Impact of sea ice on the structure of phytoplankton communities in the northern Antarctic Peninsula

Abstract

The seasonal advance and retreat of sea ice around the northern Antarctic Peninsula can have a significant impact on phytoplankton, mainly due to alterations in the availability of ice-free areas, micronutrient inputs by meltwater and variations in water column structure. The aim of this work was to evaluate the effect of sea ice conditions on phytoplankton biomass and community composition in an area off the northern Antarctic Peninsula, a region undergoing important warming processes. In two consecutive summer cruises (2013 and 2014), seawater samples were analysed for nutrients and phytoplankton (through HPLC-CHEMTAX approach), and measurements were made for water column physical structure evaluation. Two contrasting conditions were studied: a strong environmental gradient around the sea ice edge, with a marked meltwater signal (summer 2013) and the same area with little indication of meltwater and no detectable sea ice conditions (summer 2014). In the first year, the phytoplankton communities were massively dominated by nanoflagellates such as cryptophytes, small dinoflagellates and Phaeocystis antarctica, but with differences between stations with less influence of meltwater (dominance of dinoflagellates type B, mainly Gymnodinium spp., mean chlorophyll a = 1.37 mg m−3) and stations closer to the sea ice edge (dominance of cryptophytes, mean chlorophyll a = 0.98 mg m−3). In the second year, cryptophytes were apparently replaced by diatoms type B (mainly Pseudonitzschia spp., 24% contribution, mean chlorophyll a = 0.93 mg m−3), although dinoflagellates were also important. Therefore, there was a clear distinction between the phytoplankton communities under sea ice influence, where mainly cryptophytes were associated with shallow mixed layers and high water column stability in 2013 and an important presence of diatoms in 2014, associated with deeper mixed layers, lower silicic acid concentrations and higher magnitudes of both salinity and temperature, under very little sea ice influence. Gymnodinioid dinoflagellates were an important component in both years, apparently occupying sites/conditions less favourable to cryptophytes. These results support previous suggestions that climate factors leading to shortening of the sea ice season in the region do have an important impact particularly in shaping the dominance of the main phytoplankton functional groups in the region.

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