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Net Primary Production in the Ocean

Authors:
Philip W. Boyd at University of Tasmania
Philip W. Boyd
Svein Sundby at Institute of Marine Research in Norway
Svein Sundby
Hans-Otto Pörtner at Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research
Hans-Otto Pörtner
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Abstract

Net Primary Production (NPP) is the rate of photosynthetic carbon fixation minus the fraction of fixed carbon used for cellular respiration and maintenance by autotrophic planktonic microbes and benthic plants (Sections 6.2.1, 6.3.1). Environmental drivers of NPP include light, nutrients, micronutrients, CO 2 , and temperature (Figure PP-1a). These drivers, in turn, are influenced by oceanic and atmospheric processes, including cloud cover; sea ice extent; mixing by winds, waves, and currents; convection; density stratification; and various forms of upwelling induced by eddies, frontal activity, and boundary currents. Temperature has multiple roles as it influences rates of phytoplankton physiology and heterotrophic bacterial recycling of nutrients, in addition to stratification of the water column and sea ice extent (Figure PP-1a). Climate change is projected to strongly impact NPP through a multitude of ways that depend on the regional and local physical settings (WGI AR5, Chapter 3), and on ecosystem structure and functioning (medium confidence; Sections 6.3.4, 6.5.1). The influence of environmental drivers on NPP causes as much as a 10-fold variation in regional productivity with nutrient-poor subtropical waters and light-limited Arctic waters at the lower range and productive upwelling regions and highly eutrophic coastal regions at the upper range (Figure PP-1b).
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Net Primary Production in
the Ocean
Philip W. Boyd (New Zealand), Svein Sundby (Norway), Hans-Otto Pörtner (Germany)
PP
133
Net Primary Production (NPP) is the rate of photosynthetic carbon fixation minus the fraction of
fixed carbon used for cellular respiration and maintenance by autotrophic planktonic microbes
and benthic plants (Sections 6.2.1, 6.3.1). Environmental drivers of NPP include light, nutrients,
micronutrients, CO2, and temperature (Figure PP-1a). These drivers, in turn, are influenced by
oceanic and atmospheric processes, including cloud cover; sea ice extent; mixing by winds, waves,
and currents; convection; density stratification; and various forms of upwelling induced by eddies,
frontal activity, and boundary currents. Temperature has multiple roles as it influences rates
of phytoplankton physiology and heterotrophic bacterial recycling of nutrients, in addition to
stratification of the water column and sea ice extent (Figure PP-1a). Climate change is projected
to strongly impact NPP through a multitude of ways that depend on the regional and local
physical settings (WGI AR5, Chapter 3), and on ecosystem structure and functioning (medium
confidence; Sections 6.3.4, 6.5.1). The influence of environmental drivers on NPP causes as much
as a 10-fold variation in regional productivity with nutrient-poor subtropical waters and light-
limited Arctic waters at the lower range and productive upwelling regions and highly eutrophic
coastal regions at the upper range (Figure PP-1b).
The oceans currently provide ~50 × 1015 g C yr–1, or about half of global NPP (Field et al., 1998).
Global estimates of NPP are obtained mainly from satellite remote sensing (Section 6.1.2),
which provides unprecedented spatial and temporal coverage, and may be validated regionally
against oceanic measurements. Observations reveal significant changes in rates of NPP when
environmental controls are altered by episodic natural perturbations, such as volcanic eruptions
enhancing iron supply, as observed in high-nitrate low-chlorophyll waters of the Northeast Pacific
(Hamme et al., 2010). Climate variability can drive pronounced changes in NPP (Chavez et al.,
2011), such as from El Niño to La Niña transitions in Equatorial Pacific, when vertical nutrient and
trace element supply are enhanced (Chavez et al., 1999).
Multi-year time series records of NPP have been used to assess spatial trends in NPP in recent
decades. Behrenfeld et al. (2006), using satellite data, reported a prolonged and sustained global
NPP decrease of 190 × 1012 g C yr–1, for the period 1999–2005—an annual reduction of 0.57%
of global NPP. In contrast, a time series of directly measured NPP between 1988 and 2007 by
Saba et al. (2010) (i.e., in situ incubations using the radiotracer 14C-bicarbonate) revealed an
increase (2% yr–1) in NPP for two low-latitude open ocean sites. This discrepancy between in situ
and remotely sensed NPP trends points to uncertainties in either the methodology used and/
or the extent to which discrete sites are representative of oceanic provinces (Saba et al., 2010,
2011). Modeling studies have subsequently revealed that the <15-year archive of satellite-
Cross-Chapter Box
Net Primary Production in the Ocean
134
PP
Nutrient
recycling
• Nutrients
• Trace metals
Euphotic zone (0–100 m)
• Upwelling
• Vertical mixing
• Stratification
(a)
Light
• Zooplankton
• Microbes
NPP (g C m–² y–¹)
300 250 200 150 100 50 0
(b)
Latitude
Season
Cloud cover
Figure PP-1 |
(a) Environmental factors controlling Net Primary Production (NPP). NPP is controlled mainly by three basic processes: (1) light conditions in the surface ocean, that
is, the photic zone where photosynthesis occurs; (2) upward flux of nutrients and micronutrients from underlying waters into the photic zone, and (3) regeneration of nutrients and
micronutrients via the breakdown and recycling of organic material before it sinks out of the photic zone. All three processes are influenced by physical, chemical, and biological
processes and vary across regional ecosystems. In addition, water temperature strongly influences the upper rate of photosynthesis for cells that are resource-replete. Predictions of
alteration of primary productivity under climate change depend on correct parameterizations and simulations of each of these variables and processes for each region. (b) Annual
composite map of global areal NPP rates (derived from Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua satellite climatology from 2003–2012; NPP was calculated
with the Carbon-based Productivity Model (CbPM; Westberry et al., 2008)). Overlaid is a grid of (thin black lines) that represent 51 distinct global ocean biogeographical provinces
(after Longhurst, 1998 and based on Boyd and Doney, 2002). The characteristics and boundaries of each province are primarily set by the underlying regional ocean physics and
chemistry. White areas = no data. (Figure courtesy of Toby Westberry (OSU) and Ivan Lima (WHOI), satellite data courtesy of NASA Ocean Biology Processing Group.)
