[A-OS12] Physical, biogeochemical, and ecological processes and variability in the Indian Ocean

[AOS12-01] Determination of nitrogen source for phytoplankton in the eastern Indian Ocean by δ¹⁵N of chlorophyll a and divinylchlorophyll a

*Yuta Isaji¹, Chisato Yoshikawa¹, Nanako O. Ogawa¹, Kazuhiko Matsumoto¹, Naoto F. Ishikawa¹, Akiko Makabe¹, Hiroshi Ogawa², Hiroaki Saito², Makio Honda¹, Naohiko Ohkouchi¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Atmosphere and Ocean Research Institute, University of Tokyo)

Keywords:Indian Ocean, primary production, nitrogen isotope, Prochlorococcus, divinylchlorophyll

The eastern Indian Ocean is unique for its meridional gradients in physical and chemical properties with distinct biogeochemical regimes. Constraining the biogeochemical parameters in each of these regions is critical as the Indian Ocean represents 15–20% of global ocean net primary production (Behrenfeld and Falkowski, 1997). The nitrogen source assimilated by phytoplankton is of particular importance, because new production is supported by nitrate supplied from the subsurface ocean or by N₂ fixation, and regenerated production by ammonium and other reduced nitrogen species remineralized within the photic zone. Here, we conducted compound-specific nitrogen isotope analysis of chloropigments in order to determine nitrogen source assimilated by phytoplankton, and to obtain detailed snapshot of primary production in the eastern Indian Ocean. Suspended particulate materials were collected on GF-75 glass-fiber filter on board at subsurface chlorophyll maximum along 88°E transect in the eastern Indian Ocean (16°N–20°S) during the cruise KH-18-6 Leg 2. The depth of subsurface chlorophyll maximum was shallowest at the northernmost station and deepened southward, with chlorophyll a concentration showing decreasing trend toward southern stations. Compositions of chlorophylls (i.e., chlorophyll a, b, c, and divinylchlorophyll a) and carotenoids (i.e., fucoxanthin, 19’-hexanoyloxyfucoxanthin, 19’-butanoyloxyfucoxanthin, prasinoxanthin, zeaxanthin) indicated predominance of eukaryotic phytoplankton in the northern and equatorial stations. The proportion of Prochlorococcus increased in the southern stations in response to deepening of the nutricline. The δ¹⁵N values of eukaryotic phytoplankton and Prochlorococcus were estimated by those of chlorophyll a and divinylchlorophyll a, respectively. We will present meridional variation in the nitrogen source assimilated by each group of phytoplankton, and discuss its possible causes as well as implications of our results.