Nitrogen Sources for Phytoplankton in the Eastern Indian Ocean Determined From δ15N of Chlorophyll a and Divinylchlorophyll a

Nitrogen isotope analysis of chloropigments provides information on the sources of nitrogenous nutrients assimilated by phytoplankton. The abundant, ubiquitous chlorophyll a records nitrogen isotopic compositions (delta N-15) of eukaryotic phytoplankton and cyanobacteria, whereas more source-specific chloropigments, such as the divinylchlorophylls exclusively possessed by the marine picocyanobacterium Prochlorococcus, can potentially resolve isotopic variability within the phytoplankton community. In this study, we analyzed the delta N-15 of both chlorophyll a and divinylchlorophyll a isolated from suspended particulate material collected at the subsurface chlorophyll maximum (SCM) along a meridional transect at 88 degrees E in the oligotrophic eastern Indian Ocean. The nitrogen isotopic compositions of Prochlorococcus (delta N-15(PRO)) and the combined biomass of eukaryotic phytoplankton and Synechococcus (delta N-15(EU+SYN)) estimated from the delta N-15 of divinylchlorophyll a and chlorophyll a, respectively, revealed systematic variations that were not apparent from bulk isotope measurements. Whereas consistently low delta N-15(PRO) indicated reliance of Prochlorococcus on regenerated nitrogen throughout the transect, elevation in delta N-15(EU+SYN) values at several stations was interpreted to reflect assimilation of subsurface NO3- by eukaryotic phytoplankton. The delta N-15 distributions revealed subtle differences in NO3- availability at the SCM along the transect, which were consistently explained by the occurrence of mesoscale eddies in the Bay of Bengal, deepening of the mixed layer induced by a seasonal Wyrtki Jet in the equatorial region, and substantial deepening of the nutricline in the South Indian Ocean gyre. Our results highlight the utility of compound-specific isotopic measurements of multiple species of chlorophylls in obtaining essential non-incubation-based biogeochemical constraints on the primary production in the ocean.