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We explore how the iron dependence of the Si:P uptake ratio RSi:P of diatoms controls the response of the global silicon cycle and phytoplankton community structure to Southern Ocean iron fertilization. We use a data‐constrained model of the coupled Si‐P‐Fe cycles that features a mechanistic representation of nutrient colimitations for three phytoplankton classes and that is embedded in a data‐assimilated global ocean circulation model. We consider three parameterizations of the iron dependence of RSi:P, all of which are consistent with the available field data and allow equally good fits to the observed nutrient climatology but result in very different responses to iron fertilization: Depending on how sharply RSi:P decreases with increasing iron concentration, iron fertilization can either cause enhanced silicic acid leakage from the Southern Ocean or strengthened Southern Ocean silicon trapping. Enhanced silicic acid leakage occurs if decreases in RSi:P win over increases in diatom growth, while the converse causes strengthened Southern Ocean silicon trapping. Silicic acid leakage drives a floristic shift in favor of diatoms in the subtropical gyres and stimulates increased low‐latitude opal export. The diatom contribution to global phosphorus export increases, but the lower diatom silicon requirement under iron‐replete conditions reduces the global opal export. Regardless of RSi:P parameterization, the global response of the biological phosphorus and silicon pumps is dominated by the Southern Ocean. The Si isotope signature of opal flux becomes systematically lighter with increasing iron‐induced silicic acid leakage, consistent with sediment records from iron‐rich glacial periods. Key Points: Iron fertilization leads to silicic acid leakage if the Si:P uptake ratio decreases sharply enough to overwhelm increased diatom growthSilicic acid leakage increases low‐latitude opal export, while the Southern Ocean dominates increases in diatom organic matter exportSilicic acid leakage in response to iron fertilization produces isotopically lighter biogenic opal flux [ABSTRACT FROM AUTHOR]

Quantifying the relative contributions of the export of particulate organic carbon (POC), dissolved organic carbon (DOC) and active fluxes by migrating organisms is essential to understand the functioning and vulnerability of the ocean's biological pump. However, these fluxes are rarely measured at the same time. Here we provide a first simultaneous comparison of these biological pump components in the region of South Georgia. We use a combination of in-situ data and an inverse model to calculate the DOC export and the suspended POC export and compare them to the sinking POC and active export. We find that, in this region, the DOC total export contributes about 6.6% (23.0–37.5 mg C m−2 day−1) to the total export flux, the active flux has no discernible contribution, and the sinking POC flux is dominant with a mean value of 409 mg C m−2 day−1. Diapycnal fluxes of DOC obtained from the cruise data constitute only a minor fraction (0.05–1.28 mg C m−2 day−1) of the total DOC export estimated by the inverse model and are exceeded on average by the diapycnal flux of suspended POC. Our results also indicate that the total export of DOC is driven by isopycnal transport. Future fieldwork in the region of South Georgia should focus on quantifying the isopycnal flux of DOC. Future measurement campaigns should also aim to simultaneously measure the particulate, dissolved and active components of the biological pump at contrasting locations and at different times to resolve the variability of their relative contribution. • First concurrent estimate of POC, DOC & active fluxes for South Georgia from in situ data. • POC export dominates, DOC contributes 6.6% to total export, active flux is negligible. • The diapycnal flux of DOC contributes only a minor part to the total DOC flux. • Future fieldwork should prioritise isopycnal DOC fluxes and variability of mixing. [ABSTRACT FROM AUTHOR]

Greater diversity of eukaryotic phytoplankton than expected has been revealed recently through molecular techniques, but little is known about their temporal dynamics or fate in the open ocean. Here, we examined size-fractionated eukaryotic phytoplankton communities from the surface to abyssopelagic zone (5,000 m) throughout the year, by tracking sequence variants of the 18S rRNA gene in the western subtropical North Pacific. The oceanographic conditions were divided into two periods, stratification and mixing, between which the surface phytoplankton community differed. During the mixing period, the abundance of large phytoplankton (≥3 μm) increased, with diatoms and putative Pseudoscourfieldia marina dominating this fraction. Picophytoplankton (

Quantification of the actual amount of carbon export to the mesopelagic layer by both zooplankton and micronekton is at present a gap in the knowledge of the biological pump. These organisms perform diel vertical migrations exporting carbon through respiration, excretion, mortality, and egestion during their residence at depth. The role of zooplankton in active flux is nowadays partially assessed. However, micronekton active flux is scarcely known and only a few studies addressed this downward transport. Even less is known about the capacity of both communities to export carbon in the ocean. Here, we show the results of zooplankton and micronekton active flux across a productivity gradient in the tropical and subtropical Atlantic Ocean. Biomass vertical distribution from the surface up to 800 m depth by day and night was studied during April 2015 in a transect from 9°S to 25°N, covering from the quite oligotrophic zone off Brazil to the meso- and eutrophic areas of the equator, Guinea Dome, and the oceanic upwelling off Northwest Africa. Zooplankton and micronekton migrant biomass was estimated from day and night catches at different layers of the water column using MOCNESS-1 (1 m2 mouth area) and Mesopelagos (35 m2) nets, respectively. Respiratory flux was assessed by measuring the enzymatic activity of the electron transfer system (ETS) of organisms at depth. Results showed a close relationship between migrant biomass and respiratory flux in zooplankton and micronekton as expected. Using a rather conservative 50% of efficiency for the net used to capture micronekton, respiratory flux resulted in similar values for both communities. Gravitational (passive) flux measured using sediment traps increased from the oligotrophic toward the meso- and eutrophic zones. Total active flux (including respiration and estimated mortality, excretion, and gut flux) by zooplankton and micronekton accounted for about 25% of total flux (passive plus active) in oligotrophic zones. Total active flux also increased toward meso- and eutrophic zones, reaching about 80% of total flux and being at least twofold higher than passive flux. These results alert about an important underestimation of the ocean biological pump using only passive flux measurements.

Zooplankton are important consumers of sinking particles in the ocean’s twilight zone. However, the impact of different taxa depends on their feeding mode. In contrast to typical suspension-feeding zooplankton, flux-feeding taxa preferentially consume rapidly sinking particles that would otherwise penetrate into the deep ocean. To quantify the potential impact of two flux-feeding zooplankton taxa [Aulosphaeridae (Rhizaria), and Limacina helicina (euthecosome pteropod)] and the total suspension-feeding zooplankton community, we measured depth-stratified abundances of these organisms during six cruises in the California Current Ecosystem. Using allometric–scaling relationships, we computed the percentage of carbon flux intercepted by flux feeders and suspension feeders. These estimates were compared to direct measurements of carbon flux attenuation (CFA) made using drifting sediment traps and 238U–234Th disequilibrium. We found that CFA in the shallow twilight zone typically ranged from 500 to 1000 μmol organic C flux remineralized per 10-m vertical depth bin. This equated to approximately 6–10% of carbon flux remineralized/10 m. The two flux-feeding taxa considered in this study could account for a substantial proportion of this flux near the base of the euphotic zone. The mean flux attenuation attributable to Aulosphaeridae was 0.69%/10 m (median = 0.21%/10 m, interquartile range = 0.04–0.81%) at their depth of maximum abundance (∼100 m), which would equate to ∼10% of total flux attenuation in this depth range. The maximum flux attenuation attributable to Aulosphaeridae reached 4.2%/10 m when these protists were most abundant. L. helicina, meanwhile, could intercept 0.45–1.6% of carbon flux/10 m, which was slightly greater (on average) than the Aulosphaeridae. In contrast, suspension-feeding zooplankton in the mesopelagic (including copepods, euphausiids, appendicularians, and ostracods) had combined clearance rates of 2–81 L m-3 day-1 (mean of 19.6 L m-3 day-1). This implies a substantial impact on slowly sinking particles, but a negligible impact on the presumably rapidly sinking fecal pellets that comprised the majority of the material collected in sediment traps. Our results highlight the need for a greater research focus on the many taxa that potentially act as flux feeders in the oceanic twilight zone.

