Modeling reveals the shift in phytoplankton community structure in coastal ocean caused by the Three Gorges Dam

Dissolved silicate (DSi) plays an important role in the ocean silicon cycle. As one of the world's largest water conservancy projects, the Three Gorges Dam (TGD) has a great influence on the biogeochemical process at the Changjiang Estuary and adjacent coastal region, even offshore area, by regulating water and sediment discharge. In this study, we employ a coupled physical-biogeochemical model (ROMS-CoSiNE) to evaluate the TGD's decadal influences before (1999-2003, pre-TGD) and after (2004-2008, post-TGD) the water impoundment. Focusing on two distinct zones, representing the coastal and offshore regions, we found that the average summer water and sediment discharge significantly decreased from the pre-to the post-TGD period, leading to DSi reduction more serious in coastal than offshore region. Notable changes in nutrient ratios accompanied this decline with the planktonic algae composition shifts. Although diatoms still dominated the water column, they exhibited a year-to-year decline. The TGD's influence was further evidenced by a swift rise in the frequency and spatial extent of Harmful Algae Blooms (HABs) following the dam's water storage activities, with the dominant species shifting from diatoms towards dinoflagellates and other types. DSi variation caused by the TGD had profound ecological repercussions, especially in the marine environment of the Changjiang Estuary and coastal regions. To mitigate the risk of ecological catastrophes, targeted measures and solutions must be proposed and implemented.