Water quality and physicochemical conditions drive chlorophyll-a concentrations in two connected subtropical off-stream reservoirs

Abstract

The assessment of reservoir water quality is essential for ecosystem preservation and sustainable water resource management. Chlorophyll-a (Chl-a) serves as a key bioindicator of phytoplankton dynamics and trophic status in aquatic ecosystems. This study applied Generalized Additive Mixed Models to investigate the environmental drivers of Chl-a variations in two interconnected subtropical off-stream reservoirs, Ren-Yi and Lan-Tan, in Taiwan. Water temperature and rainfall were identified as primary determinants of Chl-a fluctuations in both reservoirs. However, Ren-Yi Reservoir exhibited stronger external nutrient loading effects, with ammonia and total phosphorus significantly influencing Chl-a levels. These findings coincide with elevated chemical oxygen demand, total phosphate, and total nitrogen concentrations in the Ba-Zhang River, the main inflow source. In contrast, Lan-Tan Reservoir was more influenced by depth-driven variations, reflecting thermal stratification and internal nutrient cycling. Seasonal variations in Chl-a were more pronounced in Ren-Yi Reservoir due to direct riverine inputs, while Lan-Tan exhibited attenuated but noticeable seasonal fluctuations, highlighting the importance of regulatory effect of inflow connectivity. A key finding was that dataset size influenced model performance, with seasonality emerging as a crucial factor in the larger dataset (300 data points), revealing intensified seasonal variations. This underscores the need to incorporate seasonality in predictive models. The study demonstrates that external and internal factors regulate Chl-a concentrations, emphasizing targeted reservoir management strategies. Proactive measures, such as reducing Ba-Zhang River nutrient inflows, optimizing hydrological regulation, and real-time monitoring, are recommended to mitigate phytoplankton blooms and enhance water quality. These insights contribute to a deeper understanding of Chl-a dynamics in interconnected reservoirs, informing sustainable water management strategies.