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We examined the spatial variation in size‐fractionated (0.8–51 and > 51 μm) particulate cadmium (Cd) and phosphorus (P) based on a large dataset collected during a GEOTRACES Section cruise (GP09) in the North Pacific Subtropical Gyre (NPSG) to better understand the interrelationship between Cd and P biogeochemical cycles. Concentrations of particulate Cd (0.06–2.15 pmol L−1) and P (0.33–10.19 nmol L−1) showed an initial increase with depth followed by a decrease, and were among the lowest observed in the global ocean. Bulk particulate Cd : P ratios in the euphotic zone, indicative of phytoplankton Cd assimilation, showed strong geographic variability averaging 0.05 ± 0.02 within the NPSG interior vs. 0.14 ± 0.04 pmol nmol−1 at the southern boundary. Cadmium to P remineralization ratio in the mesopelagic zone had a roughly similar stoichiometry as what was produced in the euphotic zone, being ~ 0.05 ± 0.01 in the NPSG interior vs. 0.21 ± 0.04 pmol nmol−1 at the southern boundary. Cd–P decoupling was reflected in the elements' vertical distribution, showing unsynchronized changes through the water column, consistent with Cd–P differential remineralization resulting from multiple Cd and P pools. The Cd–P relationship also differed between small and large particles, suggesting differences in Cd assimilation among phytoplankton assemblages as well as particle dynamic processes. Our results highlight complex processes fractionating Cd from P in the oligotrophic ocean and complicate the use of Cd as a palaeo‐phosphate proxy. [ABSTRACT FROM AUTHOR]

The impact of environmental factors on nitrogen (N) and oxygen (O) isotope effects during algal nitrate assimilation causes uncertainty in the field application of sedimentary N isotope records and nitrate isotopes to understand the marine nitrogen cycle. Ocean acidification is predicted to change nitrogen cycling including nitrate assimilation, but how N and O isotope effects during algal nitrate assimilation vary in response to changes in seawater pH and partial pressure CO2 (pCO2) remains unknown. We measured N and O isotope effects during nitrate assimilation and physiological states of the marine diatom Thalassiosira weissflogii and Synechococcus under different pH (8.1 or 7.8) and pCO2 (400 or 800 μatm) conditions. Low pH and/or high pCO2 equally decreased N and O isotope effects during nitrate assimilation by diatoms possibly due to reducing cellular nitrate efflux/uptake ratio and decreased isotope effects for nitrate uptake, whereas they did not affect those by Synechococcus with low intracellular nitrate concentration and limited nitrate efflux. Our results provide compelling experimental evidence showing different changes in N and O isotope effects during nitrate assimilation by marine eukaryotic and prokaryotic phytoplankton at low pH and/or high pCO2. These findings suggest new insight into environmental controls on variability in the isotope effect during algal nitrate assimilation, and have implications for improving a predictive understanding of N and O isotope tools in acidified oceans. [ABSTRACT FROM AUTHOR]

Concentrations of bioavailable inorganic nitrogen (N) and phosphorus (P) are simultaneously depleted in the (sub)tropical North Atlantic Ocean, but it remains unclear if phytoplankton growth rates are N limited or N–P co‐limited. Here we present findings from three bottle‐scale experiments using a four‐by‐four matrix of low‐level N and P additions, conducted at one site in the subtropical North Atlantic Ocean. Phytoplankton responses were assessed both in terms of bulk chlorophyll a (Chl a) concentrations and intracellular Chl a of dominant Prochlorococcus and Synechococcus groups. Two matrix experiments suggested that N was independently limiting in situ growth, with no co‐limiting role for P, while the third showed co‐limitation by both N and P in this region. This switch from N limitation to N–P co‐limitation was attributed to an episodic wet deposition event that supplied N, thereby stimulating phytoplankton growth and consuming available P. Such rapid transitions in nutrient limitation in response to environmental forcing might be common in oceanic systems with multiple depleted nutrients. [ABSTRACT FROM AUTHOR]

