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Temperature effects on export production in the open ocean
Global Biogeochemical Cycles
Abstract
A pelagic food web model was formulated with the goal of developing a quantitative understanding of the relationship between total production, export production, and environmental variables in marine ecosystems. The model assumes that primary production is partitioned through both large and small phytoplankton and that the food web adjusts to changes in the rate of allochthonous nutrient inputs in a way that maximizes stability, i.e., the ability of the system to return to steady state following a perturbation. The results of the modeling exercise indicate that ef ratios, defined as new production/totalproduction=exportproduction/totalproduction, are relatively insensitive to total production rates at temperatures greater than ~25°C and lie in the range 0.1-0.2. At moderate to high total production rates, ef ratios are insensitive to total production and negatively correlated with temperature. The maximum ef ratios are ~0.67 at high rates of production and temperatures of 0°-10°C. At temperatures less than ~20°C, there is a transition from low ef ratios to relatively high ef ratios as total production increases from low to moderate values. This transition accounts for the hyperbolic relationship often presumed to exist between ef ratios and total production. At low rates of production the model predicts a negative correlation between production and ef ratios, a result consistent with data collected at station ALOHA (22°45'N, 158°W) in the North Pacific subtropical gyre. The predictions of the model are in excellent agreement with results reported from the Joint global Ocean Flux Study (JGOFS) and from other field work. In these studies, there is virtually no correlation between total production and ef ratios, but temperature alone accounts for 86% of the variance in the ef ratios. Model predictions of the absolute and relative abundance of autotrophic and heterotrophic microorganisms are in excellent agreement with data reported from field studies. Combining the ef ratio model with estimates of ocean temperature and photosynthetic rates derived from satellite data indicates that export production on a global scale is ~20% of net photosynthesis. The results of the model have important implications for the impact of climate change on export production, particularly with respect to temperature effects.
... Particulate organic carbon (POC) export efficiency (PE eff ), defined as the sinking flux of POC (F POC ) divided by the net primary production (NPP) (Buesseler, 1998;Henson et al., 2011), represents the fraction of photosynthetically fixed carbon exported out of the euphotic zone after food-web cycling. PE eff is a critical metric for assessing the efficiency of the biological pump (Henson et al., 2019;Laws et al., 2000). ...
... The observationally constrained empirical models are constructed using either global or SO training and validation data sets and often consider different combinations of carbon pools contributing to export. For example, the export efficiency models developed by Britten et al. (2017), Arteaga et al. (2018), and Henson et al. (2011) are trained on F POC data sets, whereas the model developed by Laws et al. (2000) is trained using l5 N-labeled nitrate uptake to account for both POC and DOC export (e.g., EP TOC ; Figure 2e). The models developed by Dunne et al. (2005) and Laws ) rely on combined data sets of F POC and new production, resulting in a complex export efficiency estimate that reflects something in between the particulate and total organic carbon contributions. ...
Plain Language Summary
Microbial organisms in seawater transform carbon dioxide into different types of carbon through photosynthesis and food web cycling. These carbon types include particulate and dissolved phases, with particles being more efficiently transferred out of the sunlit ocean via gravitational sinking. The ratio of sinking particulate organic carbon to total organic carbon production, commonly referred to as the particle export efficiency, is a metric used to describe how efficiently carbon moves from the surface to the deep ocean. Using observations from a large array of robots in the Southern Ocean, we find that the different types of biogenic carbon produced control the latitudinal gradient in particle export efficiency, which is highest in regions where particulate inorganic carbon export is greatest, even when photosynthetically fixed carbon is minimal. In other areas where phytoplankton carbon production is moderate but largely comprised of dissolved organic carbon, the particle export efficiency is lower.
... Only groups whose model R² calculated by random cross-validation is >0.05 are shown (see also Supplementary Table 3). e Global and regional contribution of large mesopelagic Phaeodaria to gravitational particulate organic carbon (POC) flux attenuation, based on the ratio of their annual carbon demand to the respective median annual carbon export (i.e., transport out of the euphotic zone) as reported elsewhere [32][33][34] and summarized in Supplementary Table 5. Bars show the ratio calculated with the single export value available or the median value when more than one export value was available. In the latter case, dots show export values used to calculate the median. ...
