This study focused on the use of dominant phytoplankton species as bioindicators to assess water quality in reservoirs at a university by integrating physical, chemical, and biological parameters. The Applied Algal Research Laboratory-Phytoplankton (AARL-PP) score was used to classify the water quality from oligotrophic (clean) to hypereutrophic (highly polluted). Before a monitored event, the water quality was moderately polluted, with Phacus and Scenedesmus species indicating stable ecosystems. After the event, water quality deteriorated, with nutrient levels (NH3-N and SRP) rising significantly, leading to an increase in pollution-tolerant species like Oscillatoria are often linked to harmful algal blooms. These shifts, driven by nutrient influx from human activities, show the rapid response of phytoplankton to environmental changes. This study demonstrates the reliability of the AARL-PP score as a water quality monitoring tool and emphasizes the need for ongoing monitoring to manage the impacts of anthropogenic pollution and preserve the long-term water quality. These results provide critical insights into future water resource management and policy development.