Diatoms, tintinnids, and the protist community of the western Weddell Sea in summer: latitudinal distribution and biogeographic boundaries

Abstract

Understanding the spatial and temporal distribution of marine plankton is essential to detect trends and predict their response to climate change. Seasonality, fronts, and dispersion by currents and eddies condition the distribution from micro- to macroscales. Here, we address comparatively the structure of planktonic protist communities and the assemblages of tintinnid and diatom species in subsurface waters of the western Weddell Sea during February 2005. We analyze to which extent the biogeographic patterns based on different hierarchical levels and ecological variables reveal spatial similarities, and how these patterns reflect frontal processes. Abundance and carbon biomass were estimated for eight groups of protists and for diatom and tintinnid taxa in relation with chlorophyll-a and environmental variables. The Weddell Sea boosted the proliferation of flagellated cells. Diatoms showed an unclear distribution trend and, with the exception of a few species, those primarily contributing to abundance were clearly others than those primarily contributing to biomass. Biogeographic patterns differed according to hierarchical levels and the ecological variables considered. All levels explored showed an uneven distribution in most of the area except for the range of latitudes between ~67°S and 70°S, where the trends were notably homogeneous. Three rich carbon biomass patches were found, two of them in shelf waters (South Orkney Islands and Filchner Trough) and a major one in the Marginal Ice Zone. Based on a disruption of species distribution, the Antarctic Slope Front-Antarctic Slope Current system is suggested to be located at 73.35°S. A refinement of previous biogeographic schemes was provided.