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Oceanography
Dams have made great contributions to human society, facilitating flood control, power generation, shipping, agriculture, and industry. However, the construction of dams greatly impacts downstream ecological environments and nearby marine areas.
- dam
- river estuary
- ecosystem
- effect
1. Introduction
Dam construction has a long history, especially in China, where dams have been utilized since 3000 BC. People build dams mainly for river control, flood control, irrigation, hydropower, and shipping. According to statistics from the International Commission on Large Dams, as of April 2020, there were 68,000 large dams with heights of over 15 m or impounding more than 3 million m3 in the world (38,000 of which were located in China, accounting for 56% of the world’s dams) (Figure 1) [1,2]. There were 53,544 dams in the range of 15–30 m high (31,666 in China), accounting for 78.7% of the global dams (46.6% in China). There were 77 dams over 200 m high (20 in China) around the world, accounting for 1.13% of the total dams. The total storage capacity of these dams approached 8000 km3, which is equivalent to 10% of the annual runoff of the world’s rivers. So far, 50% of the world’s rivers have been controlled or altered by hydraulic projects before reaching the ocean [3]. The Yangtze River is the largest river in China and the third-largest in the world. It winds 6300 km through the 1.8 × 106 km2 Yangtze River Basin before reaching the East China Sea [4]. The Yangtze River Basin is an important economic zone in China with an abundance of resources, large population, and developed economy [5,6]. Therefore, the development and management of the Yangtze River are of great importance to the promotion and development of China’s economy. The Yangtze River estuary and its adjacent sea area, where brackish and freshwater intensely mix, have a unique environmental structure and serve a variety of functions [7,8]. There are 1811 large and medium-sized dam reservoirs on the Yangtze River. These dams have had a significant impact on the environment, but extensive and in-depth research is required to accurately characterize these effects [9,10,11]. At present, there are 50,000 dam reservoirs within the Yangtze River Basin, accounting for more than half of all the reservoirs in China [12]. The total water storage has reached 140 km3, which accounts for 15.6% of the annual runoff of the Yangtze River [13,14]. Dam construction has altered the runoff and sediment load of the Yangtze River, changed the flux and composition of nutrients, and affected ecological systems throughout the Yangtze River Basin. The impacts of dam construction are not limited to its direct effects on downstream hydromorphology, ecosystems, and human life, but extend to the ecological environments of the estuary and its adjacent marine area [15,16].
Figure 1. Number of registered dams worldwide and China.
Dam construction is a large part of human engineering infrastructure, which is the foundation for much of our daily lives. Damming rivers provides numerous conveniences for human societies. Dams and reservoirs can store water during rainy seasons and later release it to provide a consistent discharge and maintain sufficient flow throughout the year [2].
As a part of integrated watershed management, dam construction presents both opportunities and challenges. Dam closures convert dynamic rivers into static reservoirs, which affects the hydrography and morphological evolution of rivers by altering flow velocity, water quality, temperature, turbidity, particulate matter, and other physicochemical parameters of rivers [2,17,18]. Dams also result in major anthropogenic disturbances to the biogeochemical cycles of nutrients which affect downstream wetlands, estuaries, underwater deltas, and adjacent marine ecosystems [3,19]. In addition, the recent global warming, in terms of temperature and precipitation, may exacerbate negative effects on the ecological environment by dam construction [20,21]. In 1997, the International Commission on Large Dams published a document, ‘Position Paper on Dams and the Environment’, which pointed out that improving environmental awareness was one of the most important developments at the end of the 20th century [1]. In 2016, the Chinese government put forward the concept of “Great Protection of the Yangtze River”, which made the ecological restoration of the Yangtze River Basin a priority concept that now pervades all related work. How to balance the relationship between the ecological environment and dam construction has become the focus of people’s attention [22,23].
