Role of Macro- and Micro-nutrients in Carbon and Nitrogen Fixation across the Northern Indian Ocean

Dinitrogen (N2) fixation and carbon fixation are fundamentally essential processes in mediating production dynamics of the marine ecosystem, but clear mechanistic controls on these processes remain elusive. Also, primary production (PP) is the basis for marine food web, which sustains life in the ocean through photosynthesis by removing carbon dioxide from the atmosphere. The rates of these processes depend on several factors such as light and nutrients availability. Generally, PP and N2 fixation are sustained by a continuous supply of macro- (N, P) and micro - (such as Fe, Mo) nutrients. Previous studies from the northern Indian Ocean showed that phytoplankton growth in likely limited by Fe availability. The aim of this study was to examine the effects of these nutrients on the PP and N2 fixation. Within this context, a series of manipulation experiments by adding macro- (N, P) and micro-nutrients (Fe, Mo) were conducted in the surface waters of the Arabian Sea and the Bay of Bengal during fall intermonsoon (Sept-Nov) 2021 using 13C and 15N tracer techniques. In our results, except the Andaman Sea, no notable effect of nutrients addition has been observed on PP in the Bay of Bengal. In the Arabian Sea, separate nitrogen enrichment (as NO3-) stimulated PP but P, Fe, and Mo enrichment failed to stimulate it, which corroborate earlier studies that N can limit PP in this region. Also, the combination of nutrients (NMo, PMo, NPMo) increased PP in the Arabian Sea. In the experiments, Fe did not stimulate N2 fixation, while combined NFe showed highest increased in N2 fixation in the Bay of Bengal. Also, the combination of Fe with P and N led to increase while separate nutrients showed little effect on N2 fixation in the Arabian Sea, which likely indicated co-limitation of these nutrients.