Monsoon mysteries: Impact of rainfall-induced hydrographic changes on coastal zooplankton communities

In tropical coastal ecosystems, monsoonal rainfall is a critical driver of seasonal variability, inducing shifts in hydrological parameters such as temperature, salinity, and nutrient concentrations, which subsequently regulate zooplankton community structure. This study investigated the influence of the southwest monsoon on coastal water quality and zooplankton composition along the Mumbai coast, India. Sampling was conducted in the pre-monsoon (May 2022) and post-monsoon (September 2022) periods, corresponding to early (EMP) and later (LMP) monsoonal phases, during which the region receives 1500-2000 mm of precipitation. Analyses revealed distinct hydrological changes and a significant reorganisation of the zooplankton assemblage between phases. Total zooplankton abundance was lower in the EMP compared to the LMP. Among the 25 taxonomic groups identified, Copepoda dominated the community, represented by 27 species (23 Calanoida, 2 Harpacticoida, and 2 Cyclopoida), with families Acartidae and Paracalanidae comprising 70 % of the copepod population. Statistical and biological trait-based analyses demonstrated significant temporal shifts in copepod community composition and functional structure. Temperature, salinity, and nutrient levels were identified as the primary environmental factors shaping the distribution of copepod functional groups. Eleven copepod taxa served as indicator species for specific monsoonal phases. Trophic structure analysis showed a prevalence of omnivorous copepods during the EMP, likely due to reduced phytoplankton availability under lower nutrient concentrations and fluctuating salinity, favouring opportunistic feeding. In contrast, the LMP, characterised by elevated nutrient inputs and stabilised salinity, supported a more complex food web with a balanced representation of herbivorous, omnivorous, carnivorous, and detritivorous functional groups. Herbivores were numerically dominant in both periods. The increased co-dominance of meroplankton (pelagic larvae of benthic invertebrates) during the LMP highlighted their ecological role in coupling benthic and pelagic subsystems. These findings elucidate the mechanistic links between monsoonal hydrography and zooplankton functional ecology, providing a scientific basis for informed conservation and management of monsoon-influenced coastal ecosystems.