data for "Contrasting controls of sea-ice meltwater and meteoric water on nitrogen uptake pattern in the summer Cosmonaut Sea, Antarctica"

Nitrogen uptake by phytoplankton plays a significant role in Southern Ocean productivity and carbon export. Meltwater strongly shapes the Antarctic upper-ocean environment, yet its source-dependent impact on nitrogen uptake remains unclear. Here, isotopic evidences revealed contrasting controls of meteoric water (MW) and sea-ice meltwater (SIM) on nitrogen uptake in the Cosmonaut Sea. Depth-integrated nitrate (ρNO3) and ammonium uptake (ρNH4) rates averaged 8.0 and 1.3 mmol m‑2 d-1, with a mean f-ratio of 0.80. MW fractions were positively correlated with Chl-a concentration, micro-Chl-a proportion, and ρNO3, showing that MW input increased biomass and favored larger phytoplankton, thereby enhancing nitrate uptake. SIM fractions correlated with ambient NH4+ and ρNH4, demonstrating that sea-ice melt replenished ammonium-depleted waters and stimulated its assimilation. These opposing effects explain the observed seasonal decline in the f-ratio from spring to summer and suggest that sea-ice loss and intensified glacial melt might restructure Antarctic productivity under future warming.