Phytoplankton ecology and biogeochemistry of the warming Antarctic sea-ice zone
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Marine productivity along the western Antarctic Peninsula (WAP) is declining. The WAP is site of the fastest regional warming in the southern hemisphere, and has experienced atmospheric and oceanic temperature increases leading to increased glacial inputs and reduced winter sea-ice cover. Sea-ice is a key link between climate and phytoplankton production, as melting sea-ice stratifies the water column and provides a source of micronutrients to surface waters. Reductions in ice cover have been accompanied by declining chlorophyll (chl; a proxy for phytoplankton biomass), and a shift to smaller cell sizes in phytoplankton communities. These reductions have implications for carbon drawdown and production available to higher trophic levels. However, little is known about phytoplankton shifts at the community level, as existing studies are based on satellite records and photosynthetic pigment analyses. To elucidate the nature of the changes within phytoplankton assemblages, high-resolution time-series data of diatom speciation are coupled to environmental data from five years in Ryder Bay (Adelaide Island, WAP). Long-term monitoring at this site by the British Antarctic Survey has identified a strong relationship between chl and water column stratification, and this study spans a wide range of physical conditions and biological production. By comparing high- and low-chl phytoplankton assemblages, this study investigates the mechanisms underlying productivity changes and the manner in which these changes impact nutrient cycling, drawdown and trophic transfer. The results presented here are the first full season in-situ records documenting differences in phytoplankton and diatom assemblages between highand low-chl years. The primary difference between chl conditions is a dramatic decline in diatom abundance. This analysis indicates that the mechanism producing low-chl seasons is less stratified surface waters, where light levels are much more variable than in high-chl years. Overall production is reduced, and small increases are seen in biomass of prymnesiophytes, which are better adapted to variable light. These shifts in phytoplankton composition and size structure are consistent with a southward propagation of observed climate change effects. Within the diatom community, changes in seasonal succession and a decrease in species richness occurred following low winter sea-ice. As the main component of high productivity and that most efficiently transferred to higher trophic levels, variation in diatom production due to environmental conditions is a mechanism to explain the observed WAP ecosystem changes and chl decline. Changes in phytoplankton stocks and composition also affect nutrient use, and here the use of silicon and iron (Si and Fe, respectively, which limit productivity in large areas of the Southern Ocean) is investigated. Seasonal Si budgets estimated from Si isotopes indicate a 40 – 70% decline in Si use between high-chl and intermediate-chl years, in agreement with other indices of productivity. The consequences of reduced demand and changing supply suggest future accumulation of Si in WAP surface waters. This should increase Si export away from the WAP shelf, which may act as a mechanism to enhance productivity and carbon drawdown in the wider Southern Ocean. Sources of Fe were assessed by direct measurement and naturally occurring radioisotopes of radium. These reveal significant inputs at the surface (due to glacial sources) and to deep waters (from shelf/slope sediments), which dominate supply to the surface mixed layer at different times. Iron availability and nutrient drawdown indicate that Fe is supplied to WAP surface waters in excess of biological demand. Projected changes to Fe sources and sinks indicate that continued warming will increase the WAP Fe inventory. As for Si, this excess Fe may also be advected away from the shelf, making this region a net Fe source to the Southern Ocean.