Hydro-ecological modelling of a hypersaline lagoon ecosystem (Pulicat) in Southeast coast of India.

Climate change impacts the coastal lagoons and can affect various species reproductive cycles, feeding habits, and survival. Due to natural factors, the ecosystem habitat at Pulicat Lagoon on the east coast of India is under severe stress. This study assesses the ecosystem using a numerical model and field-collected physical, chemical, biological, and meteorological data for 2018 and 2019. A coupled hydro-ecological model was used to analyze circulation features, biological and chemical interactions, and transformation processes. The study indicates that high abnormalities in the lagoon ecosystem are caused by less exchange of fresh/coastal water from river sources and sea mouths due to less rainfall during the study period. The model output depicts the salinity variation from 15 to 58 PSU in different seasons of 2018 and 2019. Furthermore, high concentrations of Chl-a ranged from 50 μg/L to 95 μg/L during the monsoon and post-monsoon seasons, triggering phytoplankton C (70–350 mg/m3) and zooplankton C (9–43 mg/m3) at the Lagoon, resulting in an algal bloom. The monsoonal shift of Chlorophyte (9.52%–9.68%), Cyanobacteria (13.22%–14.95%), Diatoms (66.08%–63.87%), and Dinoflagellates (11.18%–11.51%) is observed in the biological field, with Diatom showing the highest abundance (65%). As a result, appropriate scenario-based model studies were conducted to conserve and strengthen the ecosystem during such extreme events. The model studies suggest that widening the lagoon inlet (800m) with a depth of 5 m (3 km length) increases the optimal tidal exchange to the ecosystem. The model output demonstrates the optimal exchange that normalizes the salinity from 12 to 34 PSU from the inner Lagoon to the mouth region. This study will support environmental authorities in conserving the ecosystem through proper management adaptations. • A coupled hydro-ecological model was used to analyse circulation features and biological and chemical interactions. • Observations reveal tidal variability of 0.8–1m, maximum current magnitude of 48cm/s at the southern mouth, and significant suspended and alongshore transport. • Tidal flux inside the lagoon from the mouth region spreads around 8 km, and river inflow water sources sustain the water balance during the monsoon. • Larger proportional shift/changes in phytoplankton species such as Ditylum sol, Diplonesis, Triporas furca, Surirella sp. with seasonal flow. • An optimized solution to mitigate and restore the ecosystem from extreme events by widening the lagoon inlet to 800m width and 5m depth, spanning a length of 1.8 km has been arrived at using scenario studies. This intervention aims to enhance tidal flux and exchange within the ecosystem. [ABSTRACT FROM AUTHOR]