Effects of summer ice coverage on phytoplankton assemblages in the Ross Sea, Antarctica

Abstract

An oceanographic cruise was conducted in the Ross Sea (Antarctica) during summer 2001 as part of the Italian National Program for Antarctic Research (PNRA). Extensive areas of pack ice occurred over the Ross Sea, atypical for summer when offshore waters are normally free of ice. The present study focuses on the effects of increased ice coverage on phytoplankton assemblages. Water samples collected at various depths at 72 hydrographical stations in offshore and coastal waters were used to determine size-fractionated phytoplankton biomass as chlorophyll a (chla) concentrations, and HPLC photosynthetic pigments. For the offshore waters, the average chla concentration was 57.8 mg m−2, approximately three times the values recorded under ice-free conditions during summer 1996. In coastal waters, the average chla concentrations were 102 and 206 mg m−2 during January and February, respectively, i.e., up to 2.5 times those of 1996. Micro- and nano-phytoplankton size fractions made up about 90% of the phytoplankton biomass over the entire study area and were composed primarily of diatoms with a pico-phytoplankton fraction dominated by prymnesiophyceans. The broken pack and melting ice was strongly coloured by an extensive algal biomass suggesting that the phytoplankton was a result of seeding from ice algal communities. The Ross Sea considered to be one of the most productive areas of the Southern Ocean, had primary production values about four-fold those of other areas. The lengthening of the ice season observed in the Western Ross Sea, associated with a considerable increase in phytoplankton biomass as observed in summer 2001, would have a major impact on the trophic structure of the entire ecosystem, and presumably, also on carbon export.