Magnitude of river discharge determines the food habit and community composition of zooplankton in the Indian estuaries during monsoon

Zooplankton of seventeen Indian estuaries were investigated using plankton net (200-μM mesh; 0.25 m2 mouth area) during the peak discharge period of 2011. Results showed that the seventeen estuaries are comparable in terms of area, and volume of river runoff. Indian estuaries differ by their level of river runoff (highest in the Godavari), suspended matter concentration (highest in the Haldia), and potential phytoplankton biomass (Chlorophyll-a) (highest in the Kaveri). Relatively higher zooplankton biovolume was observed in the estuaries which receive low discharge, than in estuaries with high discharge. Zooplankton abundance was numerically dominated by calanoids, constituting approximately 69% of the total zooplankton count. Community composition in the Northern Indian Estuaries (NIE) was dominated by Acartiella spp., and Diaptomus spp. On the other hand, taxa such as Acartia spp., Acrocalanus sp., and Paracalanus spp. were predominant in the Southern Indian Estuaries (SIE). Zooplankton community structure was driven by terrestrial organic carbon and river discharge in the NIE. Whereas both salinity and phytoplankton biomass (Chl-a) played a crucial role in the SIE. Therefore, this study suggests that Indian estuaries were driven by the magnitude of river discharge, resulting in variations in hydrological, chemical, and biological properties, which in turn amends the zooplankton community composition and food habits.