... Regional values were obtained by partitioning world predictions using Longhurst's provinces 68 . To estimate the percentage of flux attenuation, we used carbon export (i.e., out of the euphotic zone layer) values from the literature [32][33][34]69 and computed the ratios of Phaeodaria carbon demand to carbon export globally and for each oceanic region. ...
Thriving in both epipelagic and mesopelagic layers, Rhizaria are biomineralizing protists, mixotrophs or flux-feeders, often reaching gigantic sizes. In situ imaging showed their contribution to oceanic carbon stock, but left their contribution to element cycling unquantified. Here, we compile a global dataset of 167,551 Underwater Vision Profiler 5 Rhizaria images, and apply machine learning models to predict their organic carbon and biogenic silica biomasses in the uppermost 1000 m. We estimate that Rhizaria represent up to 1.7% of mesozooplankton carbon biomass in the top 500 m. Rhizaria biomass, dominated by Phaeodaria, is more than twice as high in the mesopelagic than in the epipelagic layer. Globally, the carbon demand of mesopelagic, flux-feeding Phaeodaria reaches 0.46 Pg C y⁻¹, representing 3.8 to 9.2% of gravitational carbon export. Furthermore, we show that Rhizaria are a unique source of biogenic silica production in the mesopelagic layer, where no other silicifiers are present. Our global census further highlights the importance of Rhizaria for ocean biogeochemistry.
... Overall, the original CESM2-MARBL provides a reasonable estimate of annual global POC fluxes. The simulated POC export of the global ocean is 7.8 Pg C yr 1 , which is consistent with the previous satellitederived estimates that range from ∼5 to ∼12 Pg C yr 1 (Dunne et al., 2007;Henson et al., 2011;Laws et al., 2000;Siegel et al., 2014). The model predicts high export in boundary upwelling systems and high-latitude regions, while showing obviously low export in the center of subtropical oligotrophic gyres (Figure 2), in agreement with previous estimates (Lima et al., 2014). ...
... This is because annual primary production in continental margins decreases more significantly in Type3 experiments where TLP is transported seaward along the bottom of shelves and slopes compared to Type1 and Type2 experiments where TLP is confined to estuarine water columns or transported seaward in the surface ocean. Moreover, an increase in primary production leads to higher carbon flux has been generally predicted (Dunne et al., 2007;Laws et al., 2000Laws et al., , 2011, although the coupling between primary production and sinking export depends on a complex concurrence of biological and environmental factors (Boyd & Trull, 2007;Buesseler & Boyd, 2009). There are two factors driving the changes of PP in the sensitivity experiments. ...
Lithogenic materials such as terrigenous lithogenic particles (TLP) can efficiently promote the formation and sinking of mineral‐associated marine organic matter, acting as important ballast and potentially playing an important role in the global carbon cycle. To assess the influence of TLP on fluxes of particulate organic carbon (POC) and other biogeochemical cycles, we construct TLP forcing fields based on global riverine suspended sediment data and then apply them to the Community Earth System Model, version 2 (CESM2) modified with the TLP ballasting effect term. Simulations forced by different concentrations of TLP transported in the surface ocean or along the bottom of continental shelves and slopes are conducted. When the TLP transports seaward along the bottom, simulated POC fluxes at 100 and 2,000 m decrease about 11% and 19%, respectively, for the global ocean, and about 9% and 12%, respectively, for the oceanic regions of continental margins. The initial abiotic ballast processes triggered by TLP input increase POC fluxes, causing additional removal and burial of dissolved iron in continental margins. This further enhances the accumulation of macronutrients in the upwelling regions and their advection transport to neighboring subtropical gyres, thus altering regional productivity when simulations reach quasi‐equilibrium. When consider the impacts of TLP in simulations, the simulated POC flux exhibits an increase in subtropical gyres but a decrease in tropical Pacific and mid‐high latitude regions. The present work highlights the importance of TLP in global biogeochemical cycles, suggesting that the amount of carbon sequestration might be overestimated without TLP in models.
