Interannual variability of sympagic communities in Terra Nova Bay (Ross Sea - Antarctica)

Sea ice surrounding the Antarctic continent varies in extent from 4 x 106 km2 in summer to 20 x 106 km2 in winter, representing one of the largest and most dynamic ecosystems on Earth. Annual primary production in the Antarctic sea ice has been estimated between 30 and 70 tg C; up to 400 mg Chla m-2 and 1g C m-2 d-1 of sea-ice autotrophic biomass and primary production, respectively, have been recorded similar to the values reported in ice-free waters of the most productive oceanic regions. Several forcing factors such as temperature, precipitation, wind and local circulation play a synergic role on the formation of different types of sea ice e.g.: annual, multiannual, platelet-ice. In turn, different structures and physical-chemical features of the sea ice strongly influence the ecology of organisms trapped by or colonising the sea ice. We investigated differences in structural and functional properties of the pack- and platelet-ice sympagic communities in Terra Nova Bay (Ross Sea) that occurred in spring-summer of 1997, 1999 and 2004. We analysed the variability in nutrient dynamics and physiological parameters of sympagic algae, phytoplankton biomass, photosynthetic pigment spectra, sea-ice meiofauna, micro- and mesozooplankton biodiversity as well as the quantity, composition and degradation rates of particulate organic matter (POM) and bacterial secondary production. Temporal changes in inorganic nutrients and DOM concentrations in the bottom ice increased contemporarily with a sharp increase in autotrophic biomass. The bottom sympagic flora was mainly represented by cryobenthic species, whilst the cryopelagic population was confined to the pack-platelet ice interface; diatoms dominated the sympagic communities. Bacterial production in the platelet layer was much higher than in the bottom layer. In addition, when the autotrophic biomass of the bottom ice reached its maximum, a dramatic drop of the bacterial production was observed, a phenomenon which did not occur in the platelet ice. The zooplankton community in the intact sea ice was largely dominated by copepods, Stephos longipes and Harpacticus furcifer, that accounted for more than 90% of the sympagic fauna. Interannual changes in POM composition and bacterial variables were limited, whereas the largest differences occurred between the two types of investigated sea ice. In contrast, notable differences in sea-ice autotrophic biomass and biodiversity occurred from one year to the other and pronounced spatial and temporal variability was observed for the zooplankton populations.