Influence of oceanographic features on spatial and interannual variability of phytoplankton in the Bransfield Strait, Antarctica

Spatial variability and interannual variability of phytoplankton biomass, estimated as chlorophyll-a (Chl-a) concentration and taxonomic groups, were analyzed in relation to environmental conditions in the Bransfield Strait (BS). This study is based upon both in situ (2003, 2004, 2005, 2008, 2009 and 2010) and satellite (2002-2010) data, during the austral summer. A thermohaline front was predominately observed between the colder and saltier waters under the influence of transitional water with Weddell Sea influence (TWW) in the southeastern BS, and the fresher and warmer waters associated with the presence of transitional water with Bellingshausen Sea influence (TBW) in the northwestern BS. Canonical correspondence analysis showed that the dominance of microplanktonic diatoms was associated with higher Chl-a within shallow upper mixed layers, with relatively strong pycnocline in the TBW, particularly close to the South Shetland Islands (SSI). Conversely, the TWW was primarily characterized by lower Chl-a within deeper mixed layers or a well-mixed water column and dominated by nanoplanktonic flagellates (including haptophytes and cryptophytes). Spatial variability based on both in situ and satellite data suggests that the Bransfield Strait acts as a typical Seasonal Ice Zone (SIZ) and the phytoplankton community there is governed by a combination of processes acting synergistically: the sea ice retreat, allowing for a penetration of light into the water column, and a relatively shallow upper mixed layer with strong pycnocline, primarily in the TBW, retaining organisms near the surface when light conditions are adequate. Interannual variability in Chl-a and species composition indicate an alternation between diatom-dominated and flagellate-dominated assemblages. These shifts are potentially related to the different stages of phytoplankton succession, as a result of varying water column physical features, principally influenced by the dynamic occupation of the TBW and TWW, that appear to modulate phytoplankton dynamics within the BS.