1
3
3
2
Copepod pellets
Microzooplankton
mini-pellets
Particles
Coccolithophorids
Diatoms
Other
Phytoplankton
Copepods
Plankton
Temperature
PP
Net Primary Production in the Ocean
Cross-Chapter Box
135
Arrigo, K.R. and G.L. van Dijken, 2011: Secular trends in Arctic Ocean net primary production. Journal of Geophysical Research, 116(C9), C09011,
doi:10.1029/2011JC007151.
Beaulieu, C., S.A. Henson, J.L. Sarmiento, J.P. Dunne, S.C. Doney, R.R. Rykaczewski, and L. Bopp, 2013: Factors challenging our ability to detect long-term trends in ocean
chlorophyll. Biogeosciences, 10(4), 2711-2724.
Behrenfeld, M.J., R.T. O’Malley, D.A. Siegel, C.R. McClain, J.L. Sarmiento, G.C. Feldman, A.J. Milligan, P.G. Falkowski, R.M. Letelier, and E.S. Boss, 2006: Climate-driven
trends in contemporary ocean productivity. Nature, 444(7120), 752-755.
Bopp, L., L. Resplandy, J.C. Orr, S.C. Doney, J.P. Dunne, M. Gehlen, P. Halloran, C. Heinze, T. Ilyina, R. Séférian, J. Tijiputra, and M. Vichi, 2013: Multiple stressors of ocean
ecosystems in the 21st century: projections with CMIP5 models. Biogeosciences, 10, 6225-6245.
Boyce, D.G., M.R. Lewis, and B. Worm, 2010: Global phytoplankton decline over the past century. Nature, 466(7306), 591-596.
Boyd, P.W. and S.C. Doney, 2002: Modelling regional responses by marine pelagic ecosystems to global climate change. Geophysical Research Letters, 29(16), 53-1–53-4,
doi:10.1029/2001GL014130.
Boyd, P.W., R. Strzepek, F.X. Fu, and D.A. Hutchins, 2010: Environmental control of open-ocean phytoplankton groups: now and in the future. Limnology and
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Chavez, F.P., P.G. Strutton, C.E. Friederich, R.A. Feely, G.C. Feldman, D.C. Foley, and M.J. McPhaden, 1999: Biological and chemical response of the equatorial Pacific Ocean
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References
derived NPP is insufficient to distinguish climate-change mediated shifts in NPP from those driven by natural climate variability (Henson et al.,
2010; Beaulieu et al., 2013). Although multi-decadal, the available time series of oceanic NPP measurements are also not of sufficient duration
relative to the time scales of longer-term climate variability modes as for example Atlantic Multi-decadal Oscillation (AMO), with periodicity of
60-70 years, Figure 6-1). Recent attempts to synthesize longer (i.e., centennial) records of chlorophyll as a proxy for phytoplankton stocks (e.g.,
Boyce et al., 2010) have been criticized for relying on questionable linkages between different proxies for chlorophyll over a century of records
(e.g., Rykaczewski and Dunne, 2011).
Models in which projected climate change alters the environmental drivers of NPP provide estimates of spatial changes and of the rate of
change of NPP. For example, four global coupled climate–ocean biogeochemical Earth System Models (WGI AR5 Chapter 6) projected an
increase in NPP at high latitudes as a result of alleviation of light and temperature limitation of NPP, particularly in the high-latitude biomes
(Steinacher et al., 2010). However, this regional increase in NPP was more than offset by decreases in NPP at lower latitudes and at mid-
latitudes due to the reduced input of macronutrients into the photic zone. The reduced mixed-layer depth and reduced rate of circulation may
cause a decrease in the flux of macronutrients to the euphotic zone (Figure 6-2). These changes to oceanic conditions result in a reduction in
global mean NPP by 2 to 13% by 2100 relative to 2000 under a high emission scenario (Polovina et al., 2011; SRES (Special Report on Emission
Scenarios) A2, between RCP6.0 and RCP8.5). This is consistent with a more recent analysis based on 10 Earth System Models (Bopp et al.,
2013), which project decreases in global NPP by 8.6 (±7.9), 3.9 (±5.7), 3.6 (±5.7), and 2.0 (±4.1) % in the 2090s relative to the 1990s, under
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Model projections had predicted a range of changes in global NPP from an increase (relative to preindustrial rates) of up to 8.1% under an
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between 1998 and 2009. Other regional trends in NPP are reported in Sections 30.5.1 to 30.5.6. In addition, although future model projections
of global NPP from different models (Steinacher et al., 2010; Bopp et al., 2013) are comparable, regional projections from each of the models
differ substantially. This raises concerns as to which aspect(s) of the different model NPP parameterizations are responsible for driving regional
differences in NPP, and moreover, how accurate model projections are of global NPP.
