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

Abstract 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.