... Long-term sequestration of this PP requires the organic matter to be exported to the deep ocean through a myriad of processes collectively referred to as the biological carbon pump (BCP) (Eppley and Peterson, 1979). The global range of organic carbon export is very large and estimated at 5-21 Gt C yr −1 (Laws et al., 2000;Henson et al., 2011;Siegel et al., 2014;Wang et al., 2023). The pathways for carbon export from the surface to the deep ocean by the BCP are multiple (Boyd et al., 2019;Le Moigne, 2019). ...
Diazotrophs regulate marine productivity in the oligotrophic ocean by alleviating nitrogen limitation, contributing to particulate organic carbon (POC) export to the deep ocean. Yet, the characterization of particles composing the sinking POC flux has never been explored in such ecosystems. Moreover, the contribution of the direct gravitational export of diazotrophs to the overall flux is seldom assessed. Here we explore the composition of the sinking POC flux in a hot spot of N2 fixation (the western sub-tropical South Pacific) using polyacrylamide gel-filled traps deployed at two stations (S05M and S10M) and three depths (170 m, 270 m, 1000 m) during the TONGA expedition (November-December 2019). Image analyses of particles collected in the gels was used to classify them into 5 categories (fecal aggregates, phytodetrital aggregates, mixed aggregates, cylindrical fecal pellets, and zooplankton carcasses). Fecal aggregates were the most abundant at both stations and all depths and dominated the flux (average of 56 ± 28% of the POC flux), followed by zooplankton carcasses (24 ± 19%), cylindrical fecal pellets (15 ± 14%) and mixed aggregates (5 ± 4%), whereas phytodetrital aggregates contributed less (<1%). Since N isotope budgets show that export is mainly supported by diazotrophy at these stations, these results suggest that the diazotroph-derived N has been efficiently transferred to the foodweb up to zooplankton and fecal pellets before being exported, pleading for an indirect export of diazotrophy. However, random confocal microscopy examination performed on sinking particles revealed that diazotrophs were present in several categories of exported particles, suggesting that diazotrophs are also directly exported, with a potential contribution to overall POC fluxes increasing with depth. Our results provide the first characterization of particle categories composing the sinking flux and their contribution to the overall flux in a hot spot of N2 fixation.
... In regulating the atmospheric CO 2 concentration, the ocean plays a vital role in the Earth's carbon cycle [1]. There are six (6) ocean biological carbon pumps (OBCPs) that comprise the downward pumping of biogenic carbon to the deep ocean [2], estimated to be at between 5 and 14 Pg C y −1 [3,4]. These include the mixed layer pump, the Eddy subduction pump, the large-scale subduction pump, Ekman pumping, and animalmediated pumps, and the vertical migration of zooplankton [5]. ...
This study focuses on the quantification and forecasting of the biological pump potential in the Philippine seas, specifically inside the Exclusive Economic Zone (EEZ). Variabilities and disturbances that might potentially influence ocean productivity such as increased sea surface temperature (SST), and the high frequency of typhoons in the Philippines were investigated. CHL and SST spatio-temporal maps were used to provide visualization for the trends and phenomena before, during, and after typhoon occurrence for the years 2019–2021. Integrating the NASA Ocean Color data of CHL and SST with typhoon tracks, the biological pump potential annual estimate was generated.
... The size-structure of phytoplankton communities is an important determinant of carbon and nutrient cycling, the pelagic and benthic food web structure, and carbon export to the deep sea (Tremblay et al., 1997;Laws et al., 2000). In general, higher rates of community primary production are associated with increasing abundance of larger cells, increased trophic transfer efficiency, and increased carbon export into the deep sea. ...
