Tamara Lillian Schlosser , Peter G Strutton

Over the last decade, the Antarctic sea ice extent has recorded record highs and lows. The anomalous low in 2023 suggested a new reduced sea-ice state, with unknown impacts on phytoplankton blooms, including phenology and magnitude. Analysing both Biogeochemical (BGC) and Core Argo floats, we compare annual sea ice extent (SIE) anomalies and sea ice volume (SIV) to physical and biological variables. We focus on average winter and summer variability over the circumpolar Southern Ocean and 5 subregions, and over 5°x2° longitude and latitude bins. Over the seasonal ice zone, anomalously low SIE leads to warmer and saltier surface waters, deeper mixing layers and thermocline depths, and weaker upper ocean stratification due to weaker vertical salinity gradients. These trends were generally strongest in the Indian sector and the eastern part of the Atlantic, and weakest in the Pacific and Weddell Sea. Low SIE years typically had shorter phytoplankton blooms with less average summer biomass than high SIE years. However, the anomalously low SIE in 2023 led to long blooms with large total biomass, amongst large spatial variability in bloom metrics. These results were sensitive to float spatial distributions and density of sampling, highlighting the need for a persistent and widespread BGC-Argo network in the Southern Ocean so that we may adequately monitor change in this critical environment.