2. Dam Effects the Ecosystems of Nearby Marine Areas
2.1. Effects of Dam Construction on Sediment Flux into the Ocean
Precipitation varies in different months under the influence of monsoon [66]. In recent years, frequent shifts in climate affect precipitation, which has a negative impact on agricultural production [67,68]. Dams could store water during wet seasons and release freshwater during dry seasons, stabilizing the water supply to support agricultural irrigation in delta areas [36]. A dam segments the river, changing it from a flowing whole to one impeded by a static reservoir that reduces river flow. The average, minimum, and maximum flow reduced by 31%, 21%, and 35% in the lower São Francisco Riverin after the construction of the Xingó reservoir [31]. After the construction of the Farakka dam in India, the river flow into the downstream Bengal Bay showed a deficit of 75% [37]. Similarly, the Aswan dam intercepted 90% of the Nile’s runoff upon its construction, leading to the collapse of fisheries in the Mediterranean [69]. Besides, it has been reported that the dam operation affects the size and frequency of flow alteration [70]. Dam construction could influence the hydrological regimes of rivers by reducing the peak flow and changing the flow periodicity [71].
It has been reported that, in addition to impacting river flows, dams have important impacts on sediment dynamics and geomorphic processes [30,72,73,74]. Rivers are estimated to transport about 90% of all dissolved and particulate matter that is deposited in the ocean, and the total annual sediment transported by rivers is estimated at about 19 billion tons, representing a very important driver of hydrological changes and the morphological evolution of estuaries and adjacent sea areas [75,76]. Amenuvor et al. [32] studied the hydrology of the Volta River before and after the Akosombo Dam over the period from 1936 to 2018 using Landsat remote sensing images, and the results indicated that the establishment of the dam resulted in significant hydrological changes and altered the morphological evolution of the river. Their study showed that sediment transport and river flow in the delta decreased by 92.32% and 23.23%, respectively. In major rivers of China, it has been found that the annual total sediment transport to coastal areas has decreased from 2.03 billion tons in 1955–1968 to 0.50 billion tons in 1997–2010 [38]. Similarly, studies of sediment transport in Swiss rivers [39], Russian rivers [40], European rivers [41], North African rivers [42], and South East Asian rivers [43] have shown that anthropogenic disturbances in river basins (mainly dam and reservoir construction) are the main causes of reductions in sediment transport.
Dam construction changes the topography of the riverbank, increases the erosion of the downstream riverbed, and causes erosion that degrades underwater deltas in estuaries [26,77,78]. By altering sediment transport, dams can affect the benthic environments of estuarine areas, causing retreatment of the estuarine turbidity maximum zone. It has been reported that the estuarine turbidity maximum zone in Portugal moved 8–16 km upstream compared to previous records after the construction of the Alqueva Dam [64]. More attention should be given to the effects of dam construction on geomorphic processes for its relationship with sediment flux into the ocean.
2.2. Effects of Dam Construction on Nutrient Flux to the Sea
Artificial lakes formed by dam construction will affect the biogeochemical cycles of nutrients (carbon, nitrogen, phosphorus, silicon, etc.) in water [46,53]. The nutrients in the upstream reaches of a river are intercepted by the phytoplankton that flourish in the reservoirs [19]. This removes nutrients from the water and has far-reaching ecological impacts on the global biogeosphere. The effects of dams on riverine nutrient fluxes vary from one nutrient to another. About 42–93% of river nutrients can be intercepted by reservoirs [44], especially phosphorus (P), whose uptake in reservoirs ranges between 16% and 98% [45]. After the construction of the Three Gorges Dam, eutrophication in downstream reaches has been alleviated [87]. Although the dam intercepted some nutrients, the nitrogen (N) inputs to coastal waters could increase by 20% and with a doubling of P inputs in Indonesia by 2050 due to anthropogenic sources such as domestic sewage, industrial wastewater, and agricultural fertilization [88].
Unlike other essential nutrients, silicon (Si) does not have downstream sources and is not resupplied to rivers after dam interception [48,49]. Due to the effect of anthropogenic perturbations (mainly dam construction, industrial wastewater, and use of fertilizers containing N and P nutrients), the amounts of N and P have increased by 6.7 and 6.5 times, respectively, while the amount of silicon has decreased by 30% [47,89]. In addition to nutrient retention, Si nutrient concentrations in downstream rivers and coastal marine areas can be reduced by other hydrological changes caused by dams [48,76]. Under the blocking of the dam, the flow velocity of the river decreases, which weakens the bank erosion and reduces the water-ground interaction. As a result, less Si is supplemented from the continent, and the amount of silicon in the river decreases [50]. Humborg et al. [51] found that the major cause of the reduction in land–sea Si fluxes was dams. The dissolved silicate (DSi) yield of moderately dammed rivers was only 50% of the practically undammed river. It has also been reported that 80% of Si in the ocean has been imported from rivers [52]. Therefore, the global modification of riverine Si flux directly affects the distribution of ocean basin Si concentrations, especially for coastal marine areas.
