Water quality response to monsoon dynamics and management interventions in a Philippine tropical lake

Duka MA, Tamayo LVQ, Casim NCI, De Los Reyes RB, Buela LCM, Yokoyama K. 2026. Water quality response to monsoon dynamics and management interventions in a Philippine tropical lake. Reserv. Manag. XXX–XXX.Sampaloc Lake, a small tropical crater lake in the Philippines, has long faced water quality degradation linked to unregulated aquaculture and rapid urban growth. This study assessed lake stratification and water quality dynamics from 2022 to 2024 using vertical profiles and practical indices such as the Schmidt stability index (SSI), trophic state index (TSI), and euphotic zone depth (Zeu). Seasonal patterns showed that mixing during the northeast monsoon (NEM) was associated with higher productivity and reduced water clarity, while stronger stratification during the southwest monsoon (SWM) limited nutrient exchange and improved transparency. To evaluate the longer term effects of policy interventions that followed the lake’s 2014 designation as a “threatened lake,” surface water quality data from 2006 to 2021 were also examined. Post-2014 records showed reduced phosphate and chlorophyll-a concentrations, along with declining water quality index (WQI) values, indicating some improvements in trophic status. However, low dissolved oxygen levels persisted in recent years, suggesting that internal loading and hypoxia risks remain. These results highlight the value of sustained monitoring using integrative indicators to track lake responses over time. The findings may help guide rehabilitation planning in similar tropical lake systems where seasonal mixing and legacy nutrient sources continue to affect water quality outcomes.