Investigation on phytoplankton dynamics and its environmental correlates in the Xiangjiaba channel-type reservoir

Mega reservoirs, such as the Three Gorges Reservoir on the Yangtze River, have garnered significant attention due to their environmental impacts. However, the ecological ramifications of upstream cascade reservoirs remain understudied, despite their potential influence on the Yangtze River ecosystem. The Xiangjiaba Reservoir (XJB), presents a unique case for understanding the ecological impacts of large channel-type reservoirs marked with considerable water depth and extensive runoff, which lies between the characteristics of rivers and lakes. This study delves into the phytoplankton community dynamics and associated environmental factors within the XJB. Through field surveys and laboratory analysis, we explore the drivers of phytoplankton distribution and their temporal shifts. Our findings revel significant changes in phytoplankton dominance among various phyla. Bacillariophyta and Chlorophyta dominated throughout the year, while Cryptophyta prevailed in spring and Xanthophyta peaked in autumn, indicating a unique ecological feature of XJB. The reservoir's water quality was found to be moderate. The average chlorophyll-a exhibited significant spatial-temporal variations, peaking at 26 mu g/L at the mainstream-tributary confluence. Our investigation reveals a tenfold increase in algal abundance since the reservoir's inception, accompanied by a significant transition from a Bacillariophyta-dominated community to a diverse, multi-phylum system. This shift has been significantly influenced by hydrodynamic conditions, with water stratification favoring flagellated algae like Chlorophyta and Cryptophyta. Notably, differences in phytoplankton composition between XJB and the Three Gorges Reservoir were linked to the latter's pronounced vertical mixing. The slower ecological responses of the phytoplankton community to hydrodynamic changes underscore the importance of long-term, continuous monitoring to maintain the reservoir's ecological balance. Our research offers insights into the ecological impacts of reservoir construction, highlighting the role of hydrodynamics in reservoir ecosystems and aiding in understanding reservoir functioning, water quality management, and biodiversity conservation.