Nitrite serves as a key intermediate in the nitrogen cycle, with its microbial-driven cycling regulated by various environmental factors. However, how nitrite cycling responds to changing environmental conditions remains unclear. Here, we address this question by integrating natural abundance stable isotope techniques with geochemical modeling in China's largest estuary-the Changjiang (Yangtze River) estuary-and its adjacent coastal waters, a dynamic hotspot shaped by strong anthropogenic and natural forces. From summer to autumn, shifts in water column structure-from stratification and hypoxia to mixing and reoxygenation-significantly regulate nitrite dynamics. Nitrite sources exhibit unexpected consistency across seasons, despite oxygen fluctuations from hypoxia to reoxygenation. Conversely, nitrite sinks exhibit clear seasonality and vertical structuring that are closely associated with transitions between stratification and mixing. Active nitrite oxidation is confirmed in the coastal seasonal hypoxic zone, with no clear evidence supporting the occurrence of enzymatic nitrate/nitrite isotopic exchange reaction. These findings advance our understanding of how coastal nitrogen cycling responds to seasonal environmental variability and provide a transferable framework for improving predictions of biogeochemical feedbacks under intensifying stratification and deoxygenation in a changing climate.

Biological N2 fixation is a crucial process in the marine nitrogen (N) cycle, driving primary productivity and biogeochemical cycling. As the dominant marine zooplankton, copepods play a critical role in biogeochemical fluxes and may potentially influence N2 fixation. However, the potential interactions between copepods and diazotrophs remain poorly understood. In this study, we combined natural isotope abundance (δ15N) analysis, 15N tracer assays, and nitrogenase gene (nifH) sequencing to examine the potential link between N2 fixation and copepods in the South China Sea during summer 2020. Notably, while high-throughput sequencing detected diverse nifH gene sequences (primarily Trichodesmium and proteobacteria) in copepod samples, no detectable copepod-associated N2 fixation activity was observed using the 15N2 tracer method. This decoupling between gene presence and detectable activity suggests that the copepod-associated nifH signals more likely reflected ingestion or transient retention rather than active N2 fixation within or on copepods. Molecular evidence, especially the high similarity between copepod-associated and seawater nifH sequences, is consistent with a potential trophic linkage between copepods and diazotrophs. This interpretation is also supported by isotopic patterns: copepod δ15N values were positively correlated with suspended particulate δ15N, and both pools showed clear spatial gradients decreasing from nearshore to offshore regions. These results suggest that copepods may interact with diazotrophs and diazotroph-associated particulate matter through grazing pathways. Our findings suggest that copepods may provide a potential pathway for diazotroph-derived nitrogen to enter the food web, thereby influencing the fate of newly fixed nitrogen in oligotrophic waters.

Abstract Summer nitrification's role in nitrate assimilation remains poorly quantified in the high‐latitude Southern Ocean. This study presents the first nitrate δ 15 N and δ 18 O data from the rapidly melting Amundsen Sea, showing that nitrification is active throughout the continental shelf mixed layer, while nitrate assimilation dominates in adjacent open ocean waters. Box modeling estimates that nitrification supports 20 ± 14% of mixed‐layer nitrate assimilation, with no continental shelf and open ocean difference. Redundancy analysis indicates that nitrification covaries with multiple factors, with particulate nitrogen being the strongest environmental predictor, suggesting a potential role for ammonium availability in shaping nitrification patterns. The isotope effect of nitrate assimilation varies spatially, with lower values on the continental shelf, potentially related to increased nitrification and iron availability. These findings fill a critical gap by quantifying the role of mixed‐layer nitrification in nitrate assimilation and identifying key environmental correlates of nitrification variability in the high‐latitude Southern Ocean.

