Abstract
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.
Highlights
The estuarine and coastal zone, which is a key area for human being inhabiting, is economically developed and densely populated
Through analyzing the harmful algal bloom events data from bulletins during 2000–2010 and the spatiotemporal probability of algal blooms derived from chlorophyll remote sensing data from 1998 to 2016, we investigated the long-term trend of algal bloom distribution in the Changjiang River Estuary and its adjacent water
The results showed that the frequency of algal blooms east and northeast of the Changjiang River mouth had been increased and the high-value region moved landward gradually after 2003
Summary
The estuarine and coastal zone, which is a key area for human being inhabiting, is economically developed and densely populated. Numerous attention have been paid to how the river basin engineering projects affect the physical processes and ecosystem in the receiving coastal waters (e.g., Humborg et al, 1997; Pelletier et al, 2016; Bargu et al, 2019). Several exceptions are as follows: the port expansion in the Ems River Estuary, which aggravated the estuarine eutrophication (Van Maren et al, 2015); the land reclamation in the Johor Estuary, which caused the adjacent aquatic environmental degradation (Wang et al, 2019c); and the wetlands reclamation and dike construction in the San Francisco Bay estuary which reduced the habitats for fish and waterfowl (Nichols et al, 1986)
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