From a global perspective, open ocean NPP will decrease moderately by 2100 under both low- (SRES B1 or RCP4.5) and high-emission
scenarios (medium confidence; SRES A2 or RCPs 6.0, 8.5, Sections 6.3.4, 6.5.1), paralleled by an increase in NPP at high latitudes and
a decrease in the tropics (medium confidence). However, there is limited evidence and low agreement on the direction, magnitude and
differences of a change of NPP in various ocean regions and coastal waters projected by 2100 (low confidence).
Cross-Chapter Box
Net Primary Production in the Ocean
136
PP
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This cross-chapter box should be cited as:
... 200 g.m -2 .yr -1 (Boyd et al., 2014). Two meteorological seasons can be distinguished in this tropical oceanic region of the southern hemisphere: a hot season that extends from October to March, with an average sea-surface temperature reaching between 27 ºC and 28 ºC in February; and a fresher season, extending from April to September, with an average sea-surface temperature being as low as 23 ºC in August (Donguy and Henin, 1981). ...
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Framing and Context of the Report Chapter 1 Executive Summary This special report assesses new knowledge since the IPCC 5th Assessment Report (AR5) and the Special Report on Global Warming of 1.5oC (SR15) on how the ocean and cryosphere have and are expected to change with ongoing global warming, the risks and opportunities these changes bring to ecosystems and people, and mitigation, adaptation and governance options for reducing future risks. Chapter 1 provides context on the importance of the ocean and cryosphere, and the framework for the assessments in subsequent chapters of the report. All people on Earth depend directly or indirectly on the ocean and cryosphere. The fundamental roles of the ocean and cryosphere in the Earth system include the uptake and redistribution of anthropogenic carbon dioxide and heat by the ocean, as well as their crucial involvement of in the hydrological cycle. The cryosphere also amplifies climate changes through snow, ice and permafrost feedbacks. Services provided to people by the ocean and/or cryosphere include food and freshwater, renewable energy, health and wellbeing, cultural values, trade and transport. {1.1, 1.2, 1.5} Sustainable development is at risk from emerging and intensifying ocean and cryosphere changes. Ocean and cryosphere changes interact with each of the United Nations Sustainable Development Goals (SDGs). Progress on climate action (SDG 13) would reduce risks to aspects of sustainable development that are fundamentally linked to the ocean and cryosphere and the services they provide (high confidence1). Progress on achieving the SDGs can contribute to reducing the exposure or vulnerabilities of people and communities to the risks of ocean and cryosphere change (medium confidence). {1.1} Communities living in close connection with polar, mountain, and coastal environments are particularly exposed to the current and future hazards of ocean and cryosphere change. Coasts are home to approximately 28% of the global population, including around 11% living on land less than 10 m above sea level. Almost 10% of the global population lives in the Arctic or high mountain regions. People in these regions face the greatest exposure to ocean and cryosphere change, and poor and marginalised people here are particularly vulnerable to climate-related hazards and risks (very high confidence). The adaptive capacity of people, communities and nations is shaped by social, political, cultural, economic, technological, institutional, geographical and demographic factors. {1.1, 1.5, 1.6, Cross-Chapter Box 2 in Chapter 1} Ocean and cryosphere changes are pervasive and observed from high mountains, to the polar regions, to coasts, and into the deep ocean. AR5 assessed that the ocean is warming (0 to 700 m: virtually certain2; 700 to 2,000 m: likely), sea level is rising (high confidence), and ocean acidity is increasing (high confidence). Most glaciers are shrinking (high confidence), the Greenland and Antarctic ice sheets are losing mass (high confidence), sea ice extent in the Arctic is decreasing (very high confidence), Northern Hemisphere snow cover is decreasing (very high confidence), and permafrost temperatures are increasing (high confidence). Improvements since AR5 in observation systems, techniques, reconstructions and model developments, have advanced scientific characterisation and understanding of ocean and cryosphere change, including in previously identified areas of concern such as ice sheets and Atlantic Meridional Overturning Circulation (AMOC). {1.1, 1.4, 1.8.1} Evidence and understanding of the human causes of climate warming, and of associated ocean and cryosphere changes, has increased over the past 30 years of IPCC assessments (very high confidence). Human activities are estimated to have caused approximately 1.0oC of global warming above pre-industrial levels (SR15). Areas of concern in earlier IPCC reports, such as the expected acceleration of sea level rise, are now observed (high confidence). Evidence for expected slow-down of AMOC is emerging in sustained observations and from long-term palaeoclimate reconstructions (medium confidence), and may be related with anthropogenic forcing according to model simulations, although this remains to be properly attributed. Significant sea level rise contributions from Antarctic ice sheet mass loss (very high confidence), which earlier reports did not expect to manifest this century, are already being observed. {1.1, 1.4} Ocean and cryosphere changes and risks by the end-of-century (2081–2100) will be larger under high greenhouse gas emission scenarios, compared with low emission scenarios (very high confidence). Projections and assessments of future climate, ocean and cryosphere changes in the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) are commonly based on coordinated climate model experiments from the Coupled Model Intercomparison Project Phase 5 (CMIP5) forced with Representative Concentration Pathways (RCPs) of future radiative forcing. Current emissions continue to grow at a rate consistent with a high emission future without effective climate change mitigation policies (referred to as RCP8.5). The SROCC assessment contrasts this high greenhouse gas emission future with a low greenhouse gas emission, high mitigation future (referred to as RCP2.6) that gives a two in three chance of limiting warming by the end of the century to less than 2oC above pre-industrial. {Cross-Chapter Box 1 in Chapter 1} 1 1 2 In this report, the following summary terms are used to describe