Regulation of Algal Bloom Hotspots Under Mega Estuarine Constructions in the Changjiang River Estuary

Massive large-scale engineering projects have been built in river estuaries around the world, but their effects on environments in the surrounding coastal waters were less emphasized compared to those due to the watershed projects. In this study, we used the Changjiang River Estuary as an example to show that a significant consequence can be resulted in such a situation. Through analyzing the harmful algal bloom events data and the chlorophyll satellite data, we investigate the spatiotemporal variations of algal blooms in the estuary and its adjacent water. The results indicate that the location of algal bloom hotspot changed over the period of the estuarine constructions. Furthermore, using a well-validated numerical model, we explored the mechanisms responsible for such an ecosystem regime shift. It was found that after the estuarine constructions were built, the surface chlorophyll maximum was attenuated and part of it migrated landward north of the river mouth but was strengthened south of the river mouth and extended seaward. Alternations of the nutrient concentration distribution and turbidity distribution induced by river plume deviation are responsible for the redistribution of the high chlorophyll concentration area. By contrast, the direct impact of the Three Gorges Dam through changing the runoff and sediments flux, which has been highlighted in numerous studies, was less important than expected. Given the fact that Three Gorges Dam and mega estuarine constructions were built in a similar period, any observed regime shift of hydrodynamic and ecological status outside the estuary should be interpreted with particular caution.

Automated summary

The effects of large-scale engineering projects in river estuaries on surrounding coastal waters have been less emphasized. This study uses the Changjiang River Estuary as an example to show that such projects can have significant consequences. The research analyzes the variations of algal blooms in the estuary and its adjacent water using harmful algal bloom events data and chlorophyll satellite data. It is found that the location of algal bloom hotspot changed with the construction of the estuary. A numerical model indicates that the redistribution of nutrient concentration and turbidity caused by river plume deviation is responsible for the shift in high chlorophyll concentration area. The direct impact of the Three Gorges Dam is found to be less important than expected. Therefore, caution should be exercised when interpreting hydrodynamic and ecological changes outside the estuary during the same period as these projects.