Dissolved organic nitrogen (DON) is likely comprised of multiple compound classes with varying reactivities and turnover times, resulting in numerous roles in ocean biogeochemistry. Here, we present measurements of total DON and solid-phase extracted DON (SPE-DON) concentrations and delta 15N values from sampling sites across the global ocean. An optimized SPE protocol was developed to maximize total DON recovery, with a recovery of 41.0 +/- 9.4% for surface DON across the global ocean and 56.6 +/- 11.2% for deep water DON from the Sargasso Sea. SPE-DON concentrations were 1.8-1.9 mu M across most sampling sites, in contrast to greater variation in total DON concentration (4.0-5.9 mu M). However, in the equatorial upwelling zones, SPE-DON concentrations were slightly (0.3-0.4 mu M) higher than in other regions. The delta 15N of total DON in surface waters correlated well with the delta 15N of nitrate supplied to the euphotic zone from the subsurface. SPE-DON delta 15N was also correlated with nitrate delta 15N, but SPE-DON delta 15N values were confined to a narrower range compared to those of total DON. The combined concentration and delta 15N data indicate that while SPE-DON is biased toward long-lived DON, it still retains some reactive components introduced through regional inputs in the upper ocean, and even some of its longer-lived components may be labile on the timescales of deep ocean circulation. The longer average turnover time of SPE-DON suggests that greater molecular polarity and/or charge directly increase or are otherwise correlated with the biogeochemical lability of different DON pools.

While the ocean's photosynthetic production of organic matter rivals that on land, a combination of heterotrophy and sinking prevents significant accumulation of particulate organic matter (POM) in open ocean surface waters. The origins and fates of POM in ocean surface waters are unclear, in part due to the dominance of nonliving, altered material. From the natural nitrogen isotopic composition of chlorophyll and its degradation products, we estimate the fraction of particles from eukaryotic vs. prokaryotic phytoplankton. In subtropical gyres and along the eastern North Pacific margin, the eukaryotic-to-prokaryotic ratio in particles matches that of living phytoplankton. However, in the North Atlantic outside its subtropical gyre, particles have a lower eukaryotic-to-prokaryotic ratio than do the living phytoplankton. This discrepancy at least partly arises from preferential sinking of eukaryotic biomass, consistent with the canonical but disputed paradigm that cyanobacteria disproportionately fulfill the energetic demands of the upper ocean microbial community while eukaryotes drive export production. The prokaryotic bias in surface ocean particles may also result from slow decomposition of specific components of prokaryotic biomass, a possible bottleneck in the ocean's microbial loop. The different fates of organic matter produced by eukaryotic and prokaryotic phytoplankton affect the productivity of the surface ocean, carbon export to the interior, and the signals recorded in deep-sea sediments.

The western South Indian Ocean is a highly dynamic region where tropical and subtropical waters converge. As in other ocean areas, the nutrient conditions of its thermocline play a critical role in setting regional productivity, yet they remain largely unstudied. Here, we investigate local biogeochemical processes that alter thermocline nutrient ratios in the western South Indian Ocean by applying a regional optimum multiparameter analysis to WOCE data collected across the northern and southern entrances to the Mozambique Channel, the Southeast Madagascar Current, and the Agulhas Current. We first quantify the relative contributions of three proximate source waters (Equatorial, Tropical, Subtropical) to the regional thermocline (similar to 100-900 m), along with the nutrient (i.e., nitrate and phosphate) inventories expected from isopycnal mixing of these source waters. We then use the difference between the expected and observed nutrient concentrations and ratios to diagnose the occurrence of local biogeochemical processes such as organic matter remineralization and N-2 fixation. We find that distinct regimes characterize the two entrances to the Mozambique Channel, with nutrient-rich Tropical source waters dominating the northern entrance (>55%) while nutrient-poor Subtropical source waters dominate the south (>80%). At the northern entrance, overlapping nutrient addition and removal processes drive only minor changes to the thermocline nutrient inventory relative to the nutrients supplied by the proximate source waters, with no evidence of local N-2 fixation. By contrast, south of the Mozambique Channel (>25 degrees S), including across the Southeast Madagascar Current and Agulhas Current, a strong local remineralization signal indicates nutrient addition to the Subtropical thermocline. These nutrients have a high nitrate-to-phosphate ratio relative to those supplied by the proximate and underlying source waters, which we interpret as evidence of local N-2 fixation. Our analysis shows that N-2 fixation occurs locally in the subtropical southwest Indian Ocean where it will fuel regional productivity and carbon export.

