Composition, density and size-structure of the autotrophic plankton community in a shallow coastal zone at King George Island, West Antarctic Peninsula (WAP), during early summer 2010

Resumo

The planktonic microorganism community of Admiralty Bay, Antarctic Peninsula is being monitored since 2002. This study describes composition, size-structure and biomass of the phytoplankton communities observed during early summer 2010/2011 in the area. Organisms were counted by microscopy and scored as belonging to three size classes: <10 µm, 10 - 80 µm and 80 - 400 µm. In terms of density, this phytoplankton community was dominated by cells < 10 µm (~10⁶ - 10⁷ cells L^-1), confirming the increasing presence of small planktonic autotrophs in West Antarctic Peninsula waters. Organisms > 10 µm dominated in terms of biomass (> 50% of Chlorophyll a) with densities of up to 1.6 x 10⁴ cells L^-1, which is approximately 0.6 times higher than observed for early summer in recent years. Additionally, our results confirm a previously observed phenomenon whereby large-size phytoplankton (> 80 µm), especially pennate diatoms, are replaced by mid-size centric diatoms (10- 80 µm), such as Thalassiosira spp., and small dinoflagellates (e.g. Prorocentrum antarcticum) during warmer early summers.