Nutrient transfer into the sunlit surface ocean by cyclonic eddies is potentially crucial for sustaining primary productivity in the stratified subtropical gyres. However, the nature of productivity enhancements, including the flow of matter to higher trophic levels and its impact on carbon fluxes, remain poorly resolved. Here, we report a detailed assessment of the biogeochemical response to a cyclonic eddy in the subtropical Northwest Pacific via a combination of ship‐based and autonomous platforms. Primary production was enhanced twofold within the eddy core relative to reference sites outside, whereas phytoplankton biomass even decreased. Pico‐phytoplankton (< 2 μm) dominated (> 80%) both phytoplankton biomass and primary production inside and outside the eddy. The stimulated primary production in the eddy core was accompanied by an approximately twofold increase in mesozooplankton abundance, an approximately threefold increase in particle formation in the deep chlorophyll maximum layer, as well as significantly enhanced surface oceanic CO2 uptake and net community production. We suggest these observations carry important implications for understanding carbon export in the subtropical ocean and highlight the need to include such subtropical eddy features in ocean carbon budget analyses. [ABSTRACT FROM AUTHOR]

Atmospheric aerosol deposition into the low latitude oligotrophic ocean is an important source of new nutrients for primary production. However, the resultant phytoplankton responses to aerosol deposition events, both in magnitude and changes in community composition, are poorly constrained. Here, we investigated this with 19 d of field and satellite observations for a site in the subtropical North Atlantic. During the observation period, surface dissolved aluminum concentrations alongside satellite‐derived aerosol and precipitation data demonstrated the occurrence of both a dry deposition event associated with a dust storm and a wet deposition event associated with strong rainfall. The dry deposition event did not lead to any observable phytoplankton response, whereas the wet deposition event led to an approximate doubling of chlorophyll a, with Prochlorococcus becoming more dominant at the expense of Synechococcus. Bioassay experiments showed that phytoplankton were nitrogen limited, suggesting that the wet deposition event likely provided substantial aerosol‐derived nitrogen, thereby alleviating the prevalent nutrient limitation and leading to the rapid observed phytoplankton response. These findings highlight the important role of wet deposition in driving rapid responses in both ocean productivity and phytoplankton community composition. [ABSTRACT FROM AUTHOR]

Mesoscale eddies may enhance nutrient injection into the photic zone and ultimately the magnitude and composition of particle export to depth. Using satellite altimetry, we identified 38 cyclonic eddies that passed in close proximity to the Hawaii Ocean Time‐series (HOT) Station ALOHA, located in the North Pacific Subtropical Gyre, from 1993 to 2018. Particulate carbon (C), nitrogen (N), and biogenic silica (Si) export rates, measured using free floating sediment traps deployed at 150 m as part of HOT, were then associated with either the eddy core or edge based on distance to the eddy center and time of eddy evolution. Elemental fluxes varied significantly within and among individual eddies depending on season and eddy age. Spatially, biogenic Si fluxes were enhanced relative to particulate C and N fluxes at both the cores and edges, with temporally highest particulate C, N and biogenic Si fluxes occurring during the mature stage (3–8 weeks). On average, biogenic Si fluxes were 200 ± 80% (30–270% increase) higher relative to non‐eddy and during non‐bloom periods, with modest enhanced particulate C (10–30% increase) and N (10–20% increase) fluxes. In contrast, during the bloom season (July and August), elemental fluxes were all reduced by 20% relative to non‐eddy references, suggesting that cyclonic eddies depress export during the bloom period. Our results indicate that cyclonic eddies not only increase, but differentially impact the sinking export of critical biological elements, thereby contributing to long term ecological changes in foodwebs that rely on silica as well as carbon for growth. [ABSTRACT FROM AUTHOR]