Tropospheric ozone (O3) is a harmful secondary air pollutant and an important greenhouse gas. In energy-intensive cities of the arid and semi-arid Ordos Basin in China, O3 pollution has intensified due to the combined effects of anthropogenic emissions and extreme summer heat. This study investigates O3 pollution across four representative cities during the 2022 heatwave by integrating meteorological normalization, machine learning interpretation using Shapley Additive Explanations (SHAP), and WRF-CMAQ modeling. The results show that elevated temperature, reduced humidity, and limited atmospheric dispersion promote O3 accumulation, with pollution occurring under substantially lower observed relative humidity (RH, 26.0%–38.7%) than in other major regions of China. Surface solar radiation downwards (SSRD) provides a favorable photochemical background for O3 formation. Meteorological contributions reached 25.0–34.6 μg/m3, substantially exceeding those under typical conditions. SHAP analysis identified 2 m air temperature (t2m), boundary layer height and RH as the dominant meteorological drivers of O3 variability, with O3 accumulation enhanced when t2m exceeded 23.8–25.4 °C and RH exhibiting a suppressive effect above 42.5%–54.5%. Process analysis indicates that O3 variability is governed by total diffusion, dry deposition, and gas-phase chemistry, with daytime accumulation primarily driven by enhanced diffusion and photochemical production. Physical processes dominate O3 variations (66.0%–78.2%), exceeding those in major Chinese urban clusters. Source apportionment shows that O3 pollution is mainly driven by industrial and transportation emissions, and O3 formation is predominantly VOC-limited. These findings improve understanding of O3 formation mechanisms in arid and semi-arid energy-intensive regions under extreme heat and support targeted mitigation strategies.

Antarctic coastal polynyas are among the most productive regions in the Southern Ocean, playing a crucial role in maintaining ecosystem stability and sequestering atmospheric CO2. To support high primary production, glacial meltwater (GMW) significantly contributes iron (Fe) to these coastal polynyas. However, the flux of exported Fe is poorly constrained due to the rapid recycling of Fe in the euphotic zone. In this study, we investigate the utilization of 228Ra as a tracer for GMW-derived Fe inputs in Antarctic coastal polynyas. Our results show that 228Ra exhibits significantly higher activities [(0.17 ± 0.06) Bq/m3]within the coastal polynyas compared to those outside [(0.04 ± 0.04) Bq/m3]. The elevated 228Ra activities suggest GMW inputs within the polynyas, consistent with the distribution of δ18O. Stimulated by GMW-derived Fe, primary production was enhanced. Moreover, chlorophyll a (Chl a) concentrations positively correlate with 228Ra activities in the polynyas, indicating that GMW contributed Fe and 228Ra in a fixed ratio. Based on these findings, our study highlights the potential of 228Ra as a proxy for quantifying the inputs of Fe derived from GMW in Antarctica.

Antarctic sea ice and glacier melt profoundly impacts marine ecosystems. Our study in the Cosmonaut Sea summer measures seawater oxygen isotopes, size-fractionated chlorophyll-a, and phytoplankton communities. We quantify sea ice meltwater, meteoric water, and winter water contents using a Bayesian isotope-mixing model. Contrary to common belief, our findings suggest that the reduced net export of sea ice to the north and the basal melting of ice shelves have deepened the mixed layer in coastal waters, altering the survival depth of phytoplankton. Freshwater primarily stimulates phytoplankton growth by supplying dissolved iron rather than by increasing water stability, which influences the size distribution and species composition of the phytoplankton community. These insights highlight the complex interplay between freshwater inputs, nutrient dynamics, and phytoplankton communities, and are crucial for understanding the dynamics of Antarctic ecosystem and its vulnerability to climate change.

Over the past five decades, the Ross Sea has freshened, but the mechanisms remain unclear. This study uses oxygen isotopes to trace freshwater transport from the Amundsen Sea to the Ross Sea. Results show that Winter Water (WW) in the Antarctic Slope Current (ASC) has lower δ 18 O values (<−0.7‰) due to glacial meltwater (GMW) inputs from the Amundsen Sea shelf. Modified Circumpolar Deep Water (mCDW) in the Ross Sea, formed by mixing upwelled Upper Circumpolar Deep Water and ASC WW, has intermediate salinity and δ 18 O values. High Salinity Shelf Water (HSSW) forms from mCDW during sea ice formation and is further modified by mixing with GMW, resulting in distinctly low δ 18 O of <−0.8‰. In 2020, HSSW δ 18 O averaged 0.25‰ lower than in 2000, suggesting freshwater from the Amundsen Sea and local ice shelf melting contribute to the Ross Sea's freshening. This highlights the importance of understanding marginal sea interactions in climate change.