The South Indian Ocean, one of the least explored ocean regions, is dominated by the Indian Ocean subtropical gyre (IOSG), one of the five extensive oligotrophic areas in the world’s oceans. Here, we present results from a comprehensive study on physical water column data (e.g., temperature, salinity and sigma-theta), nutrient and stable isotope data, and long-time sinking particulate matter collection retrieved between 2014 and 2019 by sediment trap moorings in the INDEX (Indian Ocean Exploration) area. Our research contributes to the main understanding of key processes of the N and C cycle and provides the first multidisciplinary approach of water mass interfingering, N sources and transformation processes and the quantity, quality and variability of sediment trap-based sinking particulate matter fluxes. With this knowledge, future impacts from deep-sea mining activities on persisting biogeochemical cycles in the South Indian Ocean will be better understood and evaluated. Numerous water samples along 3°S to 28°S allow detailed water mass analyses accompanied by a comprehensive study of nitrogen sources and cycling processes. We demonstrate the convergence and mixing of water masses of Antarctic and Subantarctic origin with water masses from the North Indian Ocean and track the changes in nutrient distribution and signatures of stable isotopes (N and O of nitrate) along these diverse water masses. This provides evidence for the injection of preformed nutrients from the Subantarctic and the Arabian Sea. Furthermore, dinitrogen (N2) fixation evidently accounts for a significant proportion of primary production, and we calculate that ~32–34% of the nitrified nitrate is contributed by N2 fixation in surface waters of the IOSG at ~20–24°S. Nevertheless, the IOSG is a strongly nutrient-limited ocean realm and is characterised by low primary production rates and hence very low sinking particulate matter fluxes. Sinking particulate matter considered here was collected during the 5-year sediment trap deployment experiment between 2014 and 2019 to provide new information on the major factors that control the particle export out of the biologically active zone, its transfer into the ocean’s interior and its potential storage in pelagic sediments of the South Indian Ocean. Comparing particulate organic carbon (POC) fluxes to global data, the IOSG reveals the lowest fluxes worldwide (0.52 ± 0.18 mg m−2 d−1 at 500 m.a.b.), even compared to other oligotrophic areas. Preliminary estimates indicate an average POC export efficiency of ε ≈ 0.02. Based on net primary production (NPP) data in surface waters of the IOSG only 0.17% of the POC produced at the sea surface reaches the sea floor. Due to intense degradation at the sediment–water interface, less than 0.01% (~0.02 mg POC m−2 d−1) accumulates in surface sediments. Assuming that the IOSG, as well as comparable ocean regions, will expand under climate warming, it is of major importance to investigate POC export fluxes and its final carbon storage in the sediments in order to study the organic carbon pump and potential changes in the global C cycle. Furthermore, we present the seasonal variability of particle fluxes in the IOSG that shows a decoupling between NPP and the amount of exported particles. In the winter season, the external nutrient supply via gyre boundaries and/or subsurface waters elevate NPP and favour a complex zooplankton-dominated food web. In a first step, this enhances the organic matter production but, on the other hand, reduces the export of particles by increasing their fragmentation into free/unprotected particles, vulnerable to remineralisation/degradation. In comparison, during the low productive summer season, a foraminifera-dominated zooplankton community increases the particle export efficiency by ballast-minerals (here CaCO3). Consequently, the final export of particulate matter is anti-correlated to NPP rates. Various factors, such as variations in the ocean mixed layer depth, impacts of physical mixing (surface wind stress, cyclonic eddies), changes in the plankton community structure and concomitant changes, impact the vertical transfer of particles to the sea floor. Understanding these complex mechanisms is challenging, but they have to be studied in detail to make robust statements about global climate predictions.
Understanding the driving mechanism of phytoplankton dynamics is key to forecasting future changes in the ocean. Here, we report an apparent “trapezoidal” relationship between chlorophyll concentrations (Chl) and surface photosynthetically available radiation (PAR(0)) at the center of the South Pacific Gyre (cSPG) based on 18 years of MODIS Aqua measurements. A comparison of Chl with a photoacclimation model revealed that photoacclimation alone could not explain the temporal dynamics of Chl. Instead, the Chl dynamics were explained by a combination of photoacclimation, nutrient limitation, and the grazing pressure of zooplankton at different times throughout the year. An annual “trapezoidal” spiral relationship between Chl and PAR(0) suggested that the steady state of phytoplankton populations at the cSPG could be influenced by the alternation of co-regulation mechanisms during a year. Because this same pattern occurs in other subtropical gyres, this understanding of the underlying mechanisms not only facilitates simulating and forecasting phytoplankton dynamics but also provides a new perspective on how multiple stressors may impact phytoplankton communities in a warmer climate.
