Temporal and spatial variation in environmental parameters and phytoplankton community structure in a semi-enclosed estuarine system in the Philippines

Shifting seasonal physicochemical parameters and contributions to nutrient loadings from natural and anthropogenic sources may influence phytoplankton community composition in a semi-enclosed embayment. In this study, three sampling campaigns were conducted to assess key environmental variables in Sorsogon Bay, a multi-use coastal zone that has previously experienced harmful algal bloom events: Wet 2022 (September), Dry 2023 (March), and Wet 2023 (September). Temporal variation was observed in both environmental conditions and phytoplankton community structure. Diatoms dominated the phytoplankton assemblage across three sampling periods, while dinoflagellates were relatively more abundant during the wet season campaigns. Environmental parameters were associated with variation in phytoplankton composition at both the community and individual taxon levels. Higher community heterogeneity was observed in both Wet 2022 and Wet 2023, likely reflecting enhanced nutrient concentrations from terrestrial run-off associated with increased precipitation. In contrast, Dry 2023 assemblage was largely dominated by Skeletonema , with population abundance correlated to light intensity and ammonium concentrations in the surface seawater. Spatial differences in environmental variables and phytoplankton community compositions were also detected, although temporal variation had a stronger influence on both parameters. Dissolved metal concentrations were not reflected in the intracellular metal composition of the phytoplankton community. This study provides new insight into the temporal phytoplankton community structure of a semi-enclosed estuary and its relationship with prevailing environmental conditions, with implications for future modeling of phytoplankton dynamics and HAB development.