The oceans play a fundamental role in the global carbon cycle, providing a sink for atmospheric carbon. Key to this role is the vertical transport of organic carbon from the surface to the deep ocean. This transport is a product of a diverse range of physical and biogeochemical processes that determine the formation and fate of this material, and in particular how much carbon is sequestered in the deep ocean. Models can be used to both diagnose biogeochemical processes and predict how the various processes will change in the future. Global biogeochemical models use simplified representations of food webs and processes but are converging on values for the export of organic carbon from the surface ocean. Other models concentrate on understanding specific processes and can be used to develop parameterizations for global models. Model development is continuing by adding representations and parameterizations of higher trophic levels and mesopelagic processes, and these are expected to improve model performance.

Greater diversity of eukaryotic phytoplankton than expected has been revealed recently through molecular techniques, but little is known about their temporal dynamics or fate in the open ocean. Here, we examined size-fractionated eukaryotic phytoplankton communities from the surface to abyssopelagic zone (5,000 m) throughout the year, by tracking sequence variants of the 18S rRNA gene in the western subtropical North Pacific. The oceanographic conditions were divided into two periods, stratification and mixing, between which the surface phytoplankton community differed. During the mixing period, the abundance of large phytoplankton (≥3 μm) increased, with diatoms and putative Pseudoscourfieldia marina dominating this fraction. Picophytoplankton (<3 μm) also increased during the mixing period and were dominated by Mamiellophyceae. Taxa belonging to prasinophytes (including Ps. marina and Mamiellophyceae) were observed in the epipelagic zone throughout the year, and thus likely seeded the seasonal bloom that occurred during the mixing period. In contrast, diatoms observed during the mixing period mostly represented taxa unique to that period, including coastal species. Numerical particle backtracking experiments indicated that water masses in the surface layer could be transported from coastal areas to the study site. Gene sequences of coastal diatoms were present in the abyssopelagic zone. Therefore, allochthonous species drove the seasonal bloom and could be transported to deep waters. In the abyssopelagic zone, the relative abundance of Ps. marina in deep waters was similar to or higher than that of diatoms during the mixing period. Among picophytoplankton, Mamiellophyceae made up a significant fraction in the abyssopelagic zone, suggesting that prasinophytes are also involved in carbon export. Our molecular survey showed that these previously overlooked phytoplankton species could contribute significantly to the seasonal bloom and biological pump in the subtropical open ocean. [ABSTRACT FROM AUTHOR]

The Eastern Tropical Pacific (ETP) is the largest oceanic source of carbon dioxide (CO2) to the atmosphere. Sinking particle fluxes at a depth of 4,950 m (50 m above the seafloor) in the ETP were monitored from 2003 to 2013, during which the Pacific decadal oscillation (PDO) shifted from a positive to negative phase. We show a disproportionate increase in the efficiency of the biological pump in this region relative to the increase in primary production that occurred during La Niña years following the shift of the PDO in 2008. Biogenic carbon export from the surface mixed layer was estimated from the observed particulate organic carbon and inorganic carbon fluxes at a depth of 4,950 m and from empirical equations of the vertical attenuation of carbon flux. Enhanced biological carbon export accounted for 2.3–5.5 mol C m–2 year–1 during the La Niña events. Despite a large uncertainty associated with these estimates, we propose that CO2 outgassing was largely suppressed by an enhanced biological pump during the La Niña events in the negative PDO phase. Plain Language Summary: We examined the composition and flux of particles sinking to the deep Eastern Tropical Pacific from 2003 to 2013. This region is known as the largest oceanic source of CO2 to the atmosphere. We observed that the flux of particulate organic and inorganic carbon to a depth of 4,950 m increased as the climate variability called the "Pacific decadal oscillation" shifted from a positive to negative phase (i.e., more La Niña events) in 2008. The biological pump efficiency (i.e., how efficiently biological production of organic matter is transported to the deep ocean) also increased during La Niña years. To investigate how biological production affects the CO2 exchange with the atmosphere, we estimated the export of carbon from the surface layer based on our flux data at 4,950 m and the current understanding of carbon flux attenuation with depth. CO2 outgassing was largely suppressed by an enhanced biological pump during the La Niña events in the negative PDO phase. Key Points: Fluxes of biogenic particles were much higher during La Niña years in the deep Eastern Tropical PacificBiogenic carbon export was estimated based on the observed particle fluxes and empirical equations for the vertical attenuation of carbon fluxCO2 outgassing was considerably suppressed by an enhanced biological pump during La Niña years [ABSTRACT FROM AUTHOR]

The influence of trophic status on the distribution of hydrophobic organic contaminants (HOCs) in different subtropical shallow waters at large spatial scales remains largely unknown. In this study, samples of surface sediments, water, total suspended particles, phytoplankton, and zooplankton were simultaneously collected from 83 sampling sites in 20 subtropical oligotrophic to hyper-eutrophic shallow lakes in China to investigate the influence of trophic status on the spatial distribution and sinking fluxes of 16 polycyclic aromatic hydrocarbons (PAHs). The total concentration of the 16 PAHs (ΣPAH 16) in the water columns of these lakes varied from 0.22 to 5.81 μg L−1, and increased with the trophic state index (TSI) and phytoplankton biomass. Phytoplankton were the dominant reservoir for the PAHs in the water column. However, the fraction of ΣPAH 16 in phytoplankton decreased with the TSI. The average sinking flux of ΣPAH 16 of the individual lakes varied from 2257.1 to 261674.1 mg m−2 d−1, and increased with the TSI of the lakes. The concentration of ΣPAH 16 in the surface sediments ranged from 385.77 to 3784.37 ng g dw −1, and increased with the TSI and the ratio of phycocyanin/sediment organic carbon. It suggested that cyanobacterial biomass affected by trophic status dominated the occurrence of the PAHs in the surface sediments of these lakes. Biomass dilution and the biological pump affected the accumulation of the PAHs in phytoplankton, and zooplankton, and had more influence on the PAHs with higher hydrophobicity. Both the bioconcentration factors and bioaccumulation factors of the PAHs decreased with the TSI. No biomagnification was observed for the PAHs from phytoplankton to zooplankton in these lakes in spring. Our study provided novel knowledge for the coupling between eutrophication and HOCs in 20 subtropical shallow lakes with different trophic status. Image 1 • ΣPAH 16 in the water column significantly increased with lake trophic state index. • Phytoplankton were the dominant reservoir for the PAHs in the lake water column. • Cyanobacterial biomass affected the occurrence of the PAHs in surface sediments. • Biomass dilution and biological pump dominated the accumulation of PAHs by plankton. • BCF and BAF of the PAHs accumulated by plankton decreased with trophic state index. Trophic status affected the occurrence of 16 PAHs in the water columns and surface sediments, and their bioaccumulation in plankton food webs of twenty Chinese lakes. [ABSTRACT FROM AUTHOR]