Globally expanding hypoxia in estuaries and coastal oceans has largely been attributed to the elevated river nutrient inputs, whereas the role of river‐delivered terrestrial organic matter (OMterr) in hypoxia formation has been less investigated. This study uses a coupled physical‐biogeochemical model and observations to investigate how OMterr directly (via remineralization) and indirectly (via the nutrients released from OMterr remineralization) promotes hypoxia development in the coastal transition zone off the Pearl River Estuary. Results show that direct contribution of OMterr remineralization to total oxygen consumption by terrestrial and marine organic matter negatively correlates with salinity, decreasing from over 60% in the upper estuary to nearly 0% in the far reaches of the river plume, and is higher in the upstream (average 30%) than the downstream region (average 18%). Nevertheless, the nutrients released from OMterr remineralization greatly sustain an indirect contribution to oxygen depletion and hypoxia formation downstream. The increasing relative importance of indirect over the direct effect of OMterr to hypoxia along the plume path is a combined result of the wind‐driven eastward shelf current and the OMterr‐released nutrients being advected farther downstream than the sinking OMterr. This highlights that without including the indirect effect of OMterr may underestimate the role of OMterr in hypoxia formation in aquatic systems. Examinations of the hypoxia response to varying riverine loads further suggest that reducing the nutrient and OMterr loads is required for hypoxia mitigation in the upstream region while reducing the nutrient load alone is more effective in mitigating hypoxia in the downstream. [ABSTRACT FROM AUTHOR]

We surveyed the carbonate system along the main channel of the Chesapeake Bay in June 2016 to elucidate carbonate dynamics and the associated sources of oxygen‐consuming organic matter. Using a two endmember mixing calculation, chemical proxies, and stoichiometry, we demonstrated that in early summer, dissolved inorganic carbon (DIC) dynamics were controlled by aerobic respiration in the water column (43%), sulfate reduction in the sediment (39%), atmospheric CO2 invasion (13%), and CaCO3 dissolution (5%). A mass balance of the DIC concentration and its stable isotope suggested that the apparent δ13C of oxygen‐consuming organic matter was −19.4 ± 0.3‰. The bulk composition of particulate organic matter also reflected a dominance of algal material (C/N = ~ 6, δ13C > −25‰). Therefore, we concluded that the decomposition of autochthonous organic matter (i.e., eutrophication‐stimulated primary production) was the dominant process consuming oxygen, while allochthonous organic matter (terrestrially derived) made minor contributions to oxygen consumption in the hypoxic zone in June 2016. These findings in the Chesapeake Bay contrast with another hypoxic estuarine ecosystem, the Pearl River Estuary in China where allochthonous organic matter contributed significantly to oxygen consumption. The differences between these two systems in terms of hydrology, quantity and quality of organic matter, and physical characteristics are discussed to yield new insights on the formation and maintenance of hypoxia. In both systems, autochthonous organic matter dominates oxygen depletion, indicating that nutrient management and reduction are useful actions to control and mitigate the occurrence of hypoxia for the restoration of ecosystem. [ABSTRACT FROM AUTHOR]

We examined the dynamics of dissolved inorganic carbon (DIC) and pH in the Pearl River Estuary (PRE) and the adjacent northern South China Sea (NSCS) shelf in summer, aiming for a better understanding of the interaction between eutrophication, hypoxia, and ocean acidification. Using a semi‐analytical diagnostic approach based on validated multiple end‐member water mass mixing models, we showed a −191 ± 54 μmol kg−1 deficit in DIC concentrations in an extensive surface plume bulge, corresponding to a significant pH increase of ∼ 0.57 ± 0.19 units relative to conservative mixing. In contrast, DIC additions in the bottom hypoxic zone reached ∼ 139 ± 21 μmol kg−1, accompanied by a decrease in pH of −0.30 ± 0.04 units. In combination with stable carbon isotopic compositions, we found biological production and CO2 outgassing to be responsible for DIC deficits in surface waters, while degradation of organic matter (OM) accounted for DIC additions in bottom waters. The PRE‐NSCS plume system as a whole served as a net source of atmospheric CO2 from the perspective of Lagrangian observations, because strong CO2 outgassing in the inner estuary overwhelmed the CO2 uptake in the plume despite strong phytoplankton blooms. Using a two‐layer box model, we further estimated that at least ∼ 45 ± 13% of eutrophication‐driven OM production in the surface plume accounted for 67 ± 18% of the DIC addition and oxygen consumption in bottom waters. Eutrophication also buffered ocean acidification in surface waters while hypoxia enhanced it in bottom waters, but their effects on acid‐base buffering capacity were secondary to the amplification of coastal ocean acidification caused by freshwater inputs. [ABSTRACT FROM AUTHOR]

