Influence of dynamic stability on chlorophyll distribution along the eastern Arabian Sea

Dynamic stability (Brunt-Vaisala frequency) has marked control on the formation of deep chlorophyll maximum (DCM) layers when it occurs in shallow euphotic zone. In this work, the monthly timeseries hydrographic data collected during December 2017 to January 2019 were used to investigate the relationship between vertical structure of dynamic stability and chlorophyll concentration. Generally, on an annual scale it was found that in terms of dynamic stability of the waters, the northern region was always unstable or neutrally stable while the southern region was relatively stable. During winter, the entire coastal region presented unstable to neutrally stable (N-2 similar to 0-5 cph) conditions while in the offshore it was observed highly stable layers starting from the surface to 100 m depth in the south and unstable waters in the north up to 100 m. Consequently, very significant DCM extends from south to north with gradual rise from about 50 m at south to surface that eventually merged with high productive waters of winter bloom in the offshore along north. During summer, the stability concentration with N-2 > 15 cph is shallow (<20 m) near the coast while it deepened to more than 50 m with about an order of magnitude less (<5 cph) in the offshore. This has resulted in high chlorophyll concentration (>5 mg/ m(3)) at shallow regions near the coast while it was absent in the offshore where Deep Stability Maximum (DSM) deepened to sub-thermocline region. The mean structures of dynamic stability and chlorophyll profiles during summer and winter indicated that DCM is associated with the DSM, which was further confirmed by statistical correlation. Vertical structure of dynamic stability apparently controlled the depth of the DCM and its chlorophyll concentration though it is not tightly coupled. It may be due to influence of light and nutrients that may have stronger control on productivity. This study was useful to assess the phytoplankton biomass and productivity potential of the eastern Arabian Sea using basin scale approach.