We estimated weekly summer net community production (NCP) in the Amundsen Sea Polynya for 2010–2011 and 2011–2012 using the self-organizing map analysis technique, which is a type of artificial neural network useful for discovering underlying structure in datasets. The net community production estimates derived with four variables (sea surface temperature, mixed layer depth, chlorophyll- a , and photosynthetically available radiation) robustly reproduced the observed net community production in the Amundsen Sea Polynya. The mean net community productions were estimated as 0.42 ± 0.09 and 0.39 ± 0.07 gC m−2 d−1 in 2010–2011 and 2011–2012, respectively. The maximum weekly net community production of 1.29 gC m−2 d−1 in 2011–2012 was greater than in 2010–2011 by 0.32 gC m−2 d−1. The net community production in 2010–2011, derived using the self-organizing map, showed weekly variation similar to the trend of satellite-derived production of the Eppley -Vertically Generalized Production Model. However, in 2011–2012, it exhibited different temporal variation both in peak timing and in magnitude of the bloom. This implies the existence of complex processes not readily resolved by the four variables used in our self-organizing map analysis. Therefore, further observations during different blooming stages are required to improve self-organizing map-derived net community production estimates. • SOM analysis successfully reproduced NCP variation in the Amundsen Sea Polynya in summer season. • Optimal NCP estimates are obtained when four variables, SST, MLD, Chl- a , and PAR, are incorporated in the analysis. • The NCP estimates suggested inter-seasonal variation between the summers of 2010–11 and 2011–12. [ABSTRACT FROM AUTHOR]

Gravitational sinking of photosynthetically fixed particulate organic carbon (POC) constitutes a key component of the biological carbon pump. The fraction of POC leaving the surface ocean depends on POC sinking velocity (SV) and remineralization rate (Cremin), both of which depend on plankton community structure. However, the key drivers in plankton communities controlling SV and Cremin are poorly constrained. In fall 2014, we conducted a 6‐week mesocosm experiment in the subtropical NE Atlantic Ocean to study the influence of plankton community structure on SV and Cremin. Oligotrophic conditions prevailed for the first 3 weeks, until nutrient‐rich deep water injected into all mesocosms stimulated diatom blooms. SV declined steadily over the course of the experiment due to decreasing CaCO3 ballast and—according to an optical proxy proposed herein—due to increasing aggregate porosity mostly during an aggregation event after the diatom bloom. Furthermore, SV was positively correlated with the contribution of picophytoplankton to the total phytoplankton biomass. Cremin was highest during a Synechococcus bloom under oligotrophic conditions and in some mesocosms during the diatom bloom after the deep water addition, while it was particularly low during harmful algal blooms. The temporal changes were considerably larger in Cremin (max. fifteenfold) than in SV (max. threefold). Accordingly, estimated POC transfer efficiency to 1,000 m was mainly dependent on how the plankton community structure affected Cremin. Our approach revealed key players and interactions in the plankton food web influencing POC export efficiency thereby improving our mechanistic understanding of the biological carbon pump. Key Points: Sinking velocity was higher during oligotrophy than during blooms, which is linked to ballast, porosity, and phytoplankton size structureRemineralization was highly variable but tended to be higher during Synechococcus or diatom blooms and lower during harmful algal bloomsPlankton community structure had a considerably larger influence on particle remineralization rate than on sinking velocity [ABSTRACT FROM AUTHOR]

The mesozooplankton in both epipelagic and mesopelagic zones is essentially important for the study of ecosystem and biological carbon pump. Previous studies showed that the diel vertical migration (DVM) pattern of mesozooplankton varied among ecosystems. However, that pattern was largely unknown in the Western Pacific Warm Pool (WPWP). The vertical distribution, DVM and community structure of mesozooplankton from the surface to 1 000 m were compared at Stas JL7K (WPWP) and MA (North Pacific Subtropical Gyre, NPSG). Two sites showed similarly low biomass in both epipelagic and mesopelagic zones, which were in accordance with oligotrophic conditions of these two ecosystems. Stronger DVM (night/day ratio) was found at JL7K (1.31) than that at MA (1.09) on surface 0–100 m, and an obvious night increase of mesopelagic biomass was observed at JL7K, which was probably due to migrators from bathypelagic zone. Active carbon flux by DVM of zooplankton was estimated to be 0.23 mmol/(m2·d) at JL7K and 0.16 mmol/(m2·d) at MA. The community structure analysis showed that calanoid copepods, cnidarians and appendicularians were the main contributors to DVM of mesozooplankton at both sites. We also compared the present result with previous studies of the two ecosystems, and suggested that the DVM of mesozooplankton was more homogeneous within the WPWP and more variable within the NPSG, though both ecosystems showed typically extremely oligotrophic conditions. The different diel vertical migration strength of mesozooplankton between NPSG and WPWP implied different efficiency of carbon pump in these two ecosystems. [ABSTRACT FROM AUTHOR]

Particulate organic carbon (POC) produced in the surface ocean sinks through the water column and is respired at depth, acting as a primary vector sequestering carbon in the abyssal ocean. Atmospheric carbon dioxide levels are sensitive to the length (depth) scale over which respiration converts POC back to inorganic carbon, because shallower waters exchange with the atmosphere more rapidly than deeper ones. However, estimates of this carbon regeneration length scale and its spatiotemporal variability are limited, hindering the ability to characterize its sensitivity to environmental conditions. Here, we present a zonal section of POC fluxes at high vertical and spatial resolution from the GEOTRACES GP16 transect in the eastern tropical South Pacific, based on normalization to the radiogenic thorium isotope 230Th. We find shallower carbon regeneration length scales than previous estimates for the oligotrophic South Pacific gyre, indicating less efficient carbon transfer to the deep ocean. Carbon regeneration is strongly inhibited within suboxic waters near the Peru coast. Canonical Martin curve power laws inadequately capture POC flux profiles at suboxic stations. We instead fit these profiles using an exponential function with flux preserved at depth, finding shallow regeneration but high POC sequestration below 1,000 m. Both regeneration length scales and POC flux at depth closely track the depths at which oxygen concentrations approach zero. Our findings imply that climate warming will result in reduced ocean carbon storage due to expanding oligotrophic gyres, but opposing effects on ocean carbon storage from expanding suboxic waters will require modeling and future work to disentangle. [ABSTRACT FROM AUTHOR]

