Feedbacks between phytoplankton and global changes in a riverine source-mainstem-estuary continuum

Global changes have led to alterations in phytoplankton community structure and dynamics in aquatic environments. However, limited information is available on the comprehensive impacts of global changes on phytoplankton communities along river systems affected by anthropogenic activities. This study explores how anthropogenic pressures and climate change affect phytoplankton community transitions and induce harmful algal blooms by employing field surveys and a 40-year historical data analysis along China's Yangtze River source-mainstem-estuary continuum. Results revealed significantly higher phytoplankton density and biodiversity in the mainstem compared to the source and estuary zones. From the river's source to its mainstem and estuary, the dominant phytoplankton community formed a transition pattern (diatoms - chlorophytes - cyanobacteria - diatoms). Similarly, phytoplankton functional groups transitioned from mixed to eutrophic groups, signaling a shift in water quality towards moderate eutrophication, although it has not yet threatened the survival of diverse phytoplankton species. Moreover, compared to climate change, anthropogenic activities have more significantly intensified the urban heat island effect and nutrient inputs, thereby promoting phytoplankton cell density and biodiversity, particularly in the case of eutrophic functional groups. However, since 2003, governmental regulations have slowed the increase in nitrogen and phosphorus transport flux from the source to the estuary, contributing to the stabilization of harmful algal blooms at low levels in the estuary and adjacent waters. Strict control of nitrogen-to-phosphorus ratios is essential for preserving biodiversity, mitigating eutrophication, and preventing harmful algal blooms, thereby ensuring ecological balance and protecting water environments along the Yangtze River.