Diatom-dominated blooms in coastal upwelling systems contribute disproportionately to global primary production. The fate of carbon captured during a diatom bloom is often influenced by species-specific ecological differences. However, successional patterns that take place during a diatom bloom are often oversimplified, and the diversity of diatom adaptations to different stages of a bloom remains poorly characterized. To improve our understanding of diatom specificity to certain conditions within a bloom, we employed microscopy, 18S rRNA amplicons, and biogeochemical analysis within a simulated upwelling mesocosm experiment. We successfully simulated a diatom bloom and found that diatoms bloomed during early and late phases of the bloom. Surprisingly, the relative abundance of congeneric diatoms with the Thalassiosira, Chaetoceros, and Pseudonitzschia displayed opposing patterns that were consistent among experimental mesocosms. The late stage of the bloom was especially interesting because some diatoms continued to bloom among mixotrophic dinoflagellate genera Akashiwo, Heterocapsa, and Prorocentrum. Additionally, Syndiniales putative parasites were correlated with several diatoms, especially in the initial phase of the bloom. The novel observations of consistent rapid successional changes within our mesocosms reflect the ability of diatom and dinoflagellate genera to occupy bloom conditions that fall outside traditional expectations. Syndiniales parasite co-occurrence with blooming diatoms may be important to successional trends of coastal diatom populations, and this parasitic interaction deserves further study in coastal upwelling systems. This study indicates there are underlying diatom traits and biotic interactions that should be considered when estimating their contribution to productivity and carbon cycling within upwelling systems.

The ocean accounts for ∼20%–30% of global nitrous oxide (N 2 O) emissions, with coastal upwelling systems estimated to contribute disproportionately to the sea‐air flux of this potent greenhouse gas. To investigate the mechanisms of and controls on N 2 O production in coastal upwelling systems, we measured the concentration and nitrogen and oxygen isotopic composition of N 2 O (δ 15 N‐N 2 O and δ 18 O‐N 2 O) along a cross‐shelf transect in the Southern Benguela Upwelling System (SBUS). At the shelf bottom, N 2 O concentrations increased from the outer shelf toward the shore (11–32 nM) inversely to dissolved oxygen (182 ± 17 to <1 μM) and in concert with the remineralization tracers, apparent oxygen utilization (108 ± 21 to 221 ± 33 μM) and nitrogen (N)‐deficit (up to 20.4 μM). These observations suggest that both nitrification and denitrification may be involved in N 2 O production on the SBUS shelf. The δ 15 N‐N 2 O implicates both processes as potential N 2 O sources on the shelf, with high δ 18 O‐N 2 O values (≤57.2‰) specifically incriminating sediments as the primary N 2 O source to the water column. Isotopic changes across the shelf delineate three discrete domains with distinct N 2 O sources. Sedimentary nitrification and/or denitrification dominate N 2 O production on the midshelf , while coupled nitrification‐denitrification explains N 2 O production on the inner‐shelf . At the shallow inner‐shelf where oxygen is depleted, both water column and sedimentary denitrification account for the production and partial consumption of N 2 O. This study illuminates the disproportionate contribution of sedimentary N cycling to N 2 O production on the SBUS shelf.