Abstract Prolonged in-situ biogeochemical observations have historically been limited in the high nitrate, low chlorophyll (HNLC) regions of the Southern Ocean (SO). Recently, large biofloat missions, including the SO Carbon and Climate Observations and Modelling (SOCCOM) program, are filling the data void. However, the standard ten-day float profiling cycle often lacks the temporal resolution required to resolve mesoscale phytoplankton bloom or export processes. This is particularly important in regions of the SO dominated by high energy mesoscale features, like the Drake Passage. In this study, we observe a naturally occurring phytoplankton bloom and subsequent carbon export event in the Drake Passage using in-situ data from an Autonomous Profiling EXplorer (APEX) biofloat that profiled the water column every two days. The fast, two-day profiling cycle meant that the biofloat's trajectory was coherent with mesoscale processes. However, because of the quasi-Lagrangian nature of the biofloat, we could not control for both spatial and temporal changes simultaneously. Therefore we explore this quasi-Lagrangian dataset as both a spatial and temporal series to understand the mesoscale processes as they relate to the observed phytoplankton bloom and its subsequent export. Our evidence suggests that eddy-driven subduction and a change in the vertical structure were important in the time period leading up to enhanced carbon export. Recent studies have shown that eddy-driven subduction may be responsible for ~20% of the total biological carbon pump in the SO. We estimate that the POC flux out of the surface ocean was 250 ± 50 mg m−2 d−1 between the peak bloom through an observed export event. In addition, we estimate that ~18 ± 10% of the particulate organic carbon observed during the peak bloom was exported out of the surface ocean via subduction and sinking. Highlights • A biofloat profiling every two days was deployed in the Drake Passage. • The biofloat observed a phytoplankton bloom and carbon export. • Following the bloom, the biofloat profiled around a cyclonic eddy. • Eddy-driven subduction likely aided post-bloom direct carbon export. [ABSTRACT FROM AUTHOR]

Abstract This study examines the sinking particle flux and composition of samples collected at three sites in the western Amundsen Sea, Antarctica: a perennial sea-ice-covered area, the central region of the Amundsen Sea polynya, and close to the Dotson Ice Shelf within the polynya. Time series sediment traps were deployed for one year at depths of 400–500 m from February and March 2012. Observations from the three sites confirm previously reported findings that the majority of annual POC (particulate organic carbon) flux in the Amundsen Sea occurs during the austral summer, with much smaller POC fluxes during other seasons. In the perennial ice-covered area, sea ice diatoms were the dominant source of sinking particles. In this region, the summertime POC flux is similar to that in the central polynya. However, the POC flux exhibited large interannual variability, with the reduction in sea ice cover and sufficient insolation being critical to enhanced sinking POC flux. Within the Amundsen Sea polynya, the sinking POC flux was higher in the central region than near the Dotson Ice Shelf, consistent with spatial variability in primary production. The site near the Dotson Ice Shelf had the lowest contribution of diatoms to sinking particles and the smallest POC flux among the three sites. Highlights • Summertime POC flux in the perennial sea ice area is mainly from sea ice diatoms. • POC flux in the perennial sea ice area is high and similar to the central polynya. • Interannual variability in the POC flux was large in the perennial sea ice area. • Reduction in sea ice and sufficient sun light were critical to enhanced POC flux. [ABSTRACT FROM AUTHOR]

The complex interplay of biological and physical mechanisms comprising the ocean's biological carbon pump has not been well characterized to date, due to the difficulty of observing these mechanisms in situ at adequate spatial and temporal resolution. An annual time series is presented of direct measurements of export production and particle properties collected using optical sediment trap‐equipped profiling floats cycling every 1.5–2.5 days. The observations indicate strong variability in particle export and bio‐optical properties, influenced by the spring bloom, mesoscale eddy activity, and the mixed layer pump. Temporal and vertical decoupling of fluxes at depths ranging from 150 to 1,000 m was also observed, and remineralization length scales were more variable than predicted by temperature‐ and oxygen‐based models. Net primary production was computed from float observations using a modification of the Carbon‐based Productivity Model and used to estimate export and export (e‐) ratios, which were compared to predictions of literature export models. Mechanistic models explicitly incorporating ecosystem processes and their depth dependence may perform better at reproducing regional observations collected at high temporal resolution. Key Points: Profiling floats captured strong variability in the biological carbon pumpFluxes from 150 to 1,000 m were decoupled temporally and verticallyMechanistic models of the biological pump best predicted the observations [ABSTRACT FROM AUTHOR]

Physical‐biological interaction within the upper ocean influences fixed organic carbon with various efficiencies leading to spatial and temporal change of carbon export in the ocean, which is important for global biogeochemical dynamics but is inadequately understood in the northern South China Sea (NSCS). Here the temporal variations of primary and export production are investigated using a 1‐D physical‐biogeochemical coupling model at a pelagic station of Xisha (XS) in the NSCS. At this station, the model reproduces the satellite sea surface observations, the depth‐integrated primary production, and the sinking flux of particulate organic carbon at 500 m from a moored sediment trap. By synthesizing these results, we investigate the long‐term dynamics of the biological pump with the focus on export production. We compared our results at XS with that at A Long‐Term Oligotrophic Habitat Assessment (ALOHA) in the North Pacific subtropical gyre. While the surface export at XS is greater than ALOHA, the stronger subsurface remineralization at XS has resulted in a similar week biological pump as ALOHA. The high surface export and subsurface flux attenuation at XS are caused by its physical dynamics that support a stronger upward nitrate flux than ALOHA. We also find a time lag of about 20 days between primary production and export production at XS, which has led to the large variability of export ratios in the subsurface layers. Further analyses suggest that the temporal variations of primary and export productions are largely driven by the change of turbulent mixing and Ekman pumping that control the vertical nutrient fluxes into the upper ocean. Plain Language Summary: Sinking flux of particles from the sea surface to the ocean interior is critical for ocean biogeochemical dynamics. Spatial and temporal variations of particle fluxes, driven by physical‐biological interaction in the ocean, can affect the efficiency of the biological pump and hence the ocean carbon cycle. The Xisha (XS) area, located between the northwestern shelf and the deep basin of the northern South China Sea, is relatively less studied compared to other regions of the northern South China Sea. Climate variability here could be controlled by the East Asian Monsoon on seasonal timescales and by the tropical Pacific El Niño–Southern Oscillation (ENSO) on interannual timescales. However, the mechanisms driving the long‐term changes of sinking organic particles and their associated biogeochemical fluxes in the XS remain less addressed. This paper describes a model‐data synthesis study of the export production at a pelagic time series station of XS. We find a generally weak biological pump at the XS site with a high surface export and a high subsurface remineralization. Key Points: Despite a high surface export, a strong subsurface remineralization leads to a weak biological pump at XishaStronger particle flux attenuation in the twilight zone of Xisha than ALOHA is due to its physical dynamics supporting a higher vertical nitrate fluxTurbulent mixing and Ekman pumping drive the temporal change of primary and export production at Xisha [ABSTRACT FROM AUTHOR]

