Seasonal similarity in rates of protistan herbivory in fjords along the Western Antarctic Peninsula

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We quantified phytoplankton growth and protistan grazing rates during late austral autumn 2013 and late austral spring 2014, in several glacio-marine fjords and connecting channels along the Western Antarctic Peninsula (WAP). During austral autumn, low and declining chlorophyll a (Chl a) concentrations (≤ 0.4 μg L−1) were almost entirely composed of pico/nanophytoplankton, whereas during austral spring, high but patchy Chl a concentrations in the fjords (up to 18.5 μg L−1) reflected a diatom bloom. These contrasting dynamics were associated with high seasonal differences in irradiance, but not temperature, and were consistent with the balance resulting from lower phytoplankton growth rates in autumn (−0.01 d−1 to 0.19 d−1) than in spring (0.06–0.93 d−1) but similar magnitudes of herbivorous grazing in both seasons. Grazing was either absent or low (0.11–0.26 d−1) and restricted to the picophytoplankton and nanophytoplankton. In the productive fjords lining the WAP, a fraction of primary production was channelled through a persistent and across-seasons equally active microbial food web, while during spring an increasing fraction of organic carbon shifted from trophic transfer and recycling to an export pathway.

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Limnology and Oceanography