Decoupling Algal Biomass and Microbial Attenuation in Wastewater-Fed Fisheries: A Spatiotemporal Analysis in an Indian Ramsar Site.

Although integrated wastewater-fed fisheries in the Ramsar-designated East Kolkata Wetlands (EKW) provide important ecosystem services, the long-term spatial and temporal dynamics of microbial attenuation in these managed pond systems remain insufficiently characterized. This study assessed the spatiotemporal dynamics of chlorophyll-a concentration and culturable bacterial abundance along a canal-to-pond transition system over 22 months. Water samples were collected from four fixed locations during 22 sampling months between September 2023 and August 2025 to determine total viable count (TVC), chlorophyll-a concentration, and chlorophyll-a-based trophic state index (TSI). TVC decreased by 79.86%, from a geometric mean of 22,409 colony-forming units (CFU/mL) at the canal source to 4513 CFU/mL at the distal pond site, and differed significantly among sampling locations (p < 0.0001). TVC was also significantly higher during the dry season than during the wet season (Mann-Whitney U = 1244.5, p = 0.017). Chlorophyll-a concentrations did not differ significantly among sampling locations (p = 0.078), although the mean concentration was 82.8% higher across the three pond stations than at the canal source. Mean TSI values ranged from 47.1 to 52.3 across the sampled gradient. Predictive modeling with Analysis of covariance (ANCOVA) showed that chlorophyll-a was not a significant predictor of log10-transformed TVC after accounting for sampling location (F = 0.48, p = 0.491). These results suggest that site-associated differences, rather than a statistically detectable chlorophyll-a effect, are associated with microbial decline within the sampled gradient. Because other physical, chemical, hydraulic, and biological factors were not measured, the observed spatial association cannot be attributed to location alone.