Recurrent jellyfish blooms and their impacts on ecosystem deliverables of coastal habitats have become a major ecological concern. In view of this, repercussions of a surge in the jellyfish population on the plankton community were studied in Cochin estuary (CE), the largest tropical estuary along the southwest coast of India. Evaluation of hydrographic attributes and plankton community of the CE during early and late pre-monsoon revealed a marked disparity in its hydrography which favoured an increase in jellyfish abundances during late pre-monsoon, eliciting distinct impacts on the plankton community. The escalation in the jellyfish abundance and their subsequent predation on the crustacean plankton released the phytoplankton community from the grazing pressure resulting in a trophic cascade in the planktonic food web. The indiscriminate feeding of jellyfishes on the ichthyoplankton, decapod larvae, and Copepoda, the primary diet component of forage fishes evoked a potent threat to the fishery potential of CE.

The influence of distinct tidal characteristics and nutrient status on mesozooplankton community was studied in six major estuaries along the west coast of India during the late-monsoon (MS) and post-monsoon (PM) periods. The macro-tidal estuaries in the north (Amba and Thane) exhibited higher nutrient concentration compared to the micro- and meso-tidal estuaries located in the south (Cochin and Nethravati) and central (Zuari and Mandovi) west coast of India. The markedly higher nitrate and phosphate levels in the macro-tidal estuaries during PM indicated anthropogenic contributions from domestic and industrial effluents, which significantly impacted the mesozooplankton community structure. Nutrient enrichments favored higher phytoplankton standing stock leading to low DO levels. In the micro- and meso-tidal estuaries, meso- and euryhaline copepods dominated whereas in the macro-tidal estuaries, the copepod community was dominated by euryhaline and coastal species. Furthermore, the high-saline eutrophic environment of macro-tidal estuaries formed congenial for the increased jellyfish preponderance during PM. The predation pressure exerted by the jellyfish population on the crustacean zooplankton and ichthyoplankton exerted an adverse impact on the potential fishery stock in the macro-tidal estuaries. Thus, the study reveals that the nutrient enrichment favoring a shift in the mesozooplankton community structure from nutritionally superior crustacean plankton to less desirable jellyfishes, which in turn, may lead to a threat on the estuarine pelagic energy transfer and ecosystem deliverables.