matter in most areas of the western Gulf. This decrease results primarily from mixing of terrestrial and marine organic matter. The terrestrially derived material in these sediments has a lignin content similar to that of grasses and tree leaves. Flowering plants contribute most of the sedimented lignin compounds. These lignins apparently occur in the form of well-mixed plant fragments that are transported to sea by rivers and deposited primarily on the inner continental shelf.
Stable isotope ratios of carbon and hydrogen were determined for six algae and one seagrass species during a 60-day decomposition experiment. Changes in the carbon ratios were in the order of 1‰. However, significant isotopic changes were observed for hydrogen in all species and in most cases these changes were substantial. These changes probably reflect differential leaching of isotopically distinct compounds.
Thirty-five years of monthly lower Mississippi River water discharge, nitrate concentration and nitrate flux have been analyzed. The potential predictability of these quantities has been evaluated. Results indicate a large amplitude, long-term cycle in the nitrate concentration that is not observed in the water discharge. A decrease in average nitrate concentration from a peak in 1983 to the present confirms that this variability is more cyclic than trend-like. River-water discharge variation was greatest in association with the annual cycle. The annual water discharge and nitrate concentration cycles were similar; high nitrate concentrations usually occurred near the vernal freshet, and low concentrations usually occurred along with autumnal low-flow conditions. Nitrate flux variations exhibited a low amplitude, long-term modulation of a dominant, annual cycle. A predictor-hindcast analysis indicates that truly skilled forecasts of all three fields are feasible.
Progress in understanding global biogeochemical cycles has been made on many fronts of organic geochemical study over the last decade. Key questions, however, remain to be answered. These include the following: 1.(1) Is kerogen completely remineralized during the weathering of sedimentary rocks?2.(2) What types of refractory biochemicals are made by plants?3.(3) What factors control export of organic matter from the land to the ocean?4.(4) What are the components, sources and fates of organic materials dissolved in seawater?5.(5) What types of organic substances accumulate in deltaic and coastal marine sediments?6.(6) Is better understanding of global biogeochemical cycles possible through new conceptual approaches?
During June 1983, surface- and bottom-water samples were collected (over a 38-h period) at two anchor stations just seaward of the Amazon River mouth. Samples were analyzed for dissolved and particulate234Th activities.234Th distribution coefficients (Kd; ratio of particulate to dissolved234Th activities) exhibited significant temporal variability, ranging from 7.4 × 103 to 1.0 × 105ml g−1 in surface waters and from 8.8 × 102 to 3.4 × 104ml g−1 in bottom waters. Environmental parameters (e.g. salinity, suspended-sediment concentration) also exhibited a high degree of temporal variability as a result of tidal processes. Based on correlation with Kd, suspended-sediment concentration and particle characteristics were the major environmental parameters controlling the scavenging of thorium. Thorium distribution coefficients were never observed to reach equilibrium values in the study area, indicating that the scavenging of thorium was limited by reaction kinetics. A kinetic (non equilibrium) approach to scavenging reveals that suspended particles (because of their short residence time in the water column) had only 1–2 days to equilibrate with any particular hydrographic regime within the anchor-station area. The residence time of dissolved234Th (with respect to scavenging) ranged from 1.8 to 3.8 days, depending on suspended-sediment concentrations and particle characteristics. The total residence time of234Th (with respect to removal from the water column) was nearly uniform (4–5 days) throughout the anchor-station area, because locations with the shortest scavenging residence times (as a result of high turbidity) had the longest particle residence times (as a result of resuspension).