St Helena Bay (SHB), a retentive zone in the productive southern Benguela Upwelling System off western South Africa, experiences seasonal hypoxia and episodic anoxic events that threaten local fisheries. To understand the drivers of oxygen variability in SHB, we queried 25 years of dissolved oxygen (DO) observations alongside high‐resolution wind and hydrographic data, and dynamical data from a high‐resolution model. At 70 m in SHB (mid‐bay), upwelling‐favorable winds in spring drove replenishment of cold, oxygenated water. Hypoxia developed in summer, becoming most severe in autumn. Bottom waters in autumn were replenished with warmer, less oxygenated water than in spring—suggesting a seasonal change in source waters upwelled into the bay. Downwelling and deep mixing in winter ventilated mid‐bay bottom waters, which reverted to hypoxic conditions during wind relaxations and reversals. In the nearshore (20 m), hypoxia occurred specifically during periods of upwelling‐favorable wind stress and was most severe in autumn. Using a statistical model, we extended basic hydrographic observations to nitrate and DO concentrations and developed metrics to identify the accumulation of excess nutrients on the shelf and nitrogen‐loss to denitrification, both of which were most prominent in autumn. A correspondence of the biogeochemical properties of hypoxic waters at 20 m to those at 70 m implicates the latter as the source waters upwelled inshore in autumn. We conclude that wind‐driven upwelling drives the replenishment of respired bottom waters in SHB with oxygenated waters, noting that less‐oxygenated water is imported later in the upwelling season, which exacerbates hypoxia.

Because nitrogen availability limits primary production over much of the global ocean, understanding the controls on the marine nitrogen inventory and supply to the surface ocean is essential for understanding biological productivity and exchange of greenhouse gases with the atmosphere. Quantifying the ocean's inputs, outputs, and internal cycling of nitrogen requires a variety of tools and approaches, including measurements of the nitrogen isotope ratio in organic and inorganic nitrogen species. The marine nitrogen cycle, which shapes nitrogen availability and speciation in the ocean, is linked to the elemental cycles of carbon, phosphorus, and trace elements. For example, the majority of nitrogen cycle oxidation and reduction reactions are mediated by enzymes that require trace metals for catalysis. Recent observations made through global -scale programs such as GEOTRACES have greatly expanded our knowledge of the marine nitrogen cycle. Though much work remains to be done, here we outline key advances in understanding the marine nitrogen cycle that have been achieved through these analyses, such as the distributions and rates of dinitrogen fixation, terrestrial nitrogen inputs, and nitrogen loss processes.

The Southern Ocean is a high-nutrient, low-chlorophyll (HNLC) region characterized by incomplete nitrate (NO3-) consumption by phytoplankton in surface waters. During this incomplete consumption, phytoplankton preferentially assimilate the 14N- versus the 15N-bearing form of NO3-, quantified as the NO3- assimilation isotope effect (15 epsilon). Previous summertime estimates of the 15 epsilon from HNLC regions range from 4 to 11 parts per thousand. While culture work has shown that the 15 epsilon varies among phytoplankton species, as well as with light and iron stress, we lack a systematic understanding of how and why the 15 epsilon varies in the field. Here we estimate the 15 epsilon from water-column profile and surface-water samples collected in the Indian sector of the Southern Ocean-the first leg of the Antarctic Circumnavigation Expedition (December 2016-January 2017) and the Crossroads transect (April 2016). Consistent with prior work in the mid-to-late summer Southern Ocean, we estimate a higher 15 epsilon (8.9 +/- 0.6 parts per thousand) for the northern Subantarctic Zone and a lower 15 epsilon (5.4 +/- 0.9 parts per thousand) at and south of the Subantarctic Front. We interpret our data in the context of coincident measurements of phytoplankton community composition and estimates of iron and light stress. Similar to prior work, we find a significant, negative relationship between the 15 epsilon and the average mixed-layer photosynthetically active radiation flux of 30-100 mu mol m-2 s-1, while above 100 mu mol m-2 s-1, 15 epsilon increases again. In addition, while we observe no robust relationship of the 15 epsilon to iron availability or phytoplankton community, mixed-layer nitrification over the Kerguelen Plateau appears to strongly influence its magnitude. The Southern Ocean, and the Subantarctic in particular, is an import region for global carbon cycling that is largely driven by microscopic algae, phytoplankton. Regarded as a high-nutrient and low-chlorophyll region, the Subantarctic surface nutrient essential for phytoplankton growth, nitrate, remains in high concentrations over the summertime growing season. During the consumption of nitrate, phytoplankton assimilates nitrate isotopes at varying ratios, likely due to environmental factors, and imparting isotopic signatures. During the Antarctic Circumnavigation Expedition and Crossroads transect, we collected samples from the Indian Sector of the Subantarctic during the mid-to-late summer. Here we use measurements of nitrate and its isotope ratios to better constrain these signatures and identify potential environmental drivers. While phytoplankton community composition and iron availability from coincident measurements may influence the nitrate isotopic assimilation ratios, we observe a strong, significant correlation between light stress across the upper ocean and isotopic signatures. This suggests that the influence of light availability for phytoplankton growth is the leading driver of nitrate isotopic ratios in the mid-to-late summer season of the Indian Sector of the Subantarctic. Nitrate assimilation isotope effects are estimated for the Indian sector of the Subantarctic and Antarctic regions of the Southern Ocean Nitrate assimilation isotope effects increase with decreasing average mixed layer PAR flux within an optimal range Dual nitrate isotopic composition profiles showed evidence of significant summertime nitrification on the Kerguelen Plateau

