Spatio-Temporal Trophic Variability and Environmental Drivers of Chlorophyll-a in a Large Freshwater Ramsar Lake, Loktak Lake, India

Chlorophyll a (Chl-a) indicates the trophic state in an aquatic ecosystem, where elevated concentrations often signify algal blooms associated with eutrophication. This study aimed at assessing the influence of water quality parameters on Chl-a in Loktak Lake using Generalized Additive Modeling (GAM) and characterizing spatio-temporal trophic variations using the Carlson Trophic State Index (CTSI) and trophic level index (TLI) while identifying the principal environmental drivers of TLI through Multiple Linear Regression (MLR). Seasonal nutrient dynamics revealed that PO43− and total phosphorus (TP) concentrations followed a similar pattern, with the highest levels during the monsoon and the lowest during the post-monsoon season, indicating eutrophic conditions in the lake. Elevated nutrient levels were accompanied by higher biological productivity, reflected in increased Chl-a and gross primary productivity (GPP). The overall mean CTSI (59.90 ± 6.75) and TLI (4.43 ± 0.48) indicated meso-eutrophic to eutrophic conditions. Seasonal eutrophication was evident during the monsoon and post-monsoon periods (52.7–76.25). One-way ANOVA and PERMANOVA confirmed significant heterogeneity in seasons and insignificant spatial variation in both indices. The Water Quality Index in the Loktak Lake during monsoon season was found “very poor”, whereas it was assessed “poor” and “good” during post-monsoon and pre-monsoon seasons respectively. GAM results identified TP and pH as key determinants of Chl-a variability. The MLR model demonstrated that TP, temperature, total nitrogen, Chl-a, and transparency significantly influenced trophic dynamics in the Loktak Lake.