data for "Contrasting associations of sea-ice meltwater and meteoric water with nitrogen uptake patterns in the summer Cosmonaut Sea, Antarctica"

Nitrogen uptake by phytoplankton regulates Southern Ocean productivity and carbon export, but how source-resolved meltwater is associated with nitrate and ammonium use remains unclear. Here, 15N tracer incubations and a δ18O-salinity mixing model were combined to examine nitrogen uptake in the early-summer Cosmonaut Sea. Depth-integrated nitrate (ρNO3) and ammonium uptake (ρNH4) 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 μm) Chl-a fraction, and ρNO3, suggesting that MW-influenced waters were associated with higher biomass, larger size structure, and enhanced nitrate uptake. Sea-ice meltwater (SIM) fractions were positively correlated with ambient NH4+ and ρNH4, 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.