Mesoscale anticyclonic eddies (ACEs) act as an important physical disturbance for marine biogeochemical cycle, but our knowledge of the dynamics of critical new nitrogen (N) sources in such environments remains ambiguous. Here, we report concurrent data on two major sources of new N, that is, the N-2 fixation rate (via N-15(2) bubble release method) and vertical diffusive nitrate flux (F-diff-NO3-), for the euphotic zone (EZ) in the northern South China Sea ACEs during summer 2020. Depth-integrated N-2 fixation rates (INF) were moderately elevated (similar to 30%) in the center of the ACEs compared with those in the outside stations, suggesting that ACEs generally provide a more favorable environment for N-2 fixation. In contrast, the upward F-diff-NO3- into the EZ were greatly lowered by an order of magnitude in the ACE center (center: 26.0 +/- 8.2 mu mol N m(-2) d(-1); outside: 124.0 +/- 127.6 mu mol N m(-2) d(-1)), thus making N-2 fixation a much more significant contributor to new production under ACE influence. Such contribution is further demonstrated in the nutrient depleted layer where substantial carbon export may be taking place. Interestingly, a significant positive correlation for the ratio of the INF to the upward F-diff-NO3- versus sea level anomaly was observed. ACE will likely leave an imprint on the isotopic composition of exported N, implying that there is possibly a need to take mesoscale forcings into account when interpreting isotopic signals from sinking particles. These findings will improve our understanding of N-2 fixation dynamics in response to mesoscale ACEs in tropical/subtropical oceanic regions, also helping better constrain biogeochemical models.

Dissolved 232Th and biologically-essential micronutrients, such as Fe, are simultaneously supplied to the open ocean mainly through the dissolution of dust aerosols. Both dust and associated Fe deposition fluxes are therefore able to be estimated by the long-lived Th isotopes (230Th and 232Th) in seawater in combination with the concentration and solubility of Th and Fe in aerosols. The vertical distribution of dissolved 230Th and 232Th in water columns and the solubilities of Fe and Th in aerosols were examined in the tropical western North Pacific during the GEOTRACES-China GP09 cruise. The solubilities of Fe ad Th in aerosols were 14.38 +/- 1.02 % and 12.22 +/- 2.46 %, respectively, giving a solubility ratio of SFe/Th of 1.24 +/- 0.34. Estimated dust deposition fluxes integrated over the upper 500 m of the water column ranged from 0.58 to 2.35 g m-2 yr-1 based on the vertical distribution of long-lived Th isotopes in seawater and measured aerosol Th solubility. The dust-borne Fe deposition flux in the tropical western North Pacific was further estimated as 4.02-12.55 mg m-2 yr-1, using the measured SFe/Th. Both estimated dust and dust-borne Fe deposition fluxes agree well with the dust deposition model. The spatial variability of surface dissolved Fe and primary production were predominantly driven by dust-borne Fe deposition as supported by their significant correlation. Dust-borne Fe input regulated the spatial distribution of Fe:N supply ratios, which, in turn, affected the growth of marine phytoplankton, notably diazotrophs, by stimulating N2-fixation. Quantification of dust-borne Fe fluxes may improve our understanding of the biogeochemical cycling of Fe and N2-fixation process in the tropical western North Pacific.

Increasing studies have recognized that the enzymatic nitrate-nitrite isotopic exchange reaction may be a potential breakthrough to update our understanding of the nitrogen cycle. However, fundamental aspects of this reaction remain poorly understood, limiting our comprehensive understanding of the nitrogen cycle. In this study, we present the first coupled measurements of nitrate and nitrite dual isotopes at natural abundance across the Antarctic summer to uncover the environmental factors influencing enzymatic isotopic exchange. East Antarctic surface waters exhibit the most anomalous nitrate and nitrite isotope signatures and a more pronounced equilibrium isotope effect compared to West Antarctica. This feature may be attributed to the regulation of enzymatic isotopic exchange reaction by temperature, and we infer that there might be a tipping point in the expression of its intensity. Given the warming of Antarctic waters due to global climate change, particularly in West Antarctic, we hypothesize that such reaction could have an amplified impact on nitrogen isotope dynamics. Further analyses incorporating data from beyond the Southern Ocean also suggest that functional differences in the nitrite oxidoreductase enzyme itself are a critical contributing factor. Overall, our study provides new insights into the mechanisms underlying the enzymatic isotopic exchange reaction, with broad implications for models of the modern upper ocean nitrogen cycle and paleoceanographic reconstructions of ancient nitrogen cycle dynamics.