2.3. Effects of Dam Construction on Ecosystems of the Estuary and Adjacent Coastal Area
Estuaries are complex amalgams of various material systems, structural systems, functional systems, and energy systems [94]. They are ecological transition zones and represent some of the most intense and complex land–sea interactions [95]. Macronutrients are carried by rivers from land to estuaries, promoting the growth and reproduction of marine and saltwater tolerant organisms and maintaining the highly complex and variable ecosystems [96]. The abundance of organic and inorganic elements in estuarine areas makes them ideal for primary productivity, as demonstrated by the plumes of highly productive areas fronting estuaries worldwide [97]. As some of the highest productivity zones in the ocean, many famous fishing grounds are associated with estuaries, such as the Lvsi and Zhoushan fishing grounds in the Yangtze River estuary. However, estuaries and adjacent sea areas are frequently densely populated with developed agriculture and industry, which makes estuarine ecosystems highly impacted by human activities.
Downstream, estuarine, and adjacent marine ecosystems will all be affected by damming. Phytoplankton in the ocean absorb nutrients in a constant ratio known as the Redfield coefficient. The deviation of the nutrient ratio from the Redfield coefficient in seawater will affect the growth and composition of phytoplankton [98,99]. Decreases in silicon concentrations and increases in nitrogen and phosphorus concentrations have been linked to changes in the growth and species composition of phytoplankton communities, as well as increases in the frequency of harmful red tide outbreaks [54,55]. Phytoplankton are important components of aquatic food webs, so changes in their abundance and composition can have effects on benthic animals, fishes, plants, and birds [56,100]. Worse still, marine products contaminated with algal toxins can cause illness or even death in humans if they are mistakenly consumed [101].
Plant species richness was significantly impacted by dam construction [57,59,102]. Most studies in the research of dam effects focused on the plants in the reservoir, downstream channel, and lake. Moreover, studies about the effects of dams on plants in coastal waters were insufficient. The water level and plants could be impacted by dam construction, which would further affect the habitat suitability of birds [103,104,105].
According to the hydrology, biology, chemistry, and sedimentation in the Yangtze River estuary, the Yangtze diluted water can be divided into plume water (salinity of 5–25 and sediment of 100–500 mg/L) and mixed water (salinity of 25–31 and sediment < 100 mg/L). The Yangtze River estuary plume is differentiated by the isohaline 25 between the two waters. The Yangtze estuary plume area is a high-quality ecological environment that promotes the growth of marine organisms. This is why the influence of dam construction on the Yangtze River (especially the Three Gorges Dam construction) on the Yangtze plume has gradually become the focus of much attention.
The land satellite images from 1974 to 2009 illustrate how the sediment flux from the Yangtze River into the sea has significantly decreased, a decrease that is strongly correlated with the construction of the Three Gorges reservoir. Upon completion of the Three Gorges Dam, the structure and distribution of nutrients near the plum changed significantly. Wang et al. analyzed the influence of the Three Gorges Dam on the biogeochemical processes in the downstream reaches of the Yangtze River and the Yangtze River estuary plume [79]. After the Three Gorges Dam finished seasonal runoff from the Yangtze River, the estuary plume decreased by 12–17% in October and increased by 5–20% in the dry season. In addition, due to the decrease of sediment fluxes, the erosion of underwater deltas and coasts, and altered benthic structure increased. From a positive perspective, the interception effect of the Three Gorges Dam on nutrients could alleviate eutrophication in the Yangtze River estuary due to the decrease of nutrient fluxes.
This entry is adapted from the peer-reviewed paper 10.3390/su14105974
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