the available evidence: limited, medium, or robust; and for the degree of agreement: low, medium or high. A level of confidence is expressed using five qualifiers: very low, low, medium, high and very high, and typeset in italics, for example, medium confidence. For a given evidence and agreement statement, different confidence levels can be assigned, but increasing levels of evidence and degrees of agreement are correlated with increasing confidence (see Section 1.9.2 and Figure 1.4 for more details). In this report, the following terms have been used to indicate the assessed likelihood of an outcome or a result: Virtually certain 99–100% probability, Very likely 90–100%, Likely 66–100%, About as likely as not 33–66%, Unlikely 0–33%, Very unlikely 0–10%, and Exceptionally unlikely 0–1%. Additional terms (Extremely likely: 95–100%, More likely than not >50–100%, and Extremely unlikely 0–5%) may also be used when appropriate. Assessed likelihood is typeset in italics, for example, very likely (see Section 1.9.2 and Figure 1.4 for more details). This Report also uses the term ‘likely range’ to indicate that the assessed likelihood of an outcome lies within the 17–83% probability range. 75 1 Characteristics of ocean and cryosphere change include thresholds of abrupt change, long-term changes that cannot be avoided, and irreversibility (high confidence). Ocean warming, acidification and deoxygenation, ice sheet and glacier mass loss, and permafrost degradation are expected to be irreversible on time scales relevant to human societies and ecosystems. Long response times of decades to millennia mean that the ocean and cryosphere are committed to long-term change even after atmospheric greenhouse gas concentrations and radiative forcing stabilise (high confidence). Ice-melt or the thawing of permafrost involve thresholds (state changes) that allow for abrupt, nonlinear responses to ongoing climate warming (high confidence). These characteristics of ocean and cryosphere change pose risks and challenges to adaptation. {1.1, Box 1.1, 1.3} Societies will be exposed, and challenged to adapt, to changes in the ocean and cryosphere even if current and future efforts to reduce greenhouse gas emissions keep global warming well below 2oC (very high confidence). Ocean and cryosphere-related mitigation and adaptation measures include options that address the causes of climate change, support biological and ecological adaptation, or enhance societal adaptation. Most ocean-based local mitigation and adaptation measures have limited effectiveness to mitigate climate change and reduce its consequences at the global scale, but are useful to implement because they address local risks, often have co-benefits such as biodiversity conservation, and have few adverse side effects. Effective mitigation at a global scale will reduce the need and cost of adaptation, and reduce the risks of surpassing limits to adaptation. Ocean-based carbon dioxide removal at the global scale has potentially large negative ecosystem consequences. {1.6.1, 1.6.2, Cross-Chapter Box 2 in Chapter 1} The scale and cross-boundary dimensions of changes in the ocean and cryosphere challenge the ability of communities, cultures and nations to respond effectively within existing governance frameworks (high confidence). Profound economic and institutional transformations are needed if climate-resilient development is to be achieved (high confidence). Changes in the ocean and cryosphere, the ecosystem services that they provide, the drivers of those changes, and the risks to marine, coastal, polar and mountain ecosystems, occur on spatial and temporal scales that may not align within existing governance structures and practices (medium confidence). This report highlights the requirements for transformative governance, international and transboundary cooperation, and greater empowerment of local communities in the governance of the ocean, coasts, and cryosphere in a changing climate. {1.5, 1.7, Cross-Chapter Box 2 in Chapter 1, Cross-Chapter Box 3 in Chapter 1} Robust assessments of ocean and cryosphere change, and the development of context-specific governance and response options, depend on utilising and strengthening all available knowledge systems (high confidence). Scientific knowledge from observations, models and syntheses provides global to local scale understandings of climate change (very high confidence). Indigenous knowledge (IK) and local knowledge (LK) provide context-specific and socio-culturally relevant understandings for effective responses and policies (medium confidence). Education and climate literacy enable climate action and adaptation (high confidence). {1.8, Cross-Chapter Box 4 in Chapter 1} Long-term sustained observations and continued modelling are critical for detecting, understanding and predicting ocean and cryosphere change, providing the knowledge to inform risk assessments and adaptation planning (high confidence). Knowledge gaps exist in scientific knowledge for important regions, parameters and processes of ocean and cryosphere change, including for physically plausible, high impact changes like high end sea level rise scenarios that would be costly if realised without effective adaptation planning and even then may exceed limits to adaptation. Means such as expert judgement, scenario building, and invoking multiple lines of evidence enable comprehensive risk assessments even in cases of uncertain future ocean and cryosphere changes. {1.8.1, 1.9.2; Cross-Chapter Box 5 in Chapter 1}
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Marine snowstorm during the Permian−Triassic mass extinction
Article
Full-text available
  • Nov 2023
  • GEOLOGY
The Permian−Triassic mass extinction (PTME) interval is marked by major excursions in both inorganic and organic carbon (C) isotopes. Carbon cycle models predict that these trends were driven by large increases in productivity, yet organic C−rich rocks are not recorded in most PTME shelf sedimentary successions. Anomalous C-rich facies have been reported from rare abyssal plains records now exposed in Japan and New Zealand, where black shales at the PTME are extraordinarily organic-rich units. We examined organic matter at the Waiheke, New Zealand, section, and results show that these deposits are dominated by lamalginites composed of unicellar solitary or colonial phytoplankton produced during algal blooms that falls as “marine snow.” We modeled the impact of ash fall from eruptions in the Siberian Traps large igneous province and argue that they fertilized the Panthalassa Ocean with P and Fe, leading to a marine “snowstorm” and significant C drawdown marking this major biobloom during the PTME.