Recent insights into isotopic exchange between nitrate and nitrite have introduced complexities to our understanding of the nitrogen cycle in the Southern Ocean. This study highlights unusual isotopic compositions in the mixed layer of the Cosmonaut Sea characterized by notably low δ 15 N and high δ 18 O values in nitrite. These anomalies challenge our expectations regarding isotopic behavior within cycling pathways, highlighting the significant role of isotopic exchange between nitrate and nitrite. Interestingly, incorporating this exchange reaction significantly altered the estimated isotope effect of nitrate assimilation for both nitrogen and oxygen based on the Rayleigh model in the nitrate‐only system. This raises questions about the reliability of previous estimates in assessing nitrate consumption in the Southern Ocean, suggesting that the nitrate + nitrite system may provide a more accurate representative of nitrate uptake. Additionally, spatial variations in the nitrate assimilation isotope effect () both for nitrogen and oxygen were also observed, with higher and values north of the southern boundary (SB) and lower values to the south. The reduced and values south of the SB may be primarily driven by elevated iron concentrations as indicated by the positive relationship between and △Si/△N, a proxy for iron limitation, rather than by nitrification, phytoplankton composition, vertical mixing, or light availability. This study uses dual isotopes of nitrate and nitrite to evaluate isotopic exchange effects on nitrate assimilation, specifically for oxygen, enhancing our understanding of the Southern Ocean's role in the global nitrogen cycle.

Increased nutrient loading in coastal waters poses a threat to marine ecosystems. To develop effective management strategies, a clearer understanding of nitrogen cycle dynamics for the main species is crucial for understudied urbanized areas. By employing stable isotopes of nitrate (δ15N and δ18O) and the rarely reported nitrite isotopes, we found a decoupling between physical mixing and microbial transformative processes in the Xiamen Bay. During the dry season, dominated by endmember mixing, the SIAR (Stable Isotope Analysis in R) model identifies manure (50%) as the primary nitrate source, followed by fertilizer, sewage, and rainfall. Microbial processes govern nitrogen cycling during the wet season, as evidenced by the relatively low ε value (∼2.4‰) using the Rayleigh fractionation model. This likely reflects distinct environmental conditions in coastal waters compared to the open ocean, such as limited light and iron availability. Nitrite isotope ratios implicate ammonia oxidation and nitrite oxidation as the primary drivers of nitrite variability during the wet season. This suggests that seasonal nitrite accumulation in summer may result from a decoupling of these processes in response to temperature fluctuations. Theoretical calculations of the nitrite reservoir, based on key parameters like temperature and substrate concentration, further support this argument. Our findings highlight the highly dynamic nature of nitrate and nitrite cycling in coastal environments. This underscores the need for further research in these understudied coastal systems, particularly in the context of intensifying human activities and climate change.

As human activities intensify, ecosystems are constantly being polluted by microplastics, which may change the microbe-driven nitrogen cycling and associated nitrous oxide emissions therein. However, the exact impact of microplastics on specific nitrogen cycling processes remains to be clarified, limiting accurate assessments of nitrous oxide production. Additionally, a gap in our understanding of the isotopic dynamics of nitrogen cycling under the impact of microplastics restricts deeper insights into nitrogen cycling in microplastic-polluted environments. Accordingly, this study represents the first integration of natural abundance isotope techniques with microcosm experiments involving various microplastics, offering a novel approach for detailed investigation into the impacts of microplastics on the nitrogen cycle dynamics and their potential role in regulating nitrous oxide production. Our results suggest that microplastics of different sizes (0.02 mm, 0.1 mm, and 1 mm) and polymer types (polypropylene, polyvinyl chloride, polyamide, and polyethylene) impact both nitrite production and consumption, highlighting the important role of size in these processes. Particularly, nitrite dual isotopic signatures help identify specific nitrogen cycling processes impacted by microplastics. More importantly, isotopic evidence indicates that nitrite may be lost from the environment primarily by reduction to gaseous products nitrous oxide or dinitrogen in polyethylene and polyvinyl chloride, especially the largest-size polyamide treatments. Conversely, polypropylene treatment, especially at large sizes, may promote nitrite oxidation, thus retaining more nitrogen within the environment. Our findings offer a new paradigm for the comprehensive assessment of the impact of microplastics on the nitrogen cycle and highlight the importance of considering microplastics when assessing greenhouse gas emissions, especially in the context of increasing microplastic pollution.

