Reservoirs are vital freshwater ecosystems that support irrigation, hydropower, fisheries, and biodiversity, but they are increasingly threatened by anthropogenic pressures and climate-induced stress. Conventional physico -chemical monitoring provides only short-term snapshots of water quality, often failing to capture ecological integrity. Biotic indicators such as fish, plankton, and macrophytes , on the other hand, integrate environmental changes over longer timescales and serve as reliable measures of ecosystem health. The present study applied a multi-metric index (MMI) framework to assess the ecological health of Poondi Reservoir, a medium-sized tropical reservoir in Southern India, by combining abiotic, biotic, and disturbance indicators across spatial (lentic , transitional, riverine) and temporal (pre-monsoon, monsoon, post-monsoon, summer) scales.
The assessment revealed significant spatio -temporal variation in ecosystem health. Fish assemblage scores ranged from 35.7 in summer to 80.0 in the monsoon, highlighting seasonal bottlenecks driven by hydrological stress and habitat connectivity. Fish production was highest in the lacustrine zone (73.3) and lowest in the riverine zone (40.0), reflecting differences in water depth and disturbance levels. Plankton indices averaged 62.9, with post-monsoon nutrient inputs enhancing productivity, while macrophyte scores varied widely (30–100), indicating both habitat value and eutrophication risk. Abiotic parameters recorded the highest average (65.7), reflecting generally good water quality, whereas disturbance scores (62.9) pointed to moderate human pressures from agriculture, domestic waste, and shoreline encroachment. Overall, the riverine zone maintained the best ecological integrity (67.3%), while the transitional zone and summer season emerged as the most stressed, with cumulative indices of 56.9.
These findings demonstrate the utility of integrative, multi-metric tools in capturing ecological health dynamics of reservoirs, especially in tropical systems where seasonal fluctuations strongly influence ecosystem function. The results underscore the need for targeted management interventions, including maintaining minimum summer water levels to reduce hypoxia, restoring riparian vegetation buffers to limit nutrient influx, and promoting community-based monitoring to address localized pressures. The study concludes that Poondi Reservoir is currently in “good” ecological condition but remains vulnerable to seasonal and spatial stressors. The multi-metric framework applied here provides a robust, evidence-based approach for reservoir health assessment and offers valuable guidance for sustainable fisheries, biodiversity conservation, and adaptive management under changing climatic and anthropogenic pressures.
Keywords: Reservoir health, multi-metric index, fish assemblage, plankton, disturbance, sustainable management