From River to Reservoir: Exploring 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. This study delves into the phytoplankton community and environmental factors of the Xiangjiaba Reservoir (XJB), a significant reservoir on the Yangtze River mainstream. Field surveys and laboratory analysis were conducted to identify factors driving algae distribution and temporal shifts. The phytoplankton exhibited dominance changes among different phyla. Bacillariophyta and Chlorophyta dominated throughout the year, while Cryptophyta prevailed in spring and Xanthophyta peaked in autumn, indicating a unique feature of the area. The water quality in XJB was moderate. The average chlorophyll-a exhibited significant spatial-temporal variations, peaking at 26 ug/L at the mainstream-tributary confluence. Since the reservoir's construction in 2006, an overall tenfold increase in algae density and a shift from Bacillariophyta-dominated system to a more diverse multi-phylum-dominance have been observed. Hydrodynamic conditions played a pivotal role, with water stratification favoring flagellated algae like Chlorophyta and Cryptophyta. Differences in phytoplankton composition between XJB and the Three Gorgeous Reservoir were linked to the latter's pronounced vertical mixing. The study underscores the swift hydrodynamic adaptations post-construction, juxtaposed with the slower biological (phytoplankton) responses, emphasizing the need for sustained monitoring to ensure the reservoir's ecological balance. This 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.