Physical gradients and spatial variability of the size structure and composition of phytoplankton in the Gerlache Strait (Antarctica)

The patterns of variability of the size structure and functional composition of phytoplankton are closely linked to physical gradients in the coastal waters of the Gerlache Strait (Antarctic Peninsula). Different stratified conditions are found at both extremes of the Strait, separated by a large, rather vertically homogeneous body of water. In the SW extreme close to Bellingshausen Sea, a cold upper layer representative of ice melting influence is dominated by microplankton (20–100 μm ESD) biomass, mainly composed of colonies or aggregates of Phaeocystis in bloom stage. In the NE extreme close to Bransfield Strait, a warm surface layer has a 10–20 μm nano planktonic community dominated by Cryptomonas and a diverse assemblage of ultraplanktonic (2–10 μm) flagellates. In this area, microplankton larger than 40 μm ESD is almost absent at the sample volume studied. Also in terms of biomass, mixed central waters are dominated by microplankton, particularly large diatoms. Our results show that diatoms and microplankton are the dominant categories under turbulent conditions, with typical flagellate blooms (Phaeocystis and Cryptomonas) under particularly stratified conditions. In terms of numerical abundance, however, ultraplanktonic flagellates usually dominate the community, giving a bell shaped size-abundance spectra in all the regions and supporting previous suggestions of flagellate dominance even under mixed-layer conditions.