The Southern Ocean accounts for similar to 30% of the ocean's CO2 sink, partly due to its biological pump that transfers surface-produced organic carbon to deeper waters. To estimate large-scale Southern Ocean carbon export potential and characterize its drivers, we measured the carbon and nitrogen isotope ratios of surface suspended particulate matter (delta C-13(SPM), delta N-15(SPM)) for samples collected in summer 2016/2017 during the Antarctic Circumnavigation Expedition (364 stations). Concurrent measurements of phytoplankton community composition revealed the dominance of large diatoms in the Antarctic and nano-phytoplankton (mainly haptophytes) in open Subantarctic waters. As expected, delta C-13(SPM) was strongly dependent on pCO(2), with local deviations in this relationship explained by phytoplankton community dynamics. delta N-15(SPM) reflected the nitrogen sources consumed by phytoplankton, with higher inferred nitrate (versus recycled ammonium) dependence generally coinciding with higher micro-phytoplankton abundances. Using delta N-15(SPM) and a two-endmember isotope mixing model, we quantified the extent of nitrate- versus ammonium-supported growth, which yields a measure of carbon export potential. We estimate that across the Southern Ocean, 41 +/- 29% of the surface-produced organic carbon was potentially exported below the seasonal mixed layer during the growth season, with maximum export potential (50%-99%) occurring in the Antarctic Circumpolar Current's southern Boundary Zone and near the (Sub)Antarctic islands, reaching a minimum in the Subtropical Zone (<33%). Alongside iron, phytoplankton community composition emerged as an important driver of the Southern Ocean's biological pump, with large diatoms dominating regions characterized by high nitrate dependence and elevated carbon export potential and smaller, mainly non-diatom taxa proliferating in waters where recycled ammonium supported most productivity.

AbstractSeawater transported into the South Atlantic from the Indian Ocean via “Agulhas leakage” modulates global ocean circulation and has been linked to glacial‐interglacial climate cycles. However, constraining past Agulhas leakage has been a challenge. We sampled a transect of the Cape Basin in winter 2017 that intersected a mature Agulhas eddy and found that the 15N/14N ratio (δ15N) of mixed‐layer nitrate, zooplankton, and foraminifera (tissue and shells) was 2‰–3‰ lower in the eddy than in the background Atlantic even though the δ15N of the underlying thermocline nitrate was indistinguishable between the two settings. We suggest that the δ15N of foraminifera and other zooplankton in the eddy reflects the original Agulhas Current thermocline nitrate, which is ∼2‰ lower than that of the South Atlantic due to N2 fixation that occurs in the Indian Ocean. Foraminifera δ15N may have been lowered further during eddy migration by in situ N2 fixation and/or recycling of low‐δ15N ammonium. The absence of low‐δ15N Agulhas nitrate in the eddy thermocline can be explained by partial assimilation of the nitrate as it was mixed into the euphotic zone during and after eddy formation, raising its δ15N. The low δ15N of eddy foraminifera, apparent even after several months of eddy migration across the Cape Basin, suggests that fossil foraminifer‐bound δ15N from the region could record variations in past Agulhas leakage.

