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

<jats:title>Abstract</jats:title><jats:p>Nitrogen isotope analysis of chloropigments provides information on the sources of nitrogenous nutrients assimilated by phytoplankton. The abundant, ubiquitous chlorophyll <jats:italic>a</jats:italic> records nitrogen isotopic compositions (δ<jats:sup>15</jats:sup>N) of eukaryotic phytoplankton and cyanobacteria, whereas more source‐specific chloropigments, such as the divinylchlorophylls exclusively possessed by the marine picocyanobacterium <jats:italic>Prochlorococcus</jats:italic>, can potentially resolve isotopic variability within the phytoplankton community. In this study, we analyzed the δ<jats:sup>15</jats:sup>N of both chlorophyll <jats:italic>a</jats:italic> and divinylchlorophyll <jats:italic>a</jats:italic> isolated from suspended particulate material collected at the subsurface chlorophyll maximum (SCM) along a meridional transect at 88°E in the oligotrophic eastern Indian Ocean. The nitrogen isotopic compositions of <jats:italic>Prochlorococcus</jats:italic> (δ<jats:sup>15</jats:sup>N<jats:sub>PRO</jats:sub>) and the combined biomass of eukaryotic phytoplankton and <jats:italic>Synechococcus</jats:italic> (δ<jats:sup>15</jats:sup>N<jats:sub>EU+SYN</jats:sub>) estimated from the δ<jats:sup>15</jats:sup>N of divinylchlorophyll <jats:italic>a</jats:italic> and chlorophyll <jats:italic>a</jats:italic>, respectively, revealed systematic variations that were not apparent from bulk isotope measurements. Whereas consistently low δ<jats:sup>15</jats:sup>N<jats:sub>PRO</jats:sub> indicated reliance of <jats:italic>Prochlorococcus</jats:italic> on regenerated nitrogen throughout the transect, elevation in δ<jats:sup>15</jats:sup>N<jats:sub>EU+SYN</jats:sub> values at several stations was interpreted to reflect assimilation of subsurface NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> by eukaryotic phytoplankton. The δ<jats:sup>15</jats:sup>N distributions revealed subtle differences in NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> 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.</jats:p>