The majority of the yearly organic carbon production int he coastal regions of the Southern ocean occurs during summer blooms. Changes in the quantity and quality of the phytoplankton standing crop during this period will likely impact organisms that occupy higher trophic levels within the ecosystem. Here we report a recurrent shift in the dominant phytoplankton taxa during the austral summer in a coastal region along the Antarctic Peninsula. The pattern was observed each year between 1991 and 1996 despite radically different biomass concentrations and meteorological and hydrographic conditions. We show that a repeated dominance of cryptophytes (Cryptophyceae) during the austral summer was significantly related (MANOVA; P << 0.001) to decreased salinities during periods of glacial melting. The transition from a system traditionally dominated by diatoms to one dominated by cryptophytes represents a fundamental decrease in the average size of water column phytoplankton. This is reason for concern, as smaller size classes are not efficiently grazed by Euphasia superba. Higher proportions of cryptophyte biomass during the critical summer months may alter zooplankton assemblages and reduce carbon available to higher trophic levels by as much as 70%. Mean air temperatures along the Antarctic Peninsula have increased significantly (2-3 C) over the past 50 years. Warmer weather will likely extend the spatial and temporal influences of glacial meltwater and may increase the importance of cryptophytes, with significant consequences for Antarctic food wed dynamics and coastal biogeochemistry.



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