Major coastal upwelling systems are among the most productive marine ecosystems in the world. They contribute disproportionately to the cycling of carbon and nutrients in the ocean and influence marine biogeochemistry beyond their productive regions. Characterized by intense microbial respiration (both aerobic and anaerobic), major coastal upwelling systems are also hotspots for the production and outgassing of potent greenhouse gases (GHG) such as CO2, N2O, and CH4. Quantifying and understanding these roles in the context of a changing climate is therefore a subject of great interest. Here we provide a short synthesis of the current knowledge of the contributions of major coastal upwelling systems to the cycling of GHG. Despite variations within and among different systems, low-latitude coastal upwelling systems typically act as a net carbon source to the atmosphere, while those at higher latitudes function as weak sinks or remain neutral regarding atmospheric CO2. These systems also significantly contribute to oceanic N2O and CH4 emissions, although the extent of their contribution to the latter remains poorly constrained. We also overview recent and future changes to upwelling systems in the context of a warmer climate and discuss uncertainties and implications for GHG production. Although rapid coastal warming is anticipated in all major coastal upwelling systems, the future changes in upwelling-favorable winds and their implications within the context of increased stratification are uncertain. Finally, we examine the major challenges that impede our ability to accurately predict how major coastal upwelling systems will respond to future climate change, and present recommendations for future research to better capture ongoing changes and disentangle natural and forced variability.

The carbon and nitrogen isotope ratios (delta C-13 and delta N-15) of marine plankton record biogeochemical processes at the base of the food web. In the Southern Ocean, such data, predominantly from summer, have been used to infer surface CO2 concentrations and the potential for biological carbon export. However, variability in plankton delta C-13 and delta N-15 remains poorly understood, with the lack of seasonal measurements from the Southern Ocean emerging as a particular limitation. Here, we investigate the delta C-13 and delta N-15 of suspended particulate matter (SPM) and zooplankton collected from the Subantarctic Ocean in winter, summer, and autumn. The low summertime delta C-13(SPM) and delta N-15(SPM) can be explained by strong diatom reliance on nitrate supplied during winter mixing. A subsequent increase in delta C-13(SPM) and decrease in delta N-15(SPM) by autumn is consistent with iron limitation in mid- to late-summer favoring diatom succession by nano-phytoplankton that consume mainly recycled ammonium. By winter, bacterial decomposition of biomass outpaces new biomass production, decreasing delta C-13(SPM) and raising delta N-15(SPM). The delta C-13 and delta N-15 of contemporaneously sampled zooplankton generally reflect the variability in delta C-13(SPM) and delta N-15(SPM), with one autumn mismatch suggesting that in situ SPM is not always the dominant zooplankton food source. Our study shows how nutrient dynamics and phytoplankton community composition shape the seasonality of the Subantarctic's isotopic baselines, emphasizing a key role for iron availability. This work has implications for isotope-based food web studies, highlighting the need to consider seasonal variability in delta C-13(SPM) and delta N-15(SPM), as well as the different turnover times of phytoplankton (i.e., SPM) vs. zooplankton biomass.

The Agulhas Current plays a major role in heat and salt exchange between the Indian and Atlantic Oceans, yet little is known of its influence on ocean fertility. To investigate carbon production and export potential in the Agulhas Current system, we measured net primary production (NPP), nitrate and ammonium uptake, N-2 fixation, and nitrification along a transect of the current and adjacent subtropical subgyre (33.4 degrees S-35.7 degrees S) in winter when nutrient supply, and thus productivity, should be highest. Phytoplankton biomass was lowest in the current core, increasing into the subgyre as surface nitrate declined, and was dominated by nanoplankton (2.7-10 mu m; 62 +/- 5.1% of total biomass). NPP and nitrate uptake were generally high across the transect and increased from the current core into the subgyre; the rates were dominated by picoplankton (<2.7 mu m; 53-93%) in the current core and nanoplankton elsewhere (63-69%). On average, euphotic zone nitrification supplied 7.6 +/- 6.4% of the nitrate consumed by phytoplankton and N-2 fixation was also low (2.1 +/- 1.3% of new production); we thus consider nitrate uptake a reasonable proxy for new production, at least in winter. Nitrate uptake was highest at the southern edge of the current core, consistent with current-associated (sub)mesoscale mixing enhancing the upward nutrient supply. The fraction of NPP available for export (i.e., the f-ratio) was high across the transect, ranging from 0.44 to 0.69. Our data thus indicate that both total and new production are elevated across the Agulhas Current system in winter and suggest that the (sub)mesoscale dynamics associated with the current system may enhance carbon production and export in the otherwise oligotrophic southwest Indian Ocean.

