Physical and biogeochemical properties in Admiralty Bay and its connection with the Bransfield Strait, Antarctica, during the austral summers of 2019, 2022, and 2023

Abstract

We investigated spatial (inside/outside) and temporal (austral summers of 2019, 2022, and 2023) variability of physical and biogeochemical properties in Admiralty Bay (AB) using in-situ data from CTD and seawater samples complemented by 2022/2023 Global Ocean Physics Analysis and Forecast product. Temperature (−1.0 to 0.2 °C), salinity (34.1 to 34.5), oxygen (246 to 340 μmol kg⁻¹) presented significative differences between stations inside and outside the AB. Salinity was higher outside for all years, while temperature/oxygen was higher/lower (317.3 vs. 304 μmol kg⁻1) inside the bay in 2019 and outside the bay in 2022 and 2023. Transitional Zonal Water with Weddell Sea influence were dominant during all years, however in 2023, bottom waters outside AB exhibited modified Circumpolar Deep Water (mCDW). Higher oxygen apparent utilization (AOU) (52.6 vs. 40.0 μmol kg⁻¹) and nitrate plus nitrite (28.0 vs. 25.0 μmol kg⁻¹) was observed outside the AB, while phosphate was higher inside (1.6 vs. 1.2 μmol kg⁻¹), resulting in distinct N:P ratios of 24:1 and 17:1, respectively. Those patterns indicate reduced respiration and more efficient vertical ventilation inside and higher oxygen consumption and remineralization outside the AB. While surrounding shelf waters showed signs of climate-driven intrusions and interannual variability, the interior of AB did not display major hydrographic shifts during the study period. The absence of mCDW signature inside the bay may be a consequence of intense vertical mixing and local circulation patterns, that can alter/inhibit mCDW intrusions. Still, high-resolution monitoring and modeling are required to resolve the mechanisms that control exchange processes and detect emerging climate signals in this Antarctic coastal system.