Environmental gradients along the tropical coast drive plankton biomass and alter food web interactions

Traditionally, the Bay of Bengal is considered as a less productive basin when compared with the neighbouring Arabian Sea. However, limited information is available for a comparative study between these two basins on plankton with relation to environmental properties of inshore waters. In order to understand the spatial variability in plankton biomass along tropical coastal waters, the analysis was carried out by collecting samples from 8 locations, covering both east and west coasts of India during dry and wet periods during 2011. Salinity showed considerable seasonal variability along west coast of India (WCI) and east coast of India (ECI); it has highly fluctuated during wet period along ECI. Suspended solids (SSC) and nitrate showed their concentrations along ECI than WCI. Phosphate and silicate exhibited significant seasonality along WCI, whereas insignificant along ECI. Inconsistency in the monsoonal runoff, physical processes and anthropogenic and terrestrial sources seems to be the major driving forces for these parameters. Nearly 3-fold higher phytoplankton biomass was noticed along WCI than ECI due to low SSC in the former region. Though the zooplankton population also followed the same pattern as phytoplankton, the difference between WCI and ECI is lesser than phytoplankton. Zooplankton abundance showed significant relation with phytoplankton and SSC along WCI and ECI, respectively. The present study reveals irrespective of nutrient concentrations; phytoplankton was dominantly constrained by light availability followed by grazers. Moreover, mesozooplankton was supported by not only phytoplankton but also alternative carbon sources; hence, variability in phytoplankton biomass and SSC leads to alterations in trophic interactions.