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Evaluating the impacts of environmental and fishery variability on the distribution of bigeye tuna in the Pacific Ocean
Article
Full-text available
  • Nov 2023
  • ICES J MAR SCI
Climate change-induced variabilities in the environment and fishing pressure affect the distribution and abundance of bigeye tuna in the Pacific Ocean. Understanding the causal relationships among these factors is complicated and challenging. We constructed a multi-output neural network model based on data from four types of bigeye tuna fisheries (longline and purse seine in the west-central and eastern Pacific Ocean, respectively) and marine environmental data, aiming to analyse the response of bigeye tuna to natural and anthropogenic factors from 1995 to 2019 in the Pacific Ocean. The input layer weights were used to explore the importance of environmental variable, while the output layer weights evaluated the contribution of fishing operations. These factors determined the final spatiotemporal distribution and abundance dynamics for bigeye tuna. The optimal model predicted a strong correlation between the locations of major habitats and El Niño southern Oscillation (ENSO) events, indicating that bigeye tuna abundance dynamics respond to the intensity of climate variability. During El Niño events, suitable conditions lead to an expansion of the main habitats east of 170◦W, while during La Niña events, the strengthening of the westward advection leads to the contraction of major habitats west of 170◦W. Furthermore, the resource abundance of bigeye tuna is predicted to be higher during moderate to weak El Niño events than during strong El Niño events. The abundances in purse seine and longline-dependent fisheries demonstrate significant different distribution patterns under different ENSO events, reflecting the unique environmental preferences at different life stages of bigeye tuna. Given the increasing frequency of climate variability and escalating fishing pressures, our findings provide beneficial insights for the sustainable development of bigeye tuna resource in the Pacific Ocean.
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Monthly Ocean Primary Productivity Forecasting by Joint Use of Seasonal Climate Prediction and Temporal Memory
Article
Full-text available
  • Mar 2023
Ocean primary productivity generated by phytoplankton is critical for ocean ecosystems and the global carbon cycle. Accurate ocean primary productivity forecasting months in advance is bene-ficial for marine management. Previous persistence-based prediction studies ignore the temporal memories of multiple relevant factors and the seasonal forecasting skill drops quickly with in-creasing lead time. On the other hand, the emerging ensemble climate forecasts are not well con-sidered as new predictability sources of ocean conditions. Here we proposed a joint forecasting model by combining the seasonal climate predictions from ten heterogeneous models and the temporal memories of relevant factors to examine the monthly predictability of ocean productivity from 0.5- to 11.5-month lead times. The results indicate that a total of ~90% and ~20% productive oceans are expected to be skillfully predicted by the combination of seasonal SST predictions and local memory at 0.5- and 4.5-month leads, respectively. The joint forecasting model improves by 10% of the skillfully predicted areas at 6.5-month lead relative to the prediction by productivity per-sistence. The hybrid data-driven and model-driven forecasting approach improves the predicta-bility of ocean productivity relative to individual predictions, of which the seasonal climate pre-dictions contribute largely to the skill improvement over the equatorial Pacific and Indian Ocean. These findings highlight the advantages of the integration of climate predictions and temporal memory for ocean productivity forecasting and may provide useful seasonal forecasting infor-mation for ocean ecosystem management.
