Biomass size espectra of the phytoplankton assemblage around the South Shetland Islands (Antarctica)

Abstract

Phytoplankton biomass - size spectrum is a general descriptor of the community structure. The slope and irregularities of the spectrum depend on the physiological behaviour of the community and the trophic organization of the planktonic ecosystem. In this work, we aim to characterize the Antarctic phytoplankton assemblage in a region around the South Shetland Islands through a comprehensive analysis of normalised biomass size spectra (NBSS), biomass-anomaly size spectra (BASS) and the taxonomic affiliations of the main size ranges. During COUPLING cruise (January 2010), a total of 60 stations were sampled from the Drake Passage to the Antarctic Sound (61-65º, Antarctic Peninsula). The sampling scenario is characterized by conditions of the late austral summer, a complete sea ice melting and a possible post-bloom stage. Our sampling covered the whole phytoplankton community from pico- to micro phytoplankton. The abundance and cell size of the species present in the samples were determined using flow cytometry for picophytoplankton and small nanoplankton, and FlowCAM analysis for large nanoplankton and microphytoplankton. Cell size was converted into biomass using Mender-Deuer and Lessard (2000) equations. The slopes of the carbon-based NBSS spectra ranged from -0.95 to -1.19, reaching the lowest values in the Drake Passage sector, where the highest surface temperatures were found as well as the lowest salinities. A unique large dome generally characterized the phytoplankton size spectra because of the lack of cyanobacteria (neither Prochlorococcus nor Synechococcus-like populations) in the study area. The BASS showed a clear predominance of nanoplankton, with a positive peak around 4-6 ¿m ESD (Equivalent Spherical Diameter). A secondary positive anomaly around 30 ¿m ESD was found in the Antarctic Sound stations. This peak was dominated by diatoms such as Thalassiosira sp. In the northeast area, some stations showed positive anomalies around 50 ¿m ESD due to the presence of Corethron sp. The spatial gradient of physicochemical variables and biological conditions from the Antarctic Sound to the nothermost stations allowed us to hypothesize a sequence of events in the assembling of the Antarctic phytoplankton community after the ice melt.