Diatoms have long been thought to dominate the marine silicon (Si) cycle, as well as play an important role in the ocean's carbon (C) export, due to density-driven particle sedimentation. Research in the past decade has shed new light on the potential importance of picocyanobacteria to C export, although the sinking mechanism is still unclear. Interestingly, the recent discovery of Si accumulation by picocyanobacteria of the genus Synechococcu s has strong implications for the marine Si cycle, which may also have profound influence on the oceanic C export. Understanding the mechanisms of Synechococcus Si accumulation and its ecological effects are therefore critical for addressing wider issues such as Si and C exports by small cells via biological pump. Here, we show that recent advances in process studies indicate that the presence of Si within picocyanobacteria may be a common and universal feature. Subsequently, we generalize four biochemical forms of Si potentially present in picocyanobacterial cells, which are all different from diatomaceous opal-A, and hypothesize that these various structures of Si phases may be several stage products of Si precipitation. At the same time, several aspects of Si dynamics in Synechococcus are also discussed emphatically. In addition, we provide a first estimate of picocyanobacteria Si stock and production for the global ocean, accounting for 12% of the global Si inventory and 45% of the global annual Si production in the surface ocean, respectively. The implication is that picocyanobacteria may exert a significant influence on the marine Si cycle, which is likely to alter our understanding of the long-term control of the oceanic Si cycling by diatoms. Finally, we summarize three possible mechanisms and pathways through which picocyanobacteria-derived Si can be transported to the deep ocean. Altogether, marine picocyanobacteria, despite very small in cell size, are a non-negligible group for the export of biomineral Si to deeper waters and ocean sediments. • The presence of Si in picocyanobacteria may be a common feature. • Four biochemical forms of Si may be present in picocyanobacterial cells. • There are three pathways for picocyanobacteria transporting Si to the depth. • Picocyanobacteria exert a significant influence on the marine Si cycle. • Quantifying the Si and C export of picocyanobacteria are important in the future. [ABSTRACT FROM AUTHOR]

Rods settling under gravity in a quiescent fluid can overcome the bottleneck associated with aggregation of equal-size spheres because they collide by virtue of their orientation-dependent settling velocity. We find the corresponding collision kernel Γrods=lβ1ΔρVrodg/(16Aμ), where l, A, and Vrod are the rods' length, aspect ratio (length divided by width), and volume, respectively, Δρ is the density difference between rods and fluid, μ is the fluid's dynamic viscosity, g is the gravitational acceleration, and β1(A) is a geometrical parameter. We apply this formula to marine snow formation following a phytoplankton bloom. Over a broad range of aspect ratios, the formula predicts a similar or higher encounter rate between rods as compared to the encounter rate between (equal volume) spheres aggregating either by differential settling or due to turbulence. Since many phytoplankton species are elongated, these results suggest that collisions induced by the orientation-dependent settling velocity can contribute significantly to marine snow formation, and that marine snow composed of elongated phytoplankton cells can form at high rates also in the absence of turbulence. [ABSTRACT FROM AUTHOR]

Pollution is one of the main anthropogenic threats to marine ecosystems. Studies analysing the accumulation and transfer of contaminants in planktonic food webs tend to rely on samples collected in discrete water bodies. Here, we assessed the representativeness of measurements at the chlorophyll-a maximum layer during the MERITE-HIPPOCAMPE cruise for the entire water column by investigating the vertical distribution of particles and plankton obtained by in-situ optical profilers at nine stations across the Mediterranean Sea. We identified specific conditions where the interpretation of results from contaminant analyses can be improved by detailing plankton size structure and vertical distributions. First, the presence of higher than usual plankton concentrations can result in sampling issues that will affect biomass estimation within each size class and therefore bias our understanding of the contaminant dynamics. Secondly, the presence of an unsampled water layer with high zooplankton biomass might imply non-resolved contaminant pathways along the trophic structure. This study lays the basis for optimizing sampling strategy in contaminant studies., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The spatiotemporal bioaccumulation, trophic transfer of antibiotics, and regulation of the phytoplankton biological pump were quantitatively evaluated in the Pearl River, South China. The occurrence of antibiotics in organisms indicated a significant spatiotemporal trend associated with the life cycle of phytoplankton. Higher temporal bioaccumulation factors (BAFs) were found in phytoplankton at the bloom site, while lower BAFs of antibiotics in organisms could not be explained by phytoplankton biomass dilution but were attributed to the low bioavailability of antibiotics, which was highly associated with distribution coefficients ( R 2 = 0.480-0.595, p < 0.05). Such lower BAFs of antibiotics in phytoplankton at higher biomass sites hampered the entry of antibiotics into food webs, and trophic dilutions were subsequently observed for antibiotics except for ciprofloxacin (CFX) and sulfamerazine (SMZ) at sites with blooms in all seasons. Distribution of CFX, norfloxacin (NFX), and sulfapyridine (SPD) showed further significant positive relationships with the plasma protein fraction ( R 2 = 0.275-0.216, p < 0.05). This study highlights the importance of the biological pump in the regulation of spatiotemporal variations in bioaccumulation and trophic transfer of antibiotics in anthropogenic-impacted eutrophic rivers in subtropical regions.R 2 = 0.661-0.741, p < 0.05). This study highlights the importance of the biological pump in the regulation of spatiotemporal variations in bioaccumulation and trophic transfer of antibiotics in anthropogenic-impacted eutrophic rivers in subtropical regions.

Global ecosystem models are essential tools for predicting climate change impacts on marine systems. Modeled biogenic carbon fluxes in the ocean often match measured data poorly and part of this could be because small copepods (<2 mm) are modeled as unicellular feeders grazing on phytoplankton and microzooplankton. The most abundant copepods from a seasonal upwelling region of the Eastern North Atlantic were sorted, and a molecular method was applied to copepod gut contents to evaluate the extent of metazoan predation under two oceanographic conditions, a trophic pathway not accounted for in global models. Scaling up the results obtained herein, based on published field and laboratory estimates, suggests that small copepods could ingest 1.79–27.20 gigatons C/year globally. This ignored metazoan‐copepod link could increase current estimates of biogeochemical fluxes (remineralization, respiration, and the biological pump) and export to higher trophic levels by 15.6%–24.4%. It could also account for global discrepancies between measured daily ingestion and copepod metabolic demand/growth. The inclusion of metazoan predation into global models could provide a more realistic role of the copepods in the ocean and if these preliminary data hold true at larger sample sizes and scales, the implications would be substantial at the global scale. Global ecosystem models (GEMs) are essential tools to make predictions about biogeochemical budgets, fisheries yields or the fate of anthropogenic carbon, but these models have a substantial mismatch between measured and modeled carbon. Copepods are the most abundant multicellular animals on Earth but GEMs currently have copepods feeding only on single cell protists, despite the fact that they also prey upon metazoans, potentially reducing the amount of carbon entering the pelagic food webs. Using a meta‐analysis on published copepod feeding rates, the metazoan‐copepod link was estimated to be between 1.79 and 27.20 gigatons of C/year at a global scale using field and laboratory estimations, which could increase current estimates of biogeochemical fluxes by 15.6%–24.4%. [ABSTRACT FROM AUTHOR]