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Spatial and temporal variability of Net Primary Production on the Agulhas Bank, 1998–2018
Article
Full-text available
  • Apr 2022
  • DEEP-SEA RES PT II
Despite the importance of Agulhas Bank (AB) marine productivity in supporting South African coastal fisheries and shelf ecosystems, there are relatively few regional-scale assessments of its spatial and temporal variability, and most productivity studies have been limited in scale. Here we use satellite-derived Net Primary Production (NPP) rates calculated using the Vertically Generalized Production Model (VGPM) to examine the spatial and temporal dynamics of NPP over the 21-year satellite record (1998–2018) on the AB. In calculating VGPM NPP we used the OCCI Chlorophyll-a product, SST from Operational-Sea-Surface-Temperature-and-Sea-Ice-Analysis (OSTIA) and PAR from GlobColour level-3 mapped products as these represent the longest datasets that fit our extended study period. We examine spatial trends between the eastern and central AB, as well as three areas of the bank (around Port Alfred, the Tsitsikamma coast, and the ‘cold ridge’) that have been previously identified as contributing significantly to the overall productivity of the AB. The AB shows only a moderate degree of seasonality in NPP calculated from the VGPM, with NPP being highest in austral summer (1.7–1.8 g C m⁻² d⁻¹) and lowest in winter (0.9–1.0 g C m⁻² d⁻¹), and remains relatively high (>1 g C m⁻² d⁻¹) throughout the year, contrasting sharply with other shelf systems. Considered annually, NPP on the bank was 516 g C m⁻² yr⁻¹ (38 Mt C yr⁻¹ when scaled to the total shelf area) which is higher than many other shelf systems though lower than the neighbouring Benguela system and is indicative of a moderately productive shelf system fuelled by perennial NPP. Comparing different sections of the AB from east to central bank, and including the three upwelling areas, highlighted that spatial differences in NPP were relatively limited; that these three upwelling areas made similar contributions to their relative proportion of the total shelf area, and that average rates of NPP are spatially similar across the bank, though notable high rates occur in some coastal upwelling areas. Interannual variability in NPP was relatively modest, varying between years by only ∼15% over the two decades assessed. Over the 21-year data set, there was a slight (∼0.26% yr⁻¹) statistically-significant decline in calculated NPP over time for the AB as a whole, which, when examined on a pixel-by-pixel basis, indicated that most of the decline was on the central bank between 100 m and 200 m isobaths. In summer, an increase in NPP occurred on the EAB (26.5–28°E). In conclusion, the AB is a significant site of perennial moderate levels of NPP, varying little interannually and with only a slight decline in NPP over time. These factors lead to a stable environment in terms of ecosystem productivity so that the AB makes a significant contribution to the productivity of South African regional fisheries.
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Database of diazotrophs in global ocean: Abundances, biomass and nitrogen fixation rates
Article
Full-text available
  • Feb 2012
Marine N<sub>2</sub> fixing microorganisms, termed diazotrophs, are a key functional group in marine pelagic ecosystems. The biological fixation of dinitrogen (N<sub>2</sub>) to bioavailable nitrogen provides an important new source of nitrogen for pelagic marine ecosystems and influences primary productivity and organic matter export to the deep ocean. As one of a series of efforts to collect biomass and rates specific to different phytoplankton functional groups, we have constructed a database on diazotrophic organisms in the global pelagic upper ocean by compiling about 12 000 direct field measurements of cyanobacterial diazotroph abundances (based on microscopic cell counts or qPCR assays targeting the nifH genes) and N<sub>2</sub> fixation rates. Biomass conversion factors are estimated based on cell sizes to convert abundance data to diazotrophic biomass. The database is limited spatially, lacking large regions of the ocean especially in the Indian Ocean. The data are approximately log-normal distributed, and large variances exist in most sub-databases with non-zero values differing 5 to 8 orders of magnitude. Lower mean N<sub>2</sub> fixation rate was found in the North Atlantic Ocean than the Pacific Ocean. Reporting the geometric mean and the range of one geometric standard error below and above the geometric mean, the pelagic N<sub>2</sub> fixation rate in the global ocean is estimated to be 62 (53–73) Tg N yr<sup>−1</sup> and the pelagic diazotrophic biomass in the global ocean is estimated to be 4.7 (2.3–9.6) Tg C from cell counts and to 89 (40–200) Tg C from nifH -based abundances. Uncertainties related to biomass conversion factors can change the estimate of geometric mean pelagic diazotrophic biomass in the global ocean by about ±70%. This evolving database can be used to study spatial and temporal distributions and variations of marine N<sub>2</sub> fixation, to validate geochemical estimates and to parameterize and validate biogeochemical models. The database is stored in PANGAEA ( http://doi.pangaea.de/10.1594/PANGAEA.774851 ).
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Factors challenging our ability to detect long-term trends in ocean chlorophyll
Article
Full-text available
  • Nov 2012
  • BGD
Global climate change is expected to affect the ocean's biological productivity. The most comprehensive information available about the global distribution of contemporary ocean primary productivity is derived from satellite data. Large spatial patchiness and interannual to multidecadal variability in chlorophyll a concentration challenges efforts to distinguish a global, secular trend given satellite records which are limited in duration and continuity. The longest ocean color satellite record comes from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), which failed in December 2010. The Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color sensors are beyond their originally planned operational lifetime. Successful retrieval of a quality signal from the current Visible Infrared Imager Radiometer Suite (VIIRS) instrument, or successful launch of the Ocean Land Colour Instrument (OLCI) in 2013 will hopefully extend the ocean color time series and increase the potential for detecting trends in ocean productivity in the future. Alternatively, a potential discontinuity in the time series of ocean chlorophyll a, introduced by a change of instrument without overlap and opportunity for cross-calibration, would make trend detection even more challenging. In this paper, we demonstrate that there are a few regions with statistically significant trends over the ten years of SeaWiFS data, but at a global scale the trend is not large enough to be distinguished from noise. We quantify the degree to which red noise (autocorrelation) especially challenges trend detection in these observational time series. We further demonstrate how discontinuities in the time series at various points would affect our ability to detect trends in ocean chlorophyll a. We highlight the importance of maintaining continuous, climate-quality satellite data records for climate-change detection and attribution studies.
