Asian Journal of Geographical Research

Tropical reservoir ecosystems exhibit distinct longitudinal gradients that influence biological communities, yet a significant data gap exists regarding phytoplankton zonation in Mindanao’s inland waters. This study evaluated the ecological status of Lumbayao Dam by analyzing longitudinal variations in phytoplankton and physicochemical parameters to establish an environmental baseline. Utilizing a multi-parametric approach across riverine (RZ), transitional (TZ), and lacustrine (LZ) zones, water quality is assessed through in-situ physicochemical measurements and laboratory BOD analysis, while phytoplankton samples were quantified using a Sedgewick-Rafter chamber. Statistical tools such as ANOVA and Pearson Correlation were applied to determine significant zonal differences and the relationships between environmental drivers and biodiversity. A total of 12 genera belonging to 3 families were identified across the system, with the highest diversity found in the RZ with 11 genera, which then stabilized to 9 genera in both the TZ and LZ. Although all physicochemical parameters remained within DENR Class C standards, a significant thermal and alkalinity increase was noted toward the dam wall, alongside a decline in Dissolved Oxygen (DO) and a rise in Biochemical Oxygen Demand (BOD). Statistical modeling established DO and BOD as the primary environmental drivers, showing a very strong positive correlation (r = 0.928) and a very strong negative correlation (r = -0.909) with species richness, respectively. Local managers should implement Integrated Water Resources Management (IWRM) focusing on the Pulangi River watershed. Establishing a localized monitoring framework using phytoplankton as bioindicators is essential to detect organic-driven hypoxia and maintain the dam’s utility for fisheries and irrigation.