The China Seas include the South China Sea, East China Sea, Yellow Sea, and Bohai Sea. Located off the Northwestern Pacific margin, covering 4700000 km2 from tropical to northern temperate zones, and including a variety of continental margins/basins and depths, the China Seas provide typical cases for carbon budget studies. The South China Sea being a deep basin and part of the Western Pacific Warm Pool is characterized by oceanic features; the East China Sea with a wide continental shelf, enormous terrestrial discharges and open margins to the West Pacific, is featured by strong cross-shelf materials transport; the Yellow Sea is featured by the confluence of cold and warm waters; and the Bohai Sea is a shallow semi-closed gulf with strong impacts of human activities. Three large rivers, the Yangtze River, Yellow River, and Pearl River, flow into the East China Sea, the Bohai Sea, and the South China Sea, respectively. The Kuroshio Current at the outer margin of the Chinese continental shelf is one of the two major western boundary currents of the world oceans and its strength and position directly affect the regional climate of China. These characteristics make the China Seas a typical case of marginal seas to study carbon storage and fluxes. This paper systematically analyzes the literature data on the carbon pools and fluxes of the Bohai Sea, Yellow Sea, East China Sea, and South China Sea, including different interfaces (land-sea, sea-air, sediment-water, and marginal sea-open ocean) and different ecosystems (mangroves, wetland, seagrass beds, macroalgae mariculture, coral reefs, euphotic zones, and water column). Among the four seas, the Bohai Sea and South China Sea are acting as CO2 sources, releasing about 0.22 and 13.86-33.60 Tg C yr−1 into the atmosphere, respectively, whereas the Yellow Sea and East China Sea are acting as carbon sinks, absorbing about 1.15 and 6.92-23.30 Tg C yr−1 of atmospheric CO2, respectively. Overall, if only the CO2 exchange at the sea-air interface is considered, the Chinese marginal seas appear to be a source of atmospheric CO2, with a net release of 6.01-9.33 Tg C yr−1, mainly from the inputs of rivers and adjacent oceans. The riverine dissolved inorganic carbon (DIC) input into the Bohai Sea and Yellow Sea, East China Sea, and South China Sea are 5.04, 14.60, and 40.14 Tg C yr−1, respectively. The DIC input from adjacent oceans is as high as 144.81 Tg C yr−1, significantly exceeding the carbon released from the seas to the atmosphere. In terms of output, the depositional fluxes of organic carbon in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea are 2.00, 3.60, 7.40, and 5.92 Tg C yr−1, respectively. The fluxes of organic carbon from the East China Sea and South China Sea to the adjacent oceans are 15.25-36.70 and 43.93 Tg C yr−1, respectively. The annual carbon storage of mangroves, wetlands, and seagrass in Chinese coastal waters is 0.36-1.75 Tg C yr−1, with a dissolved organic carbon (DOC) output from seagrass beds of up to 0.59 Tg C yr−1. Removable organic carbon flux by Chinese macroalgae mariculture account for 0.68 Tg C yr−1 and the associated POC depositional and DOC releasing fluxes are 0.14 and 0.82 Tg C yr−1, respectively. Thus, in total, the annual output of organic carbon, which is mainly DOC, in the China Seas is 81.72-104.56 Tg C yr−1. The DOC efflux from the East China Sea to the adjacent oceans is 15.00-35.00 Tg C yr−1. The DOC efflux from the South China Sea is 31.39 Tg C yr−1. Although the marginal China Seas seem to be a source of atmospheric CO2 based on the CO2 flux at the sea-air interface, the combined effects of the riverine input in the area, oceanic input, depositional export, and microbial carbon pump (DOC conversion and output) indicate that the China Seas represent an important carbon storage area. [ABSTRACT FROM AUTHOR]

Abstract The temperature of seawater can affect marine plankton in various ways, including by affecting rates of metabolic processes. This can change the way carbon and nutrients are fixed and recycled and hence the chemical and biological profile of the water column. A variety of feedbacks on global climate are possible, especially by altering patterns of uptake and return of carbon dioxide to the atmosphere. Here we summarize and synthesize recent studies in the field of ecology, oceanography and ocean carbon cycling pertaining to possible feedbacks involving metabolic processes. By altering the rates of cellular growth and respiration, temperature-dependency may affect nutrient uptake and food demand in plankton and ultimately the equilibrium of pelagic food webs, with cascade effects on the flux of organic carbon between the upper and inner ocean (the "biological carbon pump") and the global carbon cycle. Insights from modern ecology can be applied to investigate how temperature-dependent changes in ocean biogeochemical cycling over thousands to millions of years may have shaped the long-term evolution of Earth's climate and life. Investigating temperature-dependency over geological time scales, including through globally warm and cold climate states, can help to identify processes that are relevant for a variety of future scenarios. Highlights • Heterotrophic respiration rates respond twice as fast as to ocean temperature changes than photosynthesis. • This may alter the ratio of particulate organic carbon to export production with more carbon sequestered when it is cooler. • Temperature dependency of metabolic rates has potential for being a key internal feedback for Earth's climate. [ABSTRACT FROM AUTHOR]

While the Ekman drift as the cause for the high productivity of the California Current System was unraveled more than a century ago, it is less clear where the nutrients within the upwelling waters are coming from, and what the fate of the produced organic matter is. Here we address these questions using a high‐resolution simulation with a regional coupled physical/biogeochemical/ecological model within a Pacific Ocean setup. Our results, emerging from both Eulerian and Lagrangian analyses, reveal that prior to coastal production, inorganic nutrients get transported laterally over thousands of kilometers toward central California. More than 80% of the nutrients originate from central offshore or southern alongshore locations. About half of it is supplied by the California Undercurrent alone, underscoring its importance in sustaining the high coastal productivity. Even though most of the inorganic nutrients get quickly transformed into organic matter once upwelled to the euphotic layer along the coast, a substantial fraction remains unused. Together with these unused nutrients, about 36% of the organic matter produced within the nearshore 100 km gets laterally exported toward the open ocean, mostly in southwestward direction. This leakage of inorganic nutrients from the coastal zone and the continuous recycling of organic matter along the way support up to 24% of the overall observed production at a distance of 500 km from the coast. This set of processes linking the origin, biological transformation, and fate of nutrients support the growing view of the biological pump being an inherently three‐dimensional process. Plain Language Summary: The upwelling of deep, nutrient‐rich waters driven by winds facilitates high levels of biological production along the West Coast of continents. It is not well known, however, where exactly the nutrient‐rich waters come from, and where the organic matter that is produced eventually ends up. Here we use a high‐resolution numerical model to address questions about the origin, the transformation, and the fate of water masses that travel through the coastal California Current System. We find that more than 80% of the nutrients that sustain the coastal ecosystem originate from central offshore regions and from around the Southern California Bight. But biology does not completely use up the nutrient resources in the coastal environment. Together with organic matter, the unused nutrients are actually transported offshore into the open ocean. Both organic matter and nutrients arriving from coastal environments hence support the open ocean ecosystem and add to the carbon sequestration there. These processes of coastal production, offshore transport, and subsequent sequestration and subduction of carbon and nutrients, respectively, draw a fully three‐dimensional picture of how carbon and nutrients are exchanged between the open ocean and coastal regions in general, and in particular in the California Current System. Key Points: The origin and fate of nutrients supporting nearshore production along central California are strongly separated in space and timeTrajectories of central California coastal upwelling suggest a 3‐D view of the biological pump that decouples carbon export from productionOffshore export of carbon and nutrients support a large fraction of open ocean production and export downstream of coastal upwelling systems [ABSTRACT FROM AUTHOR]