The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Earth. The major role of the Agulhas Current in driving significant heat and salt fluxes is well known, yet its biogeochemical fluxes remain largely uncharacterised. Here, we use nitrate isotopes (δ15N, δ18O, and Δ(15-18) = δ15N-δ18O) to evaluate nutrient supply mechanisms that ultimately support new production in the southwest Indian Ocean. Across the greater Agulhas region, thermocline nitrate-δ15N is lower (4.9-5.8‰) than the underlying Subantarctic Mode Water source (δ15N of 6.9‰) and the upstream source regions (where nitrate-δ15N ranges from 6.4-7.0‰), which we attribute to local N2 fixation. Using a one-box model to simulate the newly-fixed nitrate flux, we estimate a local N2 fixation rate of 7-25 Tg N.a-1, amounting to ~30-95% of the whole Indian Ocean nitrogen gain estimated by models. Thermocline and mixed-layer nitrate Δ(15-18) is also low, due to both N2 fixation and coupled partial nitrate assimilation and nitrification. This local nitrogen cycling imprints an isotopic signal on Indian Ocean nitrate that persists in Agulhas rings that “leak” into the South Atlantic and are subsequently transported northwards. If this signal is retained in calcifying organisms (e.g., foraminifera) deposited on the seafloor, it could be used to trace past Agulhas leakage, yielding quantitative insights into the strength of the Atlantic Meridional Overturning Circulation over time. In addition to local N2 fixation, the nitrate isotopes reveal three physical mechanisms of subsurface nitrate supply: i) inshore upwelling driven by the current and winds, ii) entrainment at the edges of a mesoscale eddy, and iii) density-driven overturning at the current edge induced by strong horizontal velocity and density shears. All these nitrate supply mechanisms are evident as incidences of relatively high-Δ(15-18) nitrate in the thermocline and surface yet the intensity and subsurface expression of some of them is not apparent in the physical data, highlighting the utility of the nitrate isotopes for exploring physical ocean processes. The high mesoscale variability that likely drives subsurface nitrate supply to Agulhas Current surface waters is common to all western boundary currents, implying that vertical nitrate entrainment is quantitatively significant in all such systems. We posit that along with N2 fixation, physical mechanisms of upward nitrate supply enhance ocean fertility and possibly carbon export in the South Indian Ocean. Higher rates of warming, and thus thermal stratification, are expected to decrease Indian Ocean productivity more rapidly in the future than that of other ocean basins. However, a coincident increase in eddy kinetic energy across boundary currents may enhance the upward nutrient supply, partially offsetting the stratification-driven decline in productivity.

As a key biogeochemical pathway in the marine nitrogen cycle, nitrification (ammonia oxidation and nitrite oxidation) converts the most reduced form of nitrogen – ammonium–ammonia (NH4+–NH3) – into the oxidized species nitrite (NO2-) and nitrate (NO3-). In the ocean, these processes are mainly performed by ammonia-oxidizing archaea (AOA) and bacteria (AOB) and nitrite-oxidizing bacteria (NOB). By transforming nitrogen speciation and providing substrates for nitrogen removal, nitrification affects microbial community structure; marine productivity (including chemoautotrophic carbon fixation); and the production of a powerful greenhouse gas, nitrous oxide (N2O). Nitrification is hypothesized to be regulated by temperature, oxygen, light, substrate concentration, substrate flux, pH and other environmental factors. Although the number of field observations from various oceanic regions has increased considerably over the last few decades, a global synthesis is lacking, and understanding how environmental factors control nitrification remains elusive. Therefore, we have compiled a database of nitrification rates and nitrifier abundance in the global ocean from published literature and unpublished datasets. This database includes 2393 and 1006 measurements of ammonia oxidation and nitrite oxidation rates and 2242 and 631 quantifications of ammonia oxidizers and nitrite oxidizers, respectively. This community effort confirms and enhances our understanding of the spatial distribution of nitrification and nitrifiers and their corresponding drivers such as the important role of substrate concentration in controlling nitrification rates and nitrifier abundance. Some conundrums are also revealed, including the inconsistent observations of light limitation and high rates of nitrite oxidation reported from anoxic waters. This database can be used to constrain the distribution of marine nitrification, to evaluate and improve biogeochemical models of nitrification, and to quantify the impact of nitrification on ecosystem functions like marine productivity and N2O production. This database additionally sets a baseline for comparison with future observations and guides future exploration (e.g., measurements in the poorly sampled regions such as the Indian Ocean and method comparison and/or standardization). The database is publicly available at the Zenodo repository: https://doi.org/10.5281/zenodo.8355912 (Tang et al., 2023).