Biogeochemical silicon (Si) cycling in coastal systems is highly influenced by anthropogenic perturbations in recent decades. Here, we present a systematic study on the distribution of stable Si isotopes of dissolved silicate (δ30SiDSi) in a highly eutrophic coastal system, the Baltic Sea. Besides the well‐known processes, diatom production and dissolution regulating δ30SiDSi values in the water column, we combined field data with a box model to examine the role of human disturbances on Si cycling in the Baltic Sea. Results reveal that (1) damming led to increased δ30SiDSi values in water but had little impacts on their vertical distribution; (2) decrease in saltwater inflow due to enhanced thermal stratification had negligible impacts on the δ30SiDSi distribution. An atypical vertical distribution of δ30SiDSi with higher values in deep water (1.57–1.95‰) relative to those in surface water (1.24–1.68‰) was observed in the central basin. Model results suggest the role of enhanced biogenic silica (BSi) deposition and subsequently regenerated dissolved silicate (DSi) flux from sediments. Specifically, eutrophication enhances diatom production, resulting in elevated exports of highly fractionated BSi to deep water and sediments. In situ sedimentary geochemical processes, such as authigenic clay formation, further fractionate Si isotopes and increase pore‐water δ30SiDSi values, which then leads to pore‐water DSi flux carrying higher δ30SiDSi compositions into deep water. Our findings provide new quantitative information on how the isotope‐based Si cycle responds to human perturbations in coastal seas and shed lights on shifts of Si export to open ocean. [ABSTRACT FROM AUTHOR]

Nitrogen (N), as a critical element for microbial metabolisms, recycles rapidly in the euphotic ocean. Oxidation by nitrifiers is a competing pathway for phytoplankton assimilation of regenerated N (NH4+ and urea). Sharing the overlapping substrates may result in competitive exclusion, thus, niche separation for the two assemblages. Both pathways are sensitive to light, but whether light intensity will intensify or alleviate such resource competition in the euphotic zone remains poorly explored in the field at the community level. By using 15N labeling techniques, paired kinetic responses of uptake and oxidation were conducted in single bottles under manipulated light intensities for both NH4+ and urea. We found light stimulated the maximum rate (Rm) and specific affinities (αU) of both NH4+ and urea uptake. In contrast, light effects were opposite for oxidation kinetics (Rm and αO). As irradiance increased, the rapid increase in αU and concomitant decrease in αO imply a distinctive competition advantage of photosynthetic organisms over oxidizers under substrate‐limited environments. The ratio of αU/αO for NH4+ ranged from 0.8 to 3089 (5.8–46,788 for urea) showing a distinct increasing pattern as ambient light increases, demonstrating that phytoplankton overwhelms nitrifiers throughout the oligotrophic euphotic zone, driving down concentrations and maintaining short turnover times of the two regenerated N substrates. Moreover, phytoplankton relied equally on NH4+ and urea; yet, nitrifiers preferred NH4+ to urea. In the nitrate‐depleted euphotic ocean, light acts as a crucial driver for utilization pathways of regenerated N and vertical niche separation. [ABSTRACT FROM AUTHOR]