Disentangling microbial dynamics in the mesopelagic zone is crucial due to its role in processing sinking photic production, affecting carbon export to the deep ocean. The relative importance of photic zone processes versus local biogeochemical conditions in mesopelagic microbial dynamics, especially seasonal dynamics, is largely unknown. We employed rRNA gene transcript-based high-throughput sequencing on 189 samples collected from both the photic and mesopelagic zones, along with seasonal observations, to understand the South China Sea’s protistan-bacterial microbiota diversity, drivers, and mechanisms. Mesopelagic communities displayed unexpectedly greater seasonal but less vertical dynamics than photic counterparts. Temperature, dissolved oxygen, nutrients, and bacterial abundance drove mesopelagic communities vertically. Photic zone processes (using net community production and mixed layer depth as proxies) of past seasons, coinciding with strong monsoon periods, shaped seasonal fluctuations in mesopelagic communities, indicating a time-lag effect. Furthermore, certain microbes were identified as indicators for beta diversity by depth and season. This investigation deepens our understanding of how and why mesopelagic communities vary with season and depth. Recognizing the timelagged effect of photic zone processes on mesopelagic communities is crucial for understanding the current and future configurations of the ocean microbiome, especially in the context of climate change and its effect on carbon export and ocean storage.
In summer, the Madden-Julian Oscillation (MJO) greatly influences the intraseasonal variability of the South China Sea (SCS). Previous studies have revealed MJO effects on surface chlorophyll concentration, but the impact of the MJO on the ecosystem's structure and functionality remains unexplored. Here, we investigated the marine ecosystem response to the MJO by analyzing phytoplankton pigment data collected in cruises during from 2010 to 2014. The results indicated the strong influence of the MJO on the structure of phytoplankton size classes (PSCs) in the upper 50 m of the SCS basin. To further explore the ecosystem's response to MJO, we utilized a well-calibrated physical-biogeochemical model (ROMS-CoSiNE) of the SCS to conduct numerical experiments with and without MJO forcings. Our model demonstrated that MJO-induced deep mixing and upwelling increased nutrients in the upper layer, increasing the Chl concentration with a higher proportion of nanophytoplankton (15%) and a lower proportion of picophytoplankton (−20%). Moreover, The MJO-forced model experiment exhibited a substantial enhancement in primary production (56%) and export production (23%), resulting in a notable decrease in the e-ratio. This reduction in the e-ratio cannot be attributed to changes in PSCs but can be explained by the time lag between primary and export production. This lag was prolonged by the physical processes of upwelling and mixing, which slows down the particle sinking. Our results emphasize the important role of MJO in regulating the ecosystem at intraseasonal scale, thus improving our comprehension of the nonsteady dynamics of ecosystems in the SCS.
After more than thirty years of intensive research in phytoplankton ecology, our knowledge of how to measure accurately growth rates of photosynthetic organisms in nature is still very limited (1–3). Difficulties in extrapolating results from in situ bottle tests involving standard rate-measuring incubation methods (such as the 14C-labelling technique) are becoming increasingly evident (3–6); hence, we now are faced with conflicting opinions regarding the magnitude of phytoplankton growth rates in the marine environment (5,7–10).
