Seasonal succession of phytoplankton community structure from autonomous sampling at the Australian Southern Ocean Time Series (SOTS) observatory

Limited knowledge of phytoplankton community structure in the Southern Ocean hampers our understanding of ecosystem function and its response to changes expected this century from anthropogenic CO2 emissions and associated climate warming. To address this gap, we obtained records of phytoplankton community composition and nutrient concentrations, collected at 9-d intervals over the austral production season from September 2010 to April 2011 at the Southern Ocean Time Series observatory (46 degrees 56' S, 142 degrees 15' E) using an autonomous sampler, accompanied by hourly sensor-based estimates of water column structure and light levels. Satellite ocean colour and in situ fluorescence showed a moderate increase in phytoplankton biomass, 4-5 fold, from winter to mid-summer. Total cell number and biovolume increases were larger (up to 80-fold and 40-fold, respectively), reflecting the importance of heterotrophs and smaller organisms. The haptophyte Phaeocystis antarctica dominated abundances (up to 75%), but only constituted a few percent of total biovolume, which was dominated by diatoms and dinoflagellates. Ciliates contributed less than 5% of abundance, while tintinnids were rarer still. Community analysis of species similarities identified 3 distinct clusters, tracking seasonal shallowing of the mixed layer (from 500 to 50 m) and decreased silicate availability (from similar to 4 to < 1 mu M). The diatom: dinoflagellate biovolume ratio decreased more strongly than their abundance ratio, consistent with progression towards small weakly silicified diatoms in summer. Silicoflagellates were associated with elevated winter and spring silicate levels. The overall nitrate/silicate depletion ratio was similar to 2, indicating significant export by the non-diatom community. Exploratory correlative analysis of the biological diversity with environmental conditions suggests mixed layer depth and silicate levels as more likely drivers than inputs from subtropical waters or biomass accumulation.