Microbial food web and biogeochemistry of waters of the Patagonian shelf, Drake Passage and Antarctic Peninsula in post-bloom conditions

ALDER VA; FRANZOSI C; GOMEZ M. I; SANTOFERRARA L; CENTURIÓN ARAUJO P.; OLGUÍN SALINAS HF; LARA R.; KATTNER G; LIZARRALDE Z

Génova ,Italia

Simposio; CAML Final Symposium; 2009

Census of Antaritic Marine Life

The Antarctic Circumpolar Current (ACC) and the Polar Front (PF) are among the most significant oceanographic phenomena on a global scale, as they represent the connection between the Atlantic, the Indian and the Pacific oceans, as well as the separation of the high-nutrient, low-chlorophyll (HNLC) waters of the antarctic regime from the low-silicate, high-nitrate, low-chlorophyll (LSHNLC) waters of the subantarctic one. Upon running through the Drake Passage –the major chokepoint of the AAC–, waters enter the Patagonian shelf via the Malvinas/Falkland Current. Although this system exhibits a particularly high seasonality and production, up to date the structure of its food web at the first levels is practically unknown. In this framework, and for first time, the present study points to characterizing the autotrophic and heterotrophic microbial communities inhabiting the waters of the Patagonian shelf, Drake Passage and Antarctic Peninsula as to their structure, abundance, relative contribution to total C biomass, and relationships with isotopic signatures and biogeochemical conditions, as well as to assessing the ecosystemic role of the Patagonian shelf area in the production of carbon and the biogeochemical dynamics of the region. To this purpose, a multidisciplinary survey was carried out on 38 sub-surface (9 m) oceanographic stations performed by the icebreaker Alte. Irizar during austral autumn 2005, covering a mesoescale latitudinal transect from 38°S to 68°S. Two main series of samples were collected at each station in order to evaluate: 1) Cell density and carbon contribution of prokaryotes (cyanobacteria and heterotrophic bacteria), unicellular eukaryotes (nanoflagellates, dinoflagellates, picoeukaryotic algae, diatoms, silicoflagellates, ciliates, forams, radiolarians), and meroplanktonic larvae. 2) The size structure and functional role of nanoflagellates, dinoflagellates and aloricate ciliates. 3) Concentration of chlorophyll a, b and c, nutrients (nitrate, silicate, phosphate, ammonium), Total Nitrogen (TN), PON, DON, Total Organic Carbon (TOC), POC, DOC, and ∂13C and ∂15N isotopes. The results yielded by the above analyses indicate that, regardless of the hydrologic and biochemical features of the areas under study, the relative contribution of the autotrophic and heterotrophic compartments to the density and biomass of the microbial community during autumn is practically the same for Patagonian, Drake Passage and Antarctic Peninsula waters, with a clear dominance of the heterotrophic components (mainly dinoflagellates and bacteria), and with biomass increments of autotrophic protists only at three isolated sites. The 46-47°S latitude represents the boundary between two different areas in terms of regenerated and new production: North of this latitude, nutrient and POC concentrations are low and high, respectively, with a dominance of reduced N forms (ammonium and DON); southward, instead, N prevails as nitrate while there is a 64-fold increase in silicate. The latitudinal changes in both the microbial community and the biogeochemical parameters analyzed reveal the occurrence of four major zones, one located south of the PF, one in the northern Drake Passage, and two located on the Patagonian shelf (northern and southern sectors). Among these, the southern Patagonian zone (46-55°S) stands out for its highest and permanent contribution of cyanobacteria to total microbial biomass (50%).