Abstract. Transparent exopolymer particles (TEP) play a crucial role in marine carbon cycling. While phytoplankton are known to be the primary contributors to TEP, the impact of changes in phytoplankton community structure on TEP production in natural aquatic environments remains incompletely understood. This study employed multiple linear regression (MLR) modeling to quantify the contributions of two dominant phytoplankton groups, diatoms and haptophytes (primarily Phaeocystis antarctica), to TEP production in the surface waters of the Cosmonaut Sea, antarctica during the austral summer. Results demonstrate that in situ TEP production by each group can be estimated by scaling laboratory-derived theoretical values with an environmentally adjusted correction factor. These factors, primarily governed by phytoplankton community structure, reveal taxon-specific discrepancies between field and laboratory TEP production capacities. Notably, temperature, ammonium, and polysaccharide composition act as secondary modifiers of through indirect physiological effects. This study revealed that when the chlorophyll a concentration (Chl a) of P. antarctica exceeds 0.5 μg/L in the Cosmonaut Sea, its TEP production capacity surpasses that of diatoms at equivalent biomass levels – challenging the paradigm of diatom-dominated TEP contributions. In the research area, P. antarctica contributed 14.6–82.5 % (mean: 48.6 ± 15.4 %) to total TEP production, while diatoms contributed 31.0–112.0 % (mean: 55.1 ± 21.2 %; values >100 % reflect co-occurring group contributions). This highlights the pivotal role of P. antarctica in Southern Ocean carbon cycling and provides mechanistic insights for refining polar carbon budget models.

Nitrification, a central process in the marine nitrogen cycle, produces regenerated nitrate in the euphotic zone and emits N2O, a potent greenhouse gas as a by-product. The regulatory mechanisms of nitrification in the Southern Ocean, which is a critical region for CO2 sequestration and radiative benefits, remain poorly understood. Here, we investigated the in situ and dark nitrification rates in the upper 500 m and conducted substrate kinetics experiments across the Indian Sector in the Cosmonaut and Cooperation seas in the late austral summer. Our findings indicate that light inhibition of nitrification decreases exponentially with depth, exhibiting a light threshold of 0.53 4 + supply can act as a buffer against photo-inhibitory damage. Globally, substrate affinity (α) increases with depth and transitions from light to dark, decreases with increasing ambient NH 4 + , and exhibits a latitudinal distribution, reflecting substrate utilization strategies. We also reveal that upwelling in Circumpolar Deep Water (CDW) stimulates nitrification through the introduction of potentially higher iron and deep diverse nitrifying microorganisms with higher α. We conclude that although light is the primary limiting factor for nitrification in summer, coupling between substrate availability and CDW upwelling can overcome this limitation, thereby alleviating photoinhibition by up to 45

Estuarine processes regulate the transport of dissolved black carbon (DBC) and associated contaminants to the ocean. However, there is limited understanding of the geochemical behavior of DBC in estuaries. In this study, DBC in the Pearl River Estuary (PRE) and the northern shelf of the South China Sea were examined using the benzene polycarboxylic acid (BPCA) method. DBC, bulk dissolved organic carbon (DOC), and chromophoric dissolved organic matter (CDOM) exhibited distinct behaviors during their transport from the PRE to the sea. DOC and CDOM decreased during the initial mixing of river water and seawater but increased at the lower PRE. In contrast, high aromaticity DBC inputs were observed throughout the PRE, likely originating from local terrestrial sources, such as runoff from nearby islands, as indicated by the high R H/L values (i.e., the ratio of BPCA containing 5 and 6 carboxyl groups to that containing 3 and 4 carboxyl groups; 2.03–2.30). In the Pearl River-plume zone (salinity < 33.0), DOC, CDOM, and DBC showed quasi-conservative behaviors against salinity, indicating that their geochemical behaviors were primarily governed by physical mixing between plume water and seawater. Using a flux model, it was estimated that the discharge of riverine DBC from the Pearl River Delta ranged from 11.2 to 16.3 Gg year −1 , representing an important source of bio-resistant DOC to the northern South China Sea.

Atmospheric deposition of 210Pb and 210Po from Nov. 2010 to Jan. 2012 were analyzed to reveal their temporal variations, as well as applications in constraining the residence times of aerosol. The monthly depositional fluxes varied from 2.42 to 29.31 Bq/m2/mon and from 0.160 to 3.388 Bq/m2/mon for 210Pb and 210Po, respectively. High fluxes of 210Pb and 210Po were observed in the southwest monsoon prevailing months, while low fluxes corresponded to the northeast monsoon seasons, revealing the monsoon control over 210Pb and 210Po deposition on seasonal timescales. There were significant positive linear correlations between the daily depositional fluxes and precipitation, supporting the predominant removal passage of 210Pb and 210Po through rainfall. The inverse relations between specific activities and precipitation indicated that the removal efficiencies were much higher at the beginning of rainfall. In contrast, the dry deposition only accounted for 22

