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, 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 fractions were positively correlated with ambient ${\mathrm{NH}}_4^{+}$ 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.