Species composition and biogeography of diatoms in antarctic and subantarctic waters (37-78 °S)

OLGUIN SALINAS HF; ALDER V.A

Génova, Italia

Simposio; CAML Final Symposium,; 2009

Census of Antarctic Marine Life

Diatoms are among the most important single cells of the oceans because of their role in carbon fixation and exportation from surface waters, and as a tool for paleoenvironmental reconstruction. While the specific composition and abundance trends of antarctic diatoms are relatively well known, information on adjacent subantartic geographical areas is still insufficient, thus impeding to assess both the degree of interconnection between these regions and to what extent the Polar Front implies a biogeographic barrier for these organisms. During a cruise conducted on board the icebreaker Almirante Irizar during austral summer 2002, we performed 60 oceanographic stations over a mesoscale latitudinal transect covering from Argentine shelf waters to the Antarctic Peninsula and the Weddell Sea to study the specific composition of diatoms as well as the abundance, biomass and spatial distribution of each species in relation with nutrients, temperature and salinity. Identifications were made following the oxidation of part of the sample and its mounting onto glass slides using Naphrax. The density of diatoms was estimated by cell counting under an inverted microscope. Biomass values were obtained by first calculating biovolumes and subsequently converting them to carbon content. Hierarchical analyses were performed in order to identify floristic associations. A total of 168 species grouped in 61 genera were recorded for the entire area under study, with Chaetoceros and Thalassiosira being the genera best represented. The number of species per station ranged between 46 (Argentine shelf waters) and 21 (Antarctic Peninsula). While most of the species showed very low densities, only a few were numerically dominant: Chaetoceros tortissimus and Chaetoceros dichaeta represented ca. 50% of the total of counted cells (37 and 13%, respectively), followed by Fragilariopsis curta (9%), Chaetoceros socialis (4%), and Pseudo-nitzschia lineola (4%). A different trend was observed in terms of biomass, as 50% of the estimated total carbon content corresponded to Thalassiosira tumida and Coscinodiscus bouvet (18 and 10%, respectively), followed by C. tortissimus (8.3%), Porosira glacialis (7.4%) and Rhizosolenia antarctica (5%). Hierarchical analyses allowed differentiating four sectors within subantarctic shelf waters, two within the Drake Passage (divided by the Polar Front), and four within antarctic waters (Bransfield Strait, northern Antarctic Peninsula, northern Weddell Sea -encompassing the Weddell-Scotia Confluence-, and central/southern Weddell Sea). In addition, some isolated stations located in the vicinity of different islands within the antarctic region (e.g., South Shetland Islands, South Orkney Islands) evinced a weak relationship with the above mentioned sectors from the floristic point of view. The major change in specific composition was found in association with the Polar Front, but the latter does not seem to represent a strong biogeographic barrier to antarctic diatoms, as >43% of total species was found to inhabit both antarctic and subantarctic waters, with some of them even prevailing up to 44°S, in the Argentine Sea. North of this latitude the diatom community is composed by a majority of warm water species mixed with a few cold water taxa, thus denoting the influence of waters derived from the Antarctic Circumpolar Current (which enter the southern shelf via the Malvinas/Falkland Current) and the Brazil Current, respectively. Whether shelf species are the result of an expatriation process from their natural (antarctic or tropical) sources due to current transportation, or have an ample distribution range, is still under analysis. The study will also consider the relationships between the distribution trends of key diatom species vs. environmental conditions, and the calculation of biodiversity by applying the Shannon Wienner index.