Hydrochemical structure and chlorophyll-a in the Arctic Ocean based on data from the “North Pole-41” drifting expedition

Our understanding of seasonal dynamics and vertical distribution of nutrients and chlorophyll-a (Chl-a) in high-latitude Arctic regions remains limited, especially during winter. The year-round Russian drifting expedition “North Pole-41”, conducted on the ice-resistant self-propelled platform “Severniy Polus”, provided a unique in situ dataset for the central Arctic from October 2022 to April 2024 under sea-ice conditions. This paper presents an analysis of the spatiotemporal variability of thermohaline parameters, Chl-a, and nutrients (N, P, Si) in the upper 100 m water column. Our observations reveal a distinct autumn Chl-a peak (0.76 μg·L^-1) directly beneath the sea ice in mid-September. Phytoplankton blooms were recorded at late August-early September, reaching 1.22-1.28 μg·L^-1 at approximately 30 m depth. We found a noticeable negative correlation between integrated Chl-a over the upper 100 m and snow cover thickness (r=-0.67), which is consistent with the light-limitation hypothesis for phytoplankton productivity in the central Arctic Ocean. The hydrochemical structure of the Eurasian Basin reflects nutrient advection by Pacific- and Atlantic-origin waters. In the Lomonosov Ridge region, Pacific waters were identified by maximum silicate concentrations exceeding 14 μmol·L^-1 and negative N^* values (-4 μmol·L^-1). In the Nansen Basin, near the continental slope, Atlantic Water penetrated into the upper 50 m, which may partly reflect the higher winter Chl-a compared to the Amundsen Basin.