Hydrological conditions can change the effects of major nutrients and dissolved organic matter on phytoplankton community dynamics in a eutrophic river

In river ecosystems with diverse hydrological conditions, major nutrients and dissolved organic matter (DOM) play significant roles in governing the abundance and composition of phytoplankton communities. Understanding the significance of these factors is crucial for mitigating phytoplankton blooms. An extensive study was conducted to explore this relationship including an examination of 108 phytoplankton communities in the mainstream and eight tributaries of the Dongjiang River during both rainy and dry seasons. The results demonstrated the profound impact of both major nutrients and DOM on phytoplankton communities (p < 0.05). Under high-flow velocity conditions, major nutrients dominated, explaining 20 % and 2.5 % of variance during the rainy and dry seasons, respectively. DOM had a smaller influence, contributing −2.4 % and 1.1 % during these seasons. Conversely, under low-flow velocity conditions, major nutrients explained less than 8.0 % of the variation, while DOM became prominent, explaining over 11.8 % of the phytoplankton community dynamics. The dominant genera exhibited pronounced blooms in response to increased major nutrient levels across all hydrological conditions. Specific DOM components (C2, C3, and C4) were positively correlated with phytoplankton in the low flow streams (p < 0.05). These findings indicate the dual influence of major nutrients and DOM on phytoplankton communities, with their relative significance contingent upon spatial variations in hydrological conditions. As the flow velocity decreased and the potential nutrient retention time increased, the importance of DOM surpassed that of the major nutrients. These insights have substantial implications for managing stream ecosystems and preventing phytoplankton blooms in eutrophic river systems.