The marine environment is the largest contiguous habitat on Earth. It is composed of several distinct ecosystems including high-latitude (polar) zones, mid-latitude open-ocean gyres, equatorial upwelling systems and a diverse assemblage of coastal habitats (e.g. continental shelves, bays, coral reefs, mudflats and estuaries). These individual ecosystems have characteristics that collectively influence both their populations (e.g. types of organisms present, trophic structures) and their in situ rates of biogeochemical processes (e.g. primary production, respiration and nutrient cycling). This diversity makes broad generalizations about microbial processes in the sea impossible. It is also important to remember that our current understanding of ecological processes in the largest marine habitat (by volume), the deep sea, is rudimentary. An excellent example of our lack of general understanding of deep-sea processes was the unexpected discovery, in 1977, of luxuriant communities of previously unknown organisms surrounding regions of hydrothermal fluid discharge (Corliss et al., 1979). This discovery challenged our basic views on the obligate role of sunlight in global ecology, and presented a new paradigm for the existence of life in the biosphere (Karl, 1995a).
Changes in oceanic primary production, linked to changes in the network of global biogeochemical cycles, have profoundly influenced
the geochemistry of Earth for over 3 billion years. In the contemporary ocean, photosynthetic carbon fixation by marine phytoplankton
leads to formation of ∼45 gigatons of organic carbon per annum, of which 16 gigatons are exported to the ocean interior. Changes
in the magnitude of total and export production can strongly influence atmospheric CO2 levels (and hence climate) on geological time scales, as well as set upper bounds for sustainable fisheries harvest. The
two fluxes are critically dependent on geophysical processes that determine mixed-layer depth, nutrient fluxes to and within
the ocean, and food-web structure. Because the average turnover time of phytoplankton carbon in the ocean is on the order
of a week or less, total and export production are extremely sensitive to external forcing and consequently are seldom in
steady state. Elucidating the biogeochemical controls and feedbacks on primary production is essential to understanding how
oceanic biota responded to and affected natural climatic variability in the geological past, and will respond to anthropogenically
influenced changes in coming decades. One of the most crucial feedbacks results from changes in radiative forcing on the hydrological
cycle, which influences the aeolian iron flux and, in turn, affects nitrogen fixation and primary production in the oceans.
Dilution experiments were performed during two transects across the central equatorial Pacific (3°N-3°S, 140°W) to estimate growth and mortality rates of select phytoplankton groups distinguished by their characteristic pigments. The first transect was conducted in February-March 1992 during a moderate El Nino event; the second transect took place in August-September 1992, under normal upwelling conditions. Experiments with and without added nutrients (N, P, Fe, and Mn) indicated that growth rates were nutrient limited during El Nino conditions. Nevertheless, the enhanced growth rates with added nutrients during El Nino were less than rates without added nutrients during the normal upwelling period; therefore, nutrients alone did not account for all of the differences between cruises. Growth rates were different for the various algal groups. In August-September, diatoms grew at 1.7 d -1, and prochlorophytes and prymnesiophytes at ~0.5 d -1. Grazing by microzooplankton balanced growth for algal groups exhibiting the lowest growth rates (i.e. prymnesiophytes and prochlorophytes). Although microzooplankton grazing on diatoms and pelagophytes was also significant, a substantial fraction of their growth escaped consumption, accounting for most of the net chlorophyll production of the phytoplankton community.
