Phytoplankton Size as an Ecological Bioindicator in a Subtropical Fragmented River, China

This study investigates the Serial Discontinuity Concept (SDC) by analyzing the size of phytoplankton structures across 13 cascade dams in the fragmented East River, China. The results showed that dam flow-regulation minimized seasonal differences in total chlorophyll-a (Chl-a). Spatially, midstream Chl-a was higher during the dry period, whereas increased wet periods were linked to reservoir effects and nutrient conditions. Nano-phytoplankton dominated during both periods, while micro-phytoplankton declined during wet periods due to higher pH and transparency. Micro-Chl-a increased downstream in dry periods as a result of dissolved oxygen levels and silicate. Self-organizing maps revealed greater size–class variability during dry periods, with pH and conductivity identified as key regulators. Aulacoseira granulata (micro-phytoplankton) and Anabaena oscillarioides (also micro-phytoplankton) were co-dominant. This pattern suggests that the flow regulation and water impoundment by cascade dams during the wet period created localized lentic conditions with enhanced water stability, which favored the proliferation of these species, despite the increased seasonal discharge at the basin scale. These findings support the SDC in that (1) longitudinal Chl-a variations empirically validated SDC, especially during dry periods, and (2) a spatially periodic Chl-a pattern was identified, termed the Cascade Cycle of SDC (CC-SDC).