Nutrient dynamics and budgeting in a semi-enclosed coastal hypersaline lagoon.

Biogeochemical and ecological responses to limited external nutrient loading are poorly understood in tropical semi-enclosed coastal lagoons which are highly influenced by hydrological and salinity regimes. With objectives towards ecosystem sustainability via better management of the nutrient inputs, investigations were carried out to estimate the water, salt, and nutrient budget of "Pulicat" hypersaline coastal lagoon for the year 2018-2019. The budget revealed that the annual rate of precipitation and evaporation are the major driving factors regulating the annual residual flow in the lagoon. Limited exchange of water and material had resulted in a hypersaline condition with high spatial and temporal variation in salinity ranging from 20 to 103. In the absence of external loading, nutrient enrichment by internal compensation had resulted in DIN enrichment. DIN constituents are mainly contributed by ammonia, indicative of remineralisation through benthic regeneration. The extended water residence time and enhanced primary production has converted the inner lagoon into a limited phosphate system. The TRIX index also indicates a decline in the trophic status transforming the lagoon from a mesotrophic to a eutrophic system.

Hydrodynamic variability and nutrient status structuring the mesozooplankton community of the estuaries along the west coast of India.

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.