13C and 15N Natural Abundance in Southern Ocean Biota: An Investigation of Biogeochemistry and Trophic Structure

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The analysis of stable isotope ratios in organisms and in the environment is an important method for investigating the structure of food webs and the nature of biogeochemical pathways. Plankton in the Southern Ocean adjacent to Antarctica have the lowest 13C/12C ratios yet found for pelagic organisms. Despite some previous research this depletion of 13C in Antarctic plankton has not been satisfactorily explained. The answer to this question is important because this isotopic anomaly is an unambiguous indication that carbon biogeochemistry in the Southern Ocean must in some way be markedly different from the rest of the present ocean. The 13C depletion in Antarctic plankton could also have a direct bearing on the paleoceanographic interpretation of similar depletions found in the marine sedimentary record. Rau and his collaborators propose to measure 13C/12C and 15N/14N ratios in biota and particulate organic matter in the southern Atlantic Ocean, Drake Passage, Scotia Sea and Weddell Sea. The carbon isotopic measurements will address the problems mentioned above; the nitrogen isotopic measurements will provide further information on the nature of the food web and will facilitate the testing of hypotheses concerning the causes of the variations in isotopic abundances. The results of this project will directly complement the work of other scientists in the ongoing Antarctic Marine Ecosystem Research in the Ice Edge Zone (AMERIEZ) project and address an area of research recommended for special attention by the Polar Research Board.