This investigation focused on the weaker and less well understood of the two Arabian Sea monsoonal wind phases, the NE Monsoon, which persists for 3–4 months in the October to February period. Historically, this period has been characterized as a time of very low nutrient availability and low biological production. As part of the US JGOFS Arabian Sea Process Study, 17 stations were sampled on a cruise in January 1995 (late NE Monsoon) and, 15 stations were sampled on a cruise in November 1995 (early NE Monsoon). Only the southern most stations (10° and 12°N) and one shallow coastal station were as nutrient-depleted as had been expected from the few relevant prior studies in this region. Experiments were conducted to ascertain the relative importance of different nitrogenous nutrients and the sufficiency of local regeneration processes in supplying nitrogenous nutrients utilized in primary production. Except for the southern oligotrophic stations, the euphotic zone concentrations of NO3− were typically 5–10-fold greater than those of NO2− and NH4+. There was considerable variation (20–40-fold) in nutrient concentration both within and between the two sections on each cruise. All nitrogenous nutrients were more abundant (2–4-fold) later in the NE Monsoon. Strong vertical gradients in euphotic zone NH4+ concentration, with higher concentrations at depth, were common. This was in contrast to the nearly uniform euphotic zone concentrations for both NO3− and NO2−. Half-saturation constants for uptake were higher for NO3− (1.7 μmol kg−1 (s.d.=0.88, n=8)) than for NH4+ (0.47 μmol kg−1 (s.d.=0.33, n=5)). Evidence for the suppressing effect of NH4+ on NO3− uptake was widespread, although not as severe as has been noted for some other regions. Both the degree of sensitivity of NO3− uptake to NH4+ concentration and the half-saturation constant for NO3− uptake were correlated with ambient NO3− concentration. The combined effect of high affinity for low concentrations of NH4+ and the effect of NH4+ concentration on NO3− uptake resulted in similarly low f-ratios, 0.15 (s.d.=0.07, n=15) and 0.13 (s.d.=0.08, n=17), for early and late observations in the NE Monsoon, respectively. Stations with high f-ratios had the lowest euphotic zone NH4+ concentrations, and these stations were either very near shore or far from shore in the most oligotrophic waters. At several stations, particularly early in the NE Monsoon, the utilization rates for NO2− were equal to or greater than 50% the utilization rates for NO3−. When converted with a Redfield C : N value of 6.7, the total N uptake rates measured in this study were commensurate with measurements of C productivity. While nutrient concentrations at some stations approached levels low enough to limit phytoplankton growth, light was shown to be very important in regulating N uptake at all stations in this study. Diel periodicity was observed for uptake of all nitrogenous nutrients at all stations. The amplitude of this periodicity was positively correlated with nutrient concentration. The strongest of these relationships occurred with NO3−. Ammonium concentration strongly influenced the vertical profiles for NO3− uptake as well as for NH4+ uptake. Both NO2− and NH4+ were regenerated within the euphotic zone at rates comparable to rates of uptake of these nutrients, and thus maintenance of mixed layer concentrations did not require diffusive or advective fluxes from other sources. Observed turnover times for NH4+ were typically less than one day. Rapid turnover and the strong light regulation of NH4+ uptake allowed the development and maintenance of vertical structure in NH4+ concentration within the euphotic zone. In spite of the strong positive effect of light on NO2− uptake and its strong negative effect on NO2− production, the combined effects of much longer turnover times for this nutrient and mixed layer dynamics resulted in nearly uniform NO2− concentrations within the euphotic zone. Responses of the NE Monsoon planktonic community to light and nutrients, in conjunction with mixed layer dynamics, allowed for efficient recycling of N within the mixed layer. As the NE Monsoon evolved and the mixed layer deepened convectively, NO2− and NO3− concentrations increased correspondingly with the entrainment of deeper water. Planktonic N productivity increased 2-fold, but without a significant change the new vs. recycled N proportionality. Consequently, NO3− turnover time increased from about 1 month to greater than 3 months. This reflected the overriding importance of recycling processes in supplying nitrogenous nutrients for primary production throughout the duration of the NE Monsoon. As a result, NO3− supplied to the euphotic zone during the NE Monsoon is, for the most part, conserved for utilization during the subsequent intermonsoon period.
The stocks and dynamics of bacterioplankton in the upper 200 m were studied in detail providing daily temporal resolution and finescale vertical resolution during two ca 20-day occupations of a station on the equator at the U. S. JGOFS EgPac transect along 140°W longitude, in March–April and October 1992. Euphotic-zone bacterial biomass averaged 70% of phytoplankton carbon on the two cruises. This moderately high ratio was a consequence of low phytoplankton biomass, characteristic of high nutrient, low chlorophyll (HNLC) regimes, rather than high bacterial abundance, which averaged 6–8 × 108 cells l−1 in the upper 100 m. Turnover rates and thus production were low, with bacterial production (BP) averaging less than 20% of primary production (PP) for the two cruises. Biomass was higher and production was lower under the non-El Niño conditions prevailing in October, compared to the March–April observations made in El Niño conditions. Overall, bacterial production was a low fraction of primary production near the equator during both El Niño conditions. We suggest this might be the result of two possible but untested phenomena: (1) BP:PP is generally low (i.e.