Phytoplankton- and Physical-Regulated Dissolved Inorganic Nutrient Dynamics in the Amundsen Sea During Austral Summer

The Southern Ocean is critical for global marine primary productivity and ecosystem functioning. However, the spatiotemporal dynamics of dissolved nutrients in the Amundsen Sea remain poorly understood. We analyzed nutrient samples collected from the Amundsen Sea Polynya (ASP) and adjacent open ocean during the 38th Chinese National Antarctic Research Expedition. Integrating existing datasets, we identified nutrient distribution patterns and monthly variations driven by biological and physical processes. The ASP mixed layer exhibited the lowest nitrate concentrations (mean 12.07±5.94 μmol/L, minimum 1.74 μmol/L), indicating potential nitrate limitation; whereas the open ocean mixed layer displayed the lowest silicate levels (45.59±12.03 μmol/L). These contrasting nutrient regimes reflect distinct phytoplankton bloom characteristics: Phaeocystis antarctica dominates the ASP, while diatoms prevail in the open ocean. Unlike nitrate and silicate, surface phosphate concentrations were controlled by sea ice retreat, with upwelling of Circumpolar Deep Water or katabatic winds effectively replenishing phosphate in nearshore areas. Seasonal progression reveals dynamic nutrient depletion and replenishment cycles. As sea ice retreated and phytoplankton blooms progressed, dissolved inorganic nitrogen (DIN) in the ASP declined sharply from 16.11±5.85 μmol/L (December) to 9.46±4.55 μmol/L (February), while silicate decreased from 80.26±3.96 μmol/L (early-mid January) to 63.83±5.51 μmol/L (February). Both nutrients were subsequently replenished by March (DIN: 19.92±6.31 μmol/L; silicate: 78.53±9.41 μmol/L). This asynchronous depletion pattern mirrors the ecological succession from P. antarctica- to diatoms-dominated growth. These findings enhance understanding of nutrient cycling in Antarctic marginal seas and establish a critical baseline for assessing biogeochemical feedback under climate warming.