Glaciers and ice sheets are a significant source of nanoparticulate Fe, which is potentially important in sustaining the high productivity observed in the near-coastal regions proximal to terrestrial ice cover. However, the bioavailability of particulate iron is poorly understood, despite its importance in the ocean Fe inventory. We combined high-resolution imaging and spectroscopy to investigate the abundance, morphology and valence state of particulate iron in glacial sediments. Our results document the widespread occurrence of amorphous and Fe(II)-rich and Fe(II)-bearing nanoparticles in Arctic glacial meltwaters and iceberg debris, compared to Fe(III)-rich dominated particulates in an aeolian dust sample. Fe(II) is thought to be highly biolabile in marine environments. Our work shows that glacially derived Fe is more labile than previously assumed, and consequently that glaciers and ice sheets are therefore able to export potentially bioavailable Fe(II)-containing nanoparticulate material to downstream ecosystems, including those in a marine setting. Our findings provide further evidence that Greenland Ice Sheet meltwaters may provide biolabile particulate Fe that may fuel the large summer phytoplankton bloom in the Labrador Sea, and that Fe(II)-rich particulates from a region of very high productivity downstream of a polar ice sheet may be glacial in origin. [ABSTRACT FROM AUTHOR]

In contrast to substantial studies and established knowledge of aluminum (Al) effects (mainly toxicity) on freshwater organisms and terrestrial plants, and even on human health, only a few studies of Al effects on marine organisms have been reported, and our understanding of the role of Al in marine biogeochemistry is limited. In this paper, we review the results of both field and laboratory experiments on the effects of Al on marine organisms, including Al toxicity to marine phytoplankton and the beneficial effects of Al on marine phytoplankton growth, and we discuss possible links of Al to the biological pump and the global carbon cycle. We propose a revised Iron (Fe) Hypothesis, i.e., the Fe-Al Hypothesis that introduces the idea that Al as well as Fe play an important role in the glacial-interglacial change in atmospheric CO2 concentrations and climate change. We propose that Al could not only facilitate Fe utilization, dissolved organic phosphorus utilization and nitrogen fixation by marine phytoplankton, enhancing phytoplankton biomass and carbon fixation in the upper oceans, but also reduce the decomposition and decay of biogenic matter. As a result, Al allows potentially more carbon to be exported and sequestered in the ocean depths through the biological pump. We also propose that Al binds to superoxide to form an Al-superoxide complex, which could catalyze the reduction of Fe(III) to Fe(II) and thus facilitate Fe utilization by marine phytoplankton and other microbes. Further ocean fertilization experiments with Fe and Al are suggested, to clarify the role of Al in the stimulation of phytoplankton growth and carbon sequestration in the ocean depths. [ABSTRACT FROM AUTHOR]

To investigate horizontal transport and particulate export of semivolatile pollutants on the continental shelf or marginal sea, surface and depth water samples were collected across the northern South China Sea (SCS) and analyzed for polycyclic aromatic hydrocarbons (PAHs). The total concentrations of Σ 15 PAH in seawater were in the range of 1.6–14 ng L −1 (mean 5.5 ng L −1 ), and the offshore decreasing trend of PAHs revealed intense terrigenous transport on the continental shelf, which was originated from combustion activity. Vertically, PAHs in the particulate phase followed surface-enrichment and depth-depletion pattern with a concentration maximum (2.9 ng L −1 ) at the biomass maximum, while the dissolved PAH concentrations showed a minimum (1.5 ng L −1 ) at the same depth. Particulate export fluxes of PAHs were between 24 and 195 ng m −2  d −1 in the northern SCS based on 238 U/ 234 Th equilibria, which is suggested appropriate to quantify their downward flux in the upper water column. Efficient oceanic biological pump drives PAHs downward export to the deep ocean in the northern SCS. [ABSTRACT FROM AUTHOR]

Abstract: Iron is known to limit primary production in the Southern Ocean (SO). To cope with the lack of this micronutrient, diatoms, a dominant phytoplankton group in this oceanic region, have been shown in cultures to have developed an original adaptation strategy to maintain efficient growth rates despite very low cellular iron quotas, even in low light conditions. Using a global ocean biogeochemical model, we explored the consequences of this physiological adaptation for the biological pump and the seasonal variability of both surface chlorophyll concentrations and surface partial pressure of carbon dioxide (pCO2) in this key region for global climate. In the model, we implemented a low intracellular Fe:C requirement in the SO for diatoms uniquely. This results in an increase of 10% in the relative contribution of diatoms to total SO primary production. The biological pump is also strengthened, which increases the biological contribution to the seasonal evolution of pCO2 relative to the thermodynamic component. Therefore, the seasonal evolution of both surface chlorophyll and surface pCO2 is significantly impacted, with a marked improvement, in our model, in the SO polar zone compared to the observations. Our model study underscores the potentially important consequences that this adaptive physiological behavior of diatoms could have on marine biogeochemistry in the SO. It is thus critical to improve our understanding of the physiology of this key phytoplankton group, in particular in the SO. [ABSTRACT FROM AUTHOR]

Abstract: The “transfer efficiency” of organic particles from the surface to depth is a critical determinant of ocean carbon sequestration. Recently, direct observations and geochemical analyses have revealed a systematic geographical pattern of transfer efficiency, which is highest in high latitude regions and lowest in the subtropical gyres. We evaluate the possible causes of this pattern using a mechanistic model of sinking particle dynamics. The model represents the size distribution of particles, the effects of mineral ballast, seawater temperature (which influences both particle settling velocity and microbial metabolic rates), and O2. Parameters are optimized within reasonable ranges to best match the observational constraints. Our model shows that no single factor can explain the observed pattern of transfer efficiency, but the biological effect of temperature on remineralization rate and particle size effects together can reproduce most of the regional variability with both factors contributing to low transfer efficiency in the subtropical gyres and high transfer efficiency in high latitudes. Particle density from mineral ballast has a similar directional effect to temperature and size but plays a substantially smaller role in our optimum solution, due to the opposing patterns of silicate and calcium carbonate ballasting. Oxygen effects modestly improved model fit by depressing remineralization rates and thus increasing transfer efficiency in the Eastern Tropical Pacific. Our model implies that climate‐driven changes to upper ocean temperature and associated changes in surface plankton size distribution would reduce the carbon sequestration efficiency in a warmer ocean. [ABSTRACT FROM AUTHOR]

A decline in global ocean oxygen concentrations has been observed over the twentieth century and is predicted to continue under future climate change. We use a unique modeling framework to understand how the perturbed ocean circulation may influence the rate of ocean deoxygenation in response to a doubling of atmospheric CO2 and associated global warming. These simulations suggest that much of the oxygen decline under warming is due to changes in ocean mixing and O2 solubility. However, in our model, the large‐scale ocean circulation response to CO2 doubling slows the pace of future oxygen loss by 20%. Oxygen concentration changes are most sensitive to circulation perturbations in the Southern Ocean. A small stabilizing effect on oxygen arises from the reduction of export productivity and associated respiration in the ocean interior. A slowdown of the Atlantic Meridional Overturning Circulation increases the residence time of the deep Atlantic Ocean but does not cause a major oxygen decline at the time of CO2 doubling, because respiration is slow at these depths. The simulations show that the decrease in O2 solubility associated with ocean warming is greater than the realized decrease in preformed O2, particularly at high latitudes, where circulation changes reduce the proportion of undersaturated waters sinking into the ocean interior. Finally, in the tropical Pacific oxygen minimum zone, a predicted weakening of the Walker Circulation slows the regional upwelling of nutrients and the associated export productivity and respiration, preventing the intensification of hypoxia there. [ABSTRACT FROM AUTHOR]

