Interannual variability in phytoplankton in northern Marguerite Bay (WAP) is governed by both winter sea ice cover and summer stratification

The rapid warming of the West Antarctic Peninsula region has led to reduced sea ice cover and enhanced glacial melt water input. This has potential implications for marine ecosystems, notably phytoplankton growth, biomass and composition. Earlier studies suggested declining phytoplankton biomass related to reduced winter sea ice cover. In the present study we aimed to link summer and winter phytoplankton performance to the observed decline. Fifteen years (1997-2012) of year-round size fractionated chlorophyll a (chl a), environmental data, and 7 years of phytoplankton pigment fingerprinting were analyzed at the RaTS monitoring site in northern Marguerite Bay, Antarctica. Winter phytoplankton biomass was low (< 0.25 μg chlorophyll a l-1) and consisted on average of 69% diatoms, 5% cryptophytes, and 20% haptophytes. Summers following winters with low (< 65 days) sea ice cover were characterized by decreased stratification strength and relatively low (median < 4.4 μg chlorophyll a l-1) phytoplankton biomass, as compared to summers preceded by high winter sea ice cover. In addition, the summertime microphytoplankton (> 20 µm) fraction was strongly decreased in low biomass years, from 92% to 39%, coinciding with a smaller diatom fraction in favor of nanophytoplankton (< 20 µm), represented by cryptophytes and haptophytes. In contrast, diatoms dominated (> 95%) summers with average-to-high biomass. We advance a conceptual model whereby low winter sea ice cover leads to low phytoplankton biomass and enhanced proportions of nanophytoplankton, when this coincides with reduced stratification during summer. Over the observational period, the onset of spring ice melt, as indicated by a salinity decrease, was delayed 100 days when comparing high and low biomass summers. Thus, increased mixing during the winter combined with a decrease in sea ice will delay stratification further and, as such, decrease total summer phytoplankton productivity. Overall, both summer (January – February) and winter biomass declined significantly, 2.88 and 0.023 µg Chlorophyll a l-1 decade-1 respectively. These changes are likely to have a strong effect on the marine food web, including krill biomass and distribution. Given the current trends in climate change, our observed trends are not likely to be reverted within the near future.