The chlorophyll in the northern South China Sea (NSCS) shows strong interannual variability between different phases of the El Niño‐Southern Oscillation (ENSO), primarily due to the influence from Kuroshio intrusion. Chlorophyll observations also reveal that significant year‐to‐year variation remains in the same ENSO phase, but its controlling mechanism is unknown. In this study, we examined driving mechanisms for such regional ecosystem variability on a year‐to‐year timescale. Using observational data and modeling results, we found that both cyclonic eddies (CEs) and wind‐induced mixing affect phytoplankton variability, but the former is the dominant factor regulating the interannual variability of chlorophyll during La Niña years, while the latter becomes the dominant one during El Niño years. The underlying mechanisms leading to the contrast in biological responses are attributable to changes in background nutricline depth and mixed‐layer depth (MLD) during different ENSO phases. During La Niña, both thermocline and nutricline are deepened in the NSCS. Thus, wind mixing is less effective in non‐eddy areas due to the deepened nutricline, while the CE‐induced subsurface nutrient anomaly associated with increased MLD is able to control the interannual variability of chlorophyll. During El Niño, wind‐induced mixing is more effective than CEs because the surface wind can influence a larger area. [ABSTRACT FROM AUTHOR]

We examined the dynamics and production of dissolved organic carbon (DOC) on a large continental shelf in the northern South China Sea, which is largely shaped by a river plume and coastal upwelling, based on a cruise in summer 2008. The plume water extended from the mouth of the Pearl River estuary to the middle shelf and was characterized by high DOC concentrations, while the upwelled water occupying the nearshore area featured low DOC concentrations. Biological production of DOC was observed in both the river plume and the coastal upwelling zones with different behavior between regions. The system appeared to be autotrophic in terms of DOC throughout the plume, while in the upwelling circulation, the metabolism of DOC was mixed trophic. Nevertheless, the integrated net DOC production rate of 11.5 ± 6.9 mmol C m−2 d−1 in the upwelling zone was comparable to that in the plume (7.1 ± 7.0 mmol C m−2 d−1). The net DOC production correlated strongly with net consumption of dissolved inorganic carbon (DIC) and inorganic nutrients, suggesting that the net DOC production was highly coupled to net community production (NCP) in both the plume and upwelling zones. Both regimes had similar DOC/NCP partitioning, with 19-27% of NCP in the plume and 24-26% of NCP in the upwelling zones converted to DOC. A positive correlation was also found between particulate organic carbon (POC) and net DIC consumption, with higher POC production in the upwelling zones where large phytoplankton prevailed. Most NCP removal occurred through POC sinking and/or the diffusion and horizontal transport of DOC. [ABSTRACT FROM AUTHOR]

We present the first set of dissolved silicon isotope data in seawater (δ30SiSi(OH)4) from the East China Sea, a large and productive marginal sea significantly influenced by the Kuroshio Current and freshwater inputs from the Changjiang (Yangtze River). In summer (August 2009), the lowest surface δ30SiSi(OH)4 signatures of +2.1‰ corresponding to the highest Si(OH)4 concentrations (∼30.0 µmol L−1) were observed nearshore in Changjiang Diluted Water. During advection on the East China Sea inner shelf, surface δ30SiSi(OH)4 increased rapidly to +3.2‰ while Si(OH)4 became depleted, indicating increasing biological utilization of the Si(OH)4 originating from the Changjiang Diluted Water. This is also reflected in the water column profiles characterized by a general decrease of δ30SiSi(OH)4 and an increase of Si(OH)4 with depth on the East China Sea mid-shelf and slope. In winter (December 2009-January 2010), however, the δ30SiSi(OH)4 was nearly constant at +1.9‰ throughout the water column on the East China Sea shelf beyond the nearshore, which was a consequence of enhanced vertical mixing of the Kuroshio subsurface water. Horizontal admixture of Kuroshio surface water, which is highly fractionated in Si isotopes, was observed only beyond the shelf break. Significant seasonal differences in δ30SiSi(OH)4 were detected in the surface waters beyond the Changjiang Diluted Water-influenced region on the East China Sea shelf, where the winter values were ∼1.0‰ lower than those in summer, despite the same primary Si(OH)4 supply from the Kuroshio subsurface water during both seasons. This demonstrates significantly higher biological consumption and utilization of Si(OH)4 in summer than in winter. [ABSTRACT FROM AUTHOR]