This dataset provides an overview of data produced during South African voyages in the Southern Ocean aboard the R/V S.A. Agulhas II and contains a document detailing the data that are presented (AvailableData.pdf) and an .xlsx file that contains the metadata and links to the published datasets (AvailableData.xlsx). Each voyage has a dedicated sheet within the .xlsx file. This dataset will be updated, and a new version published as more data becomes available. Please refer to the most recent version on https://zenodo.org/communities/scale_south_africa/ This is a data curation document only. Any use of the data mentioned in this document should reference the original dataset and authors.

The study of Antarctic first-year sea-ice biogeochemistry has been limited by samples mostly being collected in pack ice during summer, with few winter data available. Measurements from the Antarctic marginal ice zone (AMIZ) have proven even more difficult to obtain. The AMIZ is a broad, circumpolar feature of the Southern Ocean found at different latitudes during the year where light and nutrients are high enough to sustain primary production and influence ecosystem functioning. We present the first biogeochemical data set from growing ice collected in the Atlantic AMIZ during winter 2019, including measurements from young pancake ice (YI) and consolidated first-year ice (FYI). Measurements of sea-ice temperature, salinity, crystal structure, delta 18O, chlorophyll, and nutrient concentrations were used to investigate the winter sea-ice habitat and decipher the conditions under which the ice formed and grew. Model simulations support the hypothesis that nutrient accumulation in advancing sea ice cannot be explained by passive seawater entrainment and thermodynamics alone. Our data confirm that winter sea ice is biogeochemically active and accumulates remineralized nutrients. We further propose that mechanical thickening enhances the reservoir of nutrients during the ice growth season. The biogeochemical transition from YI to FYI does not appear to be a linear progression of thickness with habitat space reduction as sea ice consolidates. Instead, FYI bulk biogeochemistry results from multiple cycles of rafting of YI, which conserves the biogeochemical properties of YI in the FYI, ultimately increasing the overall nutrient and chlorophyll content. The Antarctic marginal ice zone (AMIZ) is a region of intense air-sea interactions where sea ice begins to form during winter. It is generally assumed that little to no biological activity occurs in winter sea ice, with the chemical and biological composition of the ice instead set by seawater that is incorporated during ice formation. We sampled young ice (YI) and first-year ice (FYI; collectively, growing sea ice) in the Atlantic AMIZ in winter 2019, the first such collection from this region. Our measurements of sea-ice crystal structure and oxygen isotopes suggest that the FYI formed from repeated breaking and piling up of younger ice. This idea is supported by model experiments showing that the FYI was the result of more than just temperature-driven thickening of YI. Additionally, our measurements and modeling of nutrients and chlorophyll in the ice strongly suggest that the biological community was active during winter. Our findings challenge the assumption that winter sea ice is biologically inactive and suggest that the biological and chemical signatures of YI are conserved as growing ice is broken up and reformed. The results of our study will help to improve and validate future modeling efforts. We present the first biogeochemical data for young and growing sea ice in the Atlantic Antarctic marginal ice zoneWinter sea ice is biogeochemically active and acts as a reservoir of concentrated nutrients during the ice growth seasonThe biogeochemical signature of individual floes is conserved during rafting, leading to enhanced nutrients and chlorophyll-a in first-year sea ice

This dataset represents the averaged observations derived from the Antarctic Circumnavigation Expedition (ACE - 2016/2017) and has been utilized for the analyses presented in the publication: "A circum-Antarctic plankton isoscape: Carbon export potential across the summertime Southern Ocean". It encompasses a comprehensive suite of biogeochemical measurements, specifically the isotopic ratios (δ13C, δ15N) and concentrations of carbon and nitrogen in Suspended Particulate Matter (SPM). Additionally, it includes data from High-Performance Liquid Chromatography (HPLC) analyses (i.e., Total Chl-a and fractions of pico-, nano-, and micro-phytoplankton). This dataset also contains the outputs of our calculations based on the two-endmember isotope mixing model (Fawcett et al., 2011), complemented by the Rayleigh model (Mariotti et al., 1981) to deduce the fraction of phytoplankton biomass originating from new nitrogen source (New Production).

