Primary productivity in the northern Indian Ocean: role of nutrients stoichiometry

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 rate of primary production is dependent on several factors such as light and nutrients availability, but clear mechanistic controls on this process remain elusive. Generally, primary production is sustained by a continuous supply of nutrients like nitrogen (N) and phosphorus (P). The molar ratio of ambient inorganic nutrients or stoichiometry (N:P) is supposed to have fixed values, which is Redfield ratio (6:1). However, the observed stoichiometry has been shown to considerably vary from the Redfield values and plays a significant role in affecting PP and changes in phytoplankton ecology in the ocean. The aim of this study was to examine the effects of nutrients stoichiometry (N: P) on the PP. Within this context, a series of manipulation experiments by adding nutrients in different ratios (N: P) at different concentrations level were conducted in the surface waters of the Arabian Sea and the Bay of Bengal during fall intermonsoon (Sept-Nov) 2021 using ¹³C tracer technique. In our results, PP showed the highest increase at N: P ~ 16:1 at all concentration levels in the Bay of Bengal. Whereas, in the Arabian sea, northern stations showed no difference in PP with changing stoichiometry but southern stations showed increase in PP due to increase in ratio at higher concentration level.