Understanding critical water quality issues of a maar lake in Tropical East Asia using Longitudinal Analysis: The case of Lake Sampaloc, Luzon Is., Philippines

Tropical lakes are increasingly under threat from rapid climate change and local environmental pressures. Common foci of tropical lake studies include water quality assessments; however, the wide existing tropical lake assessments and narratives particularly in Tropical East Asia obscure critical water quality issues requiring urgent action. Our research utilizes a case study on Lake Sampaloc, a small tropical lake in Luzon Island, Philippines, to understand its critical water issues and how these impact its current lake management. We conducted a longitudinal analysis of inorganic phosphate, ammonia, nitrate, dissolved oxygen, and chlorophyll-a concentrations from 2013 to 2023 and report three major findings. First, Lake Sampaloc’s inorganic phosphate and ammonia decadal mean concentrations consistently exceed its Philippine water quality guidelines of 0.025 mg/L and 0.06 mg/L by 36 and 18 times respectively. Second, the lake’s dissolved oxygen concentration often falls below the 5.0 mg/L guideline during the Northeast monsoon months, potentially contributing to reported fish kill incidents during the same monsoon. Third, Lake Sampaloc’s inorganic phosphate and ammonia concentrations are correlated, indicating a same underlying source requiring further investigation. Our longitudinal analysis highlights the importance of identifying the most critical lake quality parameters to promote more effective lake conservation and eutrophication regulation.