It is understood that the global mean ocean nitrate δ 15 N is set by the δ 15 N of the input of fixed nitrogen (N) to the ocean (mostly N 2 fixation) and the net isotopic discrimination of fixed N loss (mostly denitrification). Here, we demonstrate that, in addition to the fixed nitrogen input/output budget, the isotopic discrimination of nitrate assimilation in the Southern Ocean also plays a role in setting the δ 15 N of both deep ocean nitrate and global mean ocean nitrate. A prognostic model is used to simulate the global overturning circulation, focusing on the Southern Ocean overturning cell that ventilates the global pycnocline. N 2 fixation and denitrification occur mainly in the model's surface and pycnocline boxes, as is appropriate given the observations. Experiments with this model indicate that, at a steady state, pycnocline nitrate δ 15 N is mostly controlled by the ocean's fixed N budget. Simultaneously, partial nitrate assimilation in the Southern Ocean sets the nitrate δ 15 N difference between the pycnocline and the deep ocean, lowering the δ 15 N of deep ocean nitrate and thus also of global mean ocean nitrate. The Southern Ocean's impact on deep and mean ocean nitrate δ 15 N depends on (a) the degree of nitrate consumption in the Southern Ocean surface waters and (b) the proportion of water that enters the pycnocline by subduction from the Southern Ocean surface. Including the effect of the Southern Ocean on mean ocean nitrate δ 15 N modestly reduces a previously calculated imbalance between fixed N inputs and outputs. Moreover, this effect has implications for paleoceanographic N isotope records.

In the southern Benguela ecosystem, regenerated production and high levels of organic matter remineralisation are expected to dominate during periods of relaxation. To study microbial growth and productivity under these remineralising conditions, we measured size fractionated (micro-nanoplankton: 10-200 & mu;m and nano-picoplankton: 0.3-10 & mu;m) net primary production, uptake rates of nitrate, ammonium, and urea, as well as nano-and picoplankton community composition and biomass over five consecutive days in autumn (March 2018). Samples were collected from three depths (1 m, 25 m, and 50 m) at a single station in St Helena Bay and abundances of nanophytoplankton, picophytoplankton and heterotrophic bacteria were determined using flow cytometry. There were differences in productivity among days but depth-differentiation was more apparent in the rates of net primary production and nitrogen uptake, with the highest rates (rate & PLUSMN; SD) (NPP: 3.36 & PLUSMN; 1.82 & mu;mol L-1 d-1; pNO3 -: 0.30 & PLUSMN; 0.18 & mu;mol L-1 d-1; pNH4+: 2.27 & PLUSMN; 0.75 & mu;mol L-1 d-1; pUrea: 1.38 & PLUSMN; 0.02 & mu;mol L-1 d-1) measured at the surface. Ammonium and urea uptake rates were two-to six-fold higher than those of nitrate, indicating that most of the biomass was produced through regenerated production with f-ratios of 0.02-0.21. Nano-picoplankton comprised 67% of the carbon biomass and were responsible for 90% of net primary pro-duction and 79-85% of total nitrogen (i.e., nitrate + ammonium + urea) uptake. Small cell size likely conferred advantages on nano-picoplankton in the nutrient-deplete euphotic zone and the low-oxygen (<89.3 & mu;mol L-1) conditions at 25 m and 50 m. Nitrite oxidation rates were fastest at deeper depths where heterotrophic bacteria were most abundant. Heterotrophic bacteria also contributed the most (95%) carbon biomass at all depths, suggesting a major role for these microorganisms in carbon and nitrogen cycling throughout the water column of St Helena Bay.