The stable carbon isotopic ratio (13C:12C) of organic matter in the water column and sediments has been used to examine the distribution and accumulation of organic carbon on the Amazon continental shelf. Near the river mouth, in 0 × 10−3 salinity surface waters, particulate organic carbon (POC) is isotopically light (−27.3 per mil). Isotopic values of POC in continental shelf waters north and east of the river mouth range from −19.5 to −25.7 per mil. The isotopic variations in water column POC can be related to productivity, turbidity, and water density. The isotopic character of water column POC is controlled, therefore, by the dynamic mixing and northwestward migration of riverine and marine shelf waters. Terrestrial organic carbon dominates the isotopic signal in surface sediments (upper 10 cm) from the river mouth to areas 400 km to the northwest. Only on the outermost and northern parts of the shelf is marine organic carbon dominant in surface sediments. A sharp boundary between terrestrial and marine isotopic values is present in continental shelf sediments at the outer edge of the prograding mud delta. This boundary is associated with an abrupt decrease in sediment accumulation rate and a change in stratigraphic setting from topset and foreset regions to bottomset regions. The average TOC content of Amazon continental shelf sediments is 0.66 ± 0.20%. Based on the average TOC of Amazon shelf sediments and current estimates of sediment accumulation rates, approximately 4.5 × 1012 g-TOC y−1 is accumulating in Amazon shelf sediments. Using δ13C-POC values at salinities of 0 and 35 × 10−3 as end-members to isotopically resolve the contribution of terrestrial and marine carbon sources, approximately 3.1 × 1012 g-TOC y−1 or 69% of the organic carbon accumulating in Amazon shelf surface sediments is from terrestrial sources. This represents 6% of current estimates for Amazon riverine carbon discharge.
Thesis--Texas A & M University, 1976. Vita. Includes bibliographical references (leaves 92-96). Microfilm of typescript. s
Thesis (Ph. D.)--University of Texas at Austin, 1982. Vita. Includes bibliographical references (leaves 236-257). Photocopy.
Thesis--Florida State University. Includes bibliographical references (leaves 90-102). Photocopy.
Specifically radiolabeled [C-lignin]lignocelluloses and [C-polysaccharide]lignocelluloses were prepared from a variety of marine and freshwater wetland plants including a grass, a sedge, a rush, and a hardwood. These [C]lignocellulose preparations and synthetic [C]lignin were incubated anaerobically with anoxic sediments collected from a salt marsh, a freshwater marsh, and a mangrove swamp. During long-term incubations lasting up to 300 days, the lignin and polysaccharide components of the lignocelluloses were slowly degraded anaerobically to CO(2) and CH(4). Lignocelluloses derived from herbaceous plants were degraded more rapidly than lignocellulose derived from the hardwood. After 294 days, 16.9% of the lignin component and 30.0% of the polysaccharide component of lignocellulose derived from the grass used (Spartina alterniflora) were degraded to gaseous end products. In contrast, after 246 days, only 1.5% of the lignin component and 4.1% of the polysaccharide component of lignocellulose derived from the hardwood used (Rhizophora mangle) were degraded to gaseous end products. Synthetic [C]lignin was degraded anaerobically faster than the lignin component of the hardwood lignocellulose; after 276 days, 3.7% of the synthetic lignin was degraded to gaseous end products. Contrary to previous reports, these results demonstrate that lignin and lignified plant tissues are biodegradable in the absence of oxygen. Although lignocelluloses are recalcitrant to anaerobic biodegradation, rates of degradation measured in aquatic sediments are significant and have important implications for the biospheric cycling of carbon from these abundant biopolymers.
Long-term changes in the Mississippi River water quality and its relationship to hypoxic continental shelf waters The Coastal Society
- R E Turner
The relationship of nitrate flux to water discharge and nitrate concentration in the Mississippi River 13C analyses of oceanic particulate organic matter
- S P And
- A Bratkovicii Eadie
- B J A ~d
- I M Jervrev
DINNELI., S. P. AND A. BRATKOVICII. In Press. The relationship of nitrate flux to water discharge and nitrate concentration in the Mississippi River. Journal of Marine Systems. EADIE, B. J. A.'~D I.. M. JErVREV. 1973. 13C analyses of oceanic particulate organic matter. Marine. Chemistry 1:199-209.
Rate-dependent distributions of lead-210 and interstitial sulfate in sediments of the Mississippi River delta. Ph.D. Dissertation, Texas A&M University, College Sta-tion Stable isotopic char-acterization of organic carbon accumulation on the Amazon continental shelf
- R E Shokes
SHOKES, R. E 1976. Rate-dependent distributions of lead-210 and interstitial sulfate in sediments of the Mississippi River delta. Ph.D. Dissertation, Texas A&M University, College Sta-tion, Texas. SHOWERS, W. J. AND D. G. ANGLE. 1986. Stable isotopic char-acterization of organic carbon accumulation on the Amazon continental shelf. Continental Shelf Research 6:227-244.