We use benthic foraminiferal accumulation rates as a proxy for productivity changes during the mid-Pleistocene climate transition (MPT) (~1.2 Ma to 0.4 Ma). Our data are chosen to test the hypothesis that longer-term cooling and the onset of 100 kyr cyclicity are linked to atmospheric CO 2 draw-down associated with an increase in primary productivity. To this end, we have constructed records from a global array of seven sites spanning major ocean basins and representing different hydrographic regimes (e.g., high and low latitudes, upwelling versus the deep western warm pools). We compare our data to published productivity proxy records from each site to identify limitations and uncertainties in the reconstructions. Results indicate that there is evidence for productivity increases during the onset of the MPT (1.2–1.0 Ma), but the changes are not globally synchronous and likely reflect regional hydrographic variability. On the orbital scale, productivity maxima tend to occur more closely related to glacial than interglacial intervals overall, but the relationships are not consistent. High interglacial productivity characterizes low latitude sites some of the time. In the obliquity band, high interglacial productivity in the eastern equatorial Pacific coincides with low interglacial productivity in the Southern Ocean, supporting a high to low latitude link via intermediate water circulation distribution of nutrients. On the regional scale, our records contribute new evidence for changes in Northern Hemisphere frontal systems during the MPT and for a close link between surface ocean production of organic matter and consumption on the ocean floor in the western tropical Atlantic. Pyrite counts at the two Southern Ocean sites provide supporting evidence for sluggish thermohaline overturn during the mid-point of the MPT at ~900 ka. Taken together, our records do not show a globally synchronous productivity signal that would support the biological pump as a driver for potential CO 2 -induced climate cooling during the MPT. Instead, we document complex regional variations in the carbon cycle, reflecting a combination of both biological and physical processes both on the longer as well as on the orbital time-scale. [ABSTRACT FROM AUTHOR]

The biological pump plays a critical role in the occurrence and fate of hydrophobic organic contaminants (HOCs) mostly in temperate and frigid oligotrophic waters. However, the factors for the long-term occurrence and fate of HOCs in subtropical eutrophic waters remain largely unknown. This study provides novel insights into biogeochemical and physical factors on the annual occurrence, bioaccumulation, and biomagnification of 16 polycyclic aromatic hydrocarbons (PAHs) in the plankton food webs of four Chinese subtropical eutrophic lakes by one-year simultaneous field observations for five compartments. The annual mean ΣPAH 16 in the water columns ranged from 359.69 ± 31.52 ng L −1 to 682.69 ± 65.41 ng L −1 , and increased with the annual mean trophic state index, and phytoplankton biomass of these lakes, but was independent on the proximity of the lakes to urban areas. Biodilution effect played an important role in the occurrence of the PAHs in both phytoplankton and zooplankton. In contrast to previous studies in oligotrophic waters, not only the biological pump but also the equilibrium partitioning and the indirect influence of eutrophication (high pH induced by phytoplankton, and phytoplankton life cycling) modulated the annual occurrence of the PAHs in the water columns of these eutrophic lakes. Biphasic correlations were found between the bioaccumulation factors of the PAHs by plankton and the temperature (n = 97–136, R 2  = 0.06–0.24, p  ≤ .008), and were related to plankton phenology. Bioaccumulation factors by plankton were dependent on the hydrophobicity of the PAHs (n = 16, R 2  = 0.27–0.31, p  ≤ .023), and decreased with plankton biomass (n = 94–103, R 2  = 0.09–0.27, p  ≤ .010). Trophic transfer of the PAHs from phytoplankton to zooplankton increased with phytoplankton biomass (n = 26, R 2  = 0.27, p  = .004), and the temperature (n = 102–135, R 2  = 0.06–0.13, p  ≤ .004), but decreased with lake trophic state index. Biomagnification only occurred during phytoplankton bloom periods. [ABSTRACT FROM AUTHOR]

Abstract: The Southern Ocean plays an important role in modulating Pleistocene atmospheric CO2 concentrations, but the underlying mechanisms are not yet fully understood. Here, we report the laser grain‐size distribution and Mn geochemical data of a 523 kyr‐long sediment record (core ANT30/P1‐02 off Prydz Bay; East Antarctica) to trace past physical changes in the deep Southern Ocean. The core sediments are predominantly composed of clay and silt‐sized material. Three grain size end‐members (EM) as well as three sensitive grain size classes (SC) were discerned, interpreted as Ice Rafted Debris (EM1 and SC1), and coarse (EM2 and SC2) and fine (EM3, SC3) materials deposited from bottom nepheloid layers, respectively. Ratios of EM2/(EM2 + EM3) and SC2/SC3 reveal changes in the local bottom current strength, which is related to the deep ocean diapycnal mixing rate, showing higher values during interglacial periods and lower values during glacial periods. MnO was enriched at each glacial termination, probably caused by abrupt elevations in Antarctic bottom water (AABW) formation rate. Lower AABW formation rate and reduced deep diapycnal mixing during glacial periods enhanced deep Southern Ocean stratification, contributing to glacial atmospheric CO2 drawdown. The elevated AABW formation and enhanced deep diapycnal mixing during glacial terminations alleviated such deep stratification, promoting deeply sequestered CO2 to outgas. [ABSTRACT FROM AUTHOR]

Persistent organic pollutants (POPs) reach Antarctica through atmospheric transport, oceanic currents, and to minor extent, by migratory animals. The Southern Ocean is a net sink for many POPs, with a key contribution of the settling fluxes of POPs bound to organic matter (biological pump). However, little is known about POP transfer through the food web in the Southern Ocean and Antarctic waters, where krill is an important ecological node. In this study, we assessed the occurrence of polychlorinated dibenzo- p -dioxins and furans (PCDD/Fs), polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs) in Antarctic krill ( Euphausia superba ) from the Bellingshausen, South Scotia and Weddell Seas around the Antarctic Peninsula. The concentrations of PCDD/Fs, PBDEs and PCBs in krill showed a large variability and the average were higher (generally within a factor 3) than those previously reported for eastern Antarctica. This result highlights regional differences related to atmospheric transport and deposition, and also probable regional sources due to human activities. Bioaccumulation and biomagnification factors for PCBs in krill were estimated using previously reported phytoplankton and seawater concentrations for this region. These suggested a near water-krill equilibrium for PCBs, which was not observed for water-phytoplankton partitioning. The estimated removal settling fluxes of PCBs due to the biological pump were several orders of magnitude higher than the estimated fluxes of PCBs transferred from phytoplankton to krill. [ABSTRACT FROM AUTHOR]