Min Chen

Nitrogen uptake by phytoplankton influences Southern Ocean productivity and carbon export, but how source‐resolved meltwater is associated with nitrate and ammonium use remains unclear. Here, ¹⁵ N tracer incubations and a δ ¹⁸ O‐salinity mixing model were combined to examine nitrogen uptake in the early‐summer Cosmonaut Sea. Depth‐integrated nitrate ( ρ NO ₃ ) and ammonium uptake ( ρ NH ₄ ) rates averaged 8.0 and 1.3 mmol m ⁻² d ⁻¹ , respectively, yielding a mean f ‐ratio of 0.80. Meteoric water (MW) fractions were positively correlated with chlorophyll a (Chl a ) concentration, the micro‐sized (> 20 μ m) Chl a fraction, and ρ NO ₃ , suggesting that MW‐influenced waters were associated with higher biomass, larger size structure, and enhanced nitrate uptake. Sea‐ice meltwater fractions were positively correlated with ambient and ρ NH ₄ , consistent with greater regenerated nitrogen availability following sea‐ice retreat and post‐bloom recycling. These source‐specific associations highlight the need to consider meltwater timing and source when assessing nitrogen assimilation pathways and biological‐pump potential in Antarctic coastal waters.

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.

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.

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.

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.

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.

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 N₂ fixation rate (via ¹⁵N₂ bubble release method) and vertical diffusive nitrate flux (F_diff‐), for the euphotic zone (EZ) in the northern South China Sea ACEs during summer 2020. Depth‐integrated N₂ fixation rates (INF) were moderately elevated (∼30%) in the center of the ACEs compared with those in the outside stations, suggesting that ACEs generally provide a more favorable environment for N₂ fixation. In contrast, the upward F_diff‐ into the EZ were greatly lowered by an order of magnitude in the ACE center (center: 26.0 ± 8.2 μmol N m⁻² d⁻¹; outside: 124.0 ± 127.6 μmol N m⁻² d⁻¹), thus making N₂ 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‐ 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 N₂ fixation dynamics in response to mesoscale ACEs in tropical/subtropical oceanic regions, also helping better constrain biogeochemical models.

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.