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Multiple stressors of ocean ecosystems in the 21st century: Projections with CMIP5 models
Article
Full-text available
  • Feb 2013
  • BG
Ocean ecosystems are increasingly stressed by human-induced changes of their physical, chemical and biological environment. Among these changes, warming, acidification, deoxygenation and changes in primary productivity by marine phytoplankton can be considered as four of the major stressors of open ocean ecosystems. Due to rising atmospheric CO2 in the coming decades, these changes will be amplified. Here, we use the most recent simulations performed in the framework of the Coupled Model Intercomparison Project 5 to assess how these stressors may evolve over the course of the 21st century. The 10 Earth System Models used here project similar trends in ocean warming, acidification, deoxygenation and reduced primary productivity for each of the IPCC's representative concentration parthways (RCP) over the 21st century. For the "business-as-usual" scenario RCP8.5, the model-mean changes in 2090s (compared to 1990s) for sea surface temperature, sea surface pH, global O2 content and integrated primary productivity amount to +2.73 °C, -0.33 pH unit, -3.45% and -8.6%, respectively. For the high mitigation scenario RCP2.6, corresponding changes are +0.71 °C, -0.07 pH unit, -1.81% and -2.0% respectively, illustrating the effectiveness of extreme mitigation strategies. Although these stressors operate globally, they display distinct regional patterns. Large decreases in O2 and in pH are simulated in global ocean intermediate and mode waters, whereas large reductions in primary production are simulated in the tropics and in the North Atlantic. Although temperature and pH projections are robust across models, the same does not hold for projections of sub-surface O2 concentrations in the tropics and global and regional changes in net primary productivity.
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Large-scale distribution of Atlantic nitrogen fixation controlled by iron availability
Article
Full-text available
  • Nov 2009
Oceanic fixed-nitrogen concentrations are controlled by the balance between nitrogen fixation and denitrification1–4. A number of factors, including iron limitation5–7, can restrict nitrogen fixation, introducing the potential for decoupling of nitrogen inputs and losses2,5,8. Such decoupling could significantly affect the oceanic fixed-nitrogen inventory and consequently the biological component of ocean carbon storage and hence air–sea partitioning of carbon dioxide2,5,8,9. However, the extent to which nutrients limit nitrogen fixation in the global ocean is uncertain. Here, we examined rates of nitrogen fixation and nutrient concentrations in the surfacewaters of the Atlantic Ocean along a north–south 10,000 km transect during October and November 2005. We show that rates of nitrogen fixation were markedly higher in the North Atlantic compared with the South Atlantic Ocean. Across the two basins, nitrogen fixation was positively correlated with dissolved iron and negatively correlated with dissolved phosphorus concentrations. We conclude that inter-basin differences in nitrogen fixation are controlled by iron supply rather than phosphorus availability. Analysis of the nutrient content of deep waters suggests that the fixed nitrogen enters North Atlantic Deep Water. Our study thus supports the suggestion that iron significantly influences nitrogen fixation5, and that subsequent interactions with ocean circulation patterns contribute to the decoupling of nitrogen fixation and loss2,4,8.
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Projected expansion of the subtropical biome and contraction of the temperate and equatorial upwelling biomes in the North Pacific under global warming
Article
Full-text available
  • Jun 2011
Polovina, J. J., Dunne, J. P., Woodworth, P. A., and Howell, E. A. 2011. Projected expansion of the subtropical biome and contraction of the temperate and equatorial upwelling biomes in the North Pacific under global warming. – ICES Journal of Marine Science, 68: 986–995. A climate model that includes a coupled ocean biogeochemistry model is used to define large oceanic biomes in the North Pacific Ocean and describe their changes over the 21st century in response to the IPCC Special Report on Emission Scenario A2 future atmospheric CO2 emissions scenario. Driven by enhanced stratification and a northward shift in the mid-latitude westerlies under climate change, model projections demonstrated that between 2000 and 2100, the area of the subtropical biome expands by ∼30% by 2100, whereas the area of temperate and equatorial upwelling (EU) biomes decreases by ∼34 and 28%, respectively, by 2100. Over the century, the total biome primary production and fish catch is projected to increase by 26% in the subtropical biome and decrease by 38 and 15% in the temperate and the equatorial biomes, respectively. Although the primary production per unit area declines slightly in the subtropical and the temperate biomes, it increases 17% in the EU biome. Two areas where the subtropical biome boundary exhibits the greatest movement is in the northeast Pacific, where it moves northwards by as much as 1000 km per 100 years and at the equator in the central Pacific, where it moves eastwards by 2000 km per 100 years. Lastly, by the end of the century, there are projected to be more than 25 million km² of water with a mean sea surface temperature of 31°C in the subtropical and EU biomes, representing a new thermal habitat. The projected trends in biome carrying capacity and fish catch suggest resource managers might have to address long-term trends in fishing capacity and quota levels.
