Phycocyanin-Derived Assessment of Cyanobacteria Abundance in Relation to Nutrient Load and Physicochemical Data in the Butuanon River, Cebu, Philippines

Cyanobacteria (blue-green algae) are major primary producers in freshwater ecosystems, but excessive growth can trigger harmful algal blooms (HABs) that degrade water quality and threaten public health. This study assessed cyanobacterial abundance in relation to nutrients and physicochemical conditions in the Butuanon River, Cebu, Philippines. Fourteen sites from upstream to downstream were sampled during the northeast (NEM) and southwest monsoons (SWM). Nutrient concentrations (nitrogen and phosphorus) were analyzed spectrophotometrically, while cyanobacterial abundance was estimated through in situ phycocyanin (BGA-PC) fluorometry. The highest BGA-PC concentration was observed at S07 (394,594 cells/mL) during SWM, while the lowest occurred at S01 (43,374 cells/mL) during NEM. Water quality declined progressively along the river continuum. Correlation analysis during SWM indicated weak positive relationships between BGA-PC and temperature (r = 0.27) and total suspended solids (r = 0.33). In contrast, NEM correlations with temperature, pH, dissolved oxygen, conductivity, and total dissolved solids were negligible (r = − 0.09 to 0.06). Nutrient associations during SWM showed a moderate positive correlation with phosphates (r = 0.48) and a strong positive correlation with ammonia (r = 0.72), while nutrient correlations during NEM were weak to slightly negative (r ≈ − 0.05 to − 0.15). These results suggest that cyanobacterial abundance is more strongly influenced by nutrient enrichment and warmer, more turbid conditions during SWM, whereas environmental factors exert limited influence during NEM. Continuous BGA-PC monitoring can support early detection and management of HAB risks in tropical urban rivers.