Atmospheric deposition of 7Be was measured at a time-series station in the southeast of China (Xiamen) from 2011 to 2013. The deposition fluxes of 7Be ranged from 0.05 Bq m−2 d−1 to 7.42 Bq m−2 d−1, averaging 1.87 ± 0.10 Bq m−2 d−1. High fluxes occurred in months with northeast monsoon, and low values were observed in the southwest monsoon prevailing months. The significant correlations between 7Be deposition and precipitation, existing in both northeast and southwest monsoon seasons, suggested the dominant removal of atmospheric 7Be via precipitation. However, the correlations showed a large slope for the northeast monsoon season, indicating higher 7Be contents in the atmosphere during the northeast monsoon prevailing months, supported by the precipitation-normalized 7Be and the temporal variability of 7Be/210Pb ratios. Such a scenario revealed more intensive exchange of air mass between the stratosphere and troposphere during the northeast monsoon prevailing months. Together with the high pollutant concentrations in ambient air observed in these seasons, the results indicated that the pollutants in Xiamen might enter into the upper troposphere via vertical air mass exchange.

The Amundsen Sea Polynya (ASP) is the most biologically productive area around Antarctica due to the input of iron-rich glacial meltwater (GMW). However, the source and path of GMW in the ASP, and how these have changed since the Dotson Ice Shelf (DIS), a primary GMW supplier, began experiencing a cooling period after 2011, remain unclear. This study presents the distribution of GMW in the ASP during the austral summer of 2020. Subsurface GMW proportions were estimated using a composite tracer derived from potential temperature, salinity, and dissolved oxygen, while surface GMW were using 226Ra and 228Ra. The results indicate that GMW in the ASP originates from DIS and Pine Island Bay. Surface GMW upwelled from the melting basal cavity of DIS is transported northwestward by katabatic winds, while subsurface GMW is transported northwestward beneath the mixed layer, with the upper portion upwelling to the surface in the ASP center along isopycnals (σθ) of 27.40 to 27.45 kg/m3. Depth variations of these isopycnals correlate well with brine inventories released by winter sea ice formation, suggesting that sea ice formation influences seawater σθ structures and consequently the transport path of subsurface GMW. Compared to 2011, the GMW content in the ASP in 2020 decreased by nearly half, and the transport routes have also changed. These changes align with the significantly reduced GMW discharge from the DIS after 2012. Our study confirms that the GMW system in the ASP has undergone significant changes following the onset of the cooling period experienced by the DIS.

This study examined particulate organic carbon (POC) and its isotopic composition (δ13CPOC) in the Cosmonaut and Cooperation Seas in the Antarctica during the summer of 2019. Our results show that the spatial variation of POC concentration in summer surface water generally mirrors that of δ13CPOC, with higher POC and δ13CPOC values in the Cosmonaut Sea compared to the Cooperation Sea. The δ13CPOC values in both seas were positively correlated with the proportion of Chl-a in smaller particles (< 20 μm). However, the relationship with the proportion of biogenic POC in smaller particles (< 20 μm) differed between the two seas. This discrepancy is attributed to differences in the dominant phytoplankton species. In the Cosmonaut Sea, smaller phytoplankton (nano- and pico-phytoplankton) were dominated by Phaeocystis antarctica, whereas in the Cooperation Sea, they were dominated by pennate diatoms. The δ13CPOC in deep waters of both seas increased with depth, reflecting the effects of organic remineralization. The carbon isotope fractionation factors during remineralization, estimated using Rayleigh model, were 1.5 ± 0.2‰ and 1.6 ± 0.2‰ in the Cosmonaut Sea and the Cooperation Sea, respectively. These small isotope effects indicate that the isotope signals of organic matter exported from the upper layer are well preserved in the deep ocean. Additionally, anomalously high δ13CPOC values were observed in the bottom water outside the Cape Darnley polynya in the Cooperation Sea, reflecting the input of ice algae-derived organic matter from the shelf during AABW formation. A simple isotopic mass balance estimate suggests that 6–19% of the POC in the AABW of the Cooperation Sea is contributed by ice algae. Our study highlights the complexity of factors affecting δ13CPOC in the Southern Ocean, emphasizing the importance of phytoplankton community composition.