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Comparative Assessment of Nitrogen Fixation Methodologies, Conducted in the Oligotrophic North Pacific Ocean
Article
Full-text available
Resolution of the nitrogen (N) cycle in the marine environment requires an accurate assessment of dinitrogen (N2) fixation. We present here an update on progress in conducting field measurements of acetylene reduction (AR) and 15N2 tracer assimilation in the oligotrophic North Pacific Subtropical Gyre (NPSG). The AR assay was conducted on discrete seawater samples using a headspace analysis system, followed by quantification of ethylene (C2H4) with a reducing compound photodetector. The rates of C2H4 production were measurable for nonconcentrated seawater samples after an incubation period of 3 to 4 h. The 15N2 tracer measurements compared the addition of 15N2 as a gas bubble and dissolved as 15N2 enriched seawater. On all sampling occasions and at all depths, a 2- to 6-fold increase in the rate of 15N2 assimilation was measured when 15N2-enriched seawater was added to the seawater sample compared to the addition of 15N2 as a gas bubble. In addition, we show that the 15N2-enriched seawater can be prepared prior to its use with no detectable loss (<1.7%) of dissolved 15N2 during 4 weeks of storage, facilitating its use in the field. The ratio of C2H4 production to 15N2 assimilation varied from 7 to 27 when measured simultaneously in surface seawater samples. Collectively, the modifications to the AR assay and the 15N2 assimilation technique present opportunities for more accurate and high frequency measurements (e.g., diel scale) of N2 fixation, providing further insight into the contribution of different groups of diazotrophs to the input of N in the global oceans.
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Modelling regional responses by marine pelagic ecosystems to global change. Geophys Res Lett 29 (16): 10.1029/2001GL014130
Article
Current coupled ocean-atmosphere model (COAM) projections of future oceanic anthropogenic carbon uptake suggest reduced rates due to surface warming, enhanced stratification, and slowed thermohaline overturning. Such models rely on simple, bulk biogeochemical parameterisations, whereas recent ocean observations indicate that floristic shifts may be induced by climate variability, are widespread, complex, and directly impact biogeochemical cycles. We present a strategy to incorporate ecosystem function in COAM's and to evaluate the results in relation to region-specific ecosystem dynamics and interannual variability using a template of oceanic biogeographical provinces. Illustrative simulations for nitrogen fixers with an off-line multi-species, functional group model suggest significant changes by the end of this century in ecosystem structure, with some of the largest regional impacts caused by shifts in the areal extent of biomes.
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Toward an Ecological Geography of the Sea
Book
  • Jan 1998
This book presents an in-depth discussion of the biological and ecological geography of the oceans. It synthesizes locally restricted studies of the ocean to generate a global geography of the vast marine world. Based on patterns of algal ecology, the book divides the ocean into four primary compartments, which are then subdivided into secondary compartments. *Includes color insert of the latest in satellite imagery showing the world's oceans, their similarities and differences *Revised and updated to reflect the latest in oceanographic research *Ideal for anyone interested in understanding ocean ecology -- accessible and informative.
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Rates of dinitrogen fixation and the abundance of diazotrophs in North Americancoastal waters between Cape Hatteras and Georges Bank
Article
  • Jul 2012
We coupled dinitrogen (N2) fixation rate estimates with molecular biological methods to determine the activity and abundance of diazotrophs in coastal waters along the temperate North American Mid-Atlantic continental shelf during multiple seasons and cruises. Volumetric rates of N2 fixation were as high as 49.8 nmol N L−1 d−1 and areal rates as high as 837.9 µmol N m−2 d−1 in our study area. Our results suggest that N2 fixation occurs at high rates in coastal shelf waters that were previously thought to be unimportant sites of N2 fixation and so were excluded from calculations of pelagic marine N2 fixation. Unicellular N2-fixing group A cyanobacteria were the most abundant diazotrophs in the Atlantic coastal waters and their abundance was comparable to, or higher than, that measured in oceanic regimes where they were discovered. High rates of N2 fixation and the high abundance of diazotrophs along the North American Mid-Atlantic continental shelf highlight the need to revise marine N budgets to include coastal N2 fixation. Integrating areal rates of N2 fixation over the continental shelf area between Cape Hatteras and Nova Scotia, the estimated N2 fixation in this temperate shelf system is about 0.02 Tmol N yr−1, the amount previously calculated for the entire North Atlantic continental shelf. Additional studies should provide spatially, temporally, and seasonally resolved rate estimates from coastal systems to better constrain N inputs via N2 fixation from the neritic zone.
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Carbon-based primary productivity modeling with vertically resolved photoacclimation
Article
  • Jun 2008
1] Net primary production (NPP) is commonly modeled as a function of chlorophyll concentration (Chl), even though it has been long recognized that variability in intracellular chlorophyll content from light acclimation and nutrient stress confounds the relationship between Chl and phytoplankton biomass. It was suggested previously that satellite estimates of backscattering can be related to phytoplankton carbon biomass (C) under conditions of a conserved particle size distribution or a relatively stable relationship between C and total particulate organic carbon. Together, C and Chl can be used to describe physiological state (through variations in Chl:C ratios) and NPP. Here, we fully develop the carbon-based productivity model (CbPM) to include information on the subsurface light field and nitracline depths to parameterize photoacclimation and nutrient stress throughout the water column. This depth-resolved approach produces profiles of biological properties (Chl, C, NPP) that are broadly consistent with observations. The CbPM is validated using regional in situ data sets of irradiance-derived products, phytoplankton chlorophyll:carbon ratios, and measured NPP rates. CbPM-based distributions of global NPP are significantly different in both space and time from previous Chl-based estimates because of the distinction between biomass and physiological influences on global Chl fields. The new model yields annual, areally integrated water column production of $52 Pg C a À1 for the global oceans.
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