The Impact of Artificial Water Diversion–induced Algal Blooms on Carbon Balance in a Semi-enclosed Bay

Artificial water diversion is widely used to address water security; yet, its impacts on phytoplankton communities and coastal carbon balance remain poorly understood. Using a seasonal diversion project in a semi-enclosed bay as a case study, we analyzed phytoplankton composition via morphological methods and assessed carbon balance through simultaneous measurements of primary production (P), ecosystem respiration rate (R), and production-to-respiration (PP/R) ratio. Our results showed that artificial water diversion activities during the wet month enhanced hydrological connectivity and phytoplankton homogeneity, triggering a mixed diatom-dinoflagellate bloom. Phytoplankton abundance during the wet month increased by sevenfold (surface layer) and 26.5-fold (bottom layer) compared to dry month values. This simultaneously resulted in the PP value of the wet month being more than twice that of the dry month. Although R rose with increasing phytoplankton abundance, no significant correlation was observed between them. Instead, dry-month R was primarily driven by pH and dissolved organic carbon, whereas wet-month R showed minimal environmental linkages. PP/R ratios of surface and bottom layers were always less than 1, implying Meishan bay was a net heterotrophic ecosystem, despite significant changes in phytoplankton community structure induced by artificial water diversion and associated algal bloom. Furthermore, our results strongly suggest that changes in PP, but not in R, control the PP/R ratio of Meishan bay. This study offers valuable guidance for the ecological management of artificial water diversions and can serve as a reference for similar water diversion projects in other semi-enclosed bays.