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Trace metals are required for aquatic life but in higher concentrations heavy metals such as iron, lead, mercury, aluminum, and magnesium are toxic to fish, especially at low pHs (PA FBC). One reason metal toxicity is such a problem is that no natural processes exist to neutralize or remove them (Chapman, 1996). Metals also tend to accumulate in bottom sediments (Chapman, 1996), which presents a problem if those sediments are later disturbed. Industrial wastewater discharges (point source) and mining are common metal sources, although metals like lead (from automobiles) can also come from atmospheric deposition. Aluminum, cadmium, chromium, copper, iron, mercury, manganese, nickel, lead, zinc, arsenic, and selenium are the commonly monitored "metals" although beryllium, thallium, vanadium, antimony, and molybdenum are also very toxic and important to monitor if a pollutant source is likely to discharge them (Chapman,1996).
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In the 1970s, there was a major boom in dam construction, especially in China, the United States, Japan, Spain, and India (WCD, 2000). Currently, of the thousands of large dams (defined by the International Commission on Large Dams as dams with a head height of over fifteen meters) two two thirds are in developing countries (WCD, 2000). These dams fulfill a variety of functions including but not limited to water storage, hydroelectric power generation, and flood control. The figure at the right shows the distribution of dams by functions. One third of countries rely on hydropower for over half their energy needs (WCD, 2000). Overall, it is easy to see the incredible importance and economic impact of dams. However, there are many environmental and social problems associated with dams. Dams have a significant impact on the marine fisheries, either directly by destroying spawning habitat or blocking migration or indirectly by increasing pressures on marine fisheries.
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According to the World Commission on Dams, 46% of the 106 primary watersheds on earth are affected by dams. These effects can include temperature changes (water held in a reservoir warms, while water which is released over the dam's head is cooled) and dissolved oxygen level changes (the warmer water in a dam's reservoir will have lower dissolved oxygen levels resulting from higher water temperatures and slower water velocity, while water below the dam may become super-saturated with oxygen and poison fish). These changes often favor invasive species, which can then outcompete the native biota. Dams also change the natural flow regimes, which are important triggers for biological cycles. Flow levels can enhance or suppress reproductive success for many species, as well serving to redistribute substrates and bed-loads (Young, 1997). Furthermore, starvation of sediments because of retention by dams can alter the substrate composition downstream with huge effects on fish; studies on the Colorado River indicated that natural reproduction of fish species was suppressed because sandbar formation had ceased due to a lack of sediments (Young, 1997). The WCD reports that in many cases wetlands dry out and recharge of groundwater is diminished. Besides "trapping" water behind them, dams also act as particle traps, holding back nutrients and sediment. The downstream ecosystems that rely on these nutrients can suffer severely; the crash of Kokanee salmon was attributed to the drastic decrease in nutrient loading caused by the construction of two dams (Wuest). The changes in sediment transport can heavily influence the channel, floodplain, and delta morphology. In coastal areas, the erosion caused by waves is no longer counteracted by deposition of sediment; the WCD reports that the coastline of Togo and Benin has decreased by 10-15 meters per year after the Akosombo Dam on the Volta River was completed. There are indications that this erosion may also result in a lack of floodplain fertility.
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However, it is not just by obstructing fish passage that dams affect marine fisheries. Dams have been shown to decrease catches of fish in upstream portions of rivers (ex. Senegral such as the Senegal and Niger Rivers, Nile Delta, and Zambezi River ) which again may put more stress on marine fisheries (WCD, 2000). Downstream, changes in flows of fresh water and in nutrient levels can influence the estuarine habitats where many marine fish come to spawn. Lowered nutrient levels can result in lowered overall productivity from a diminished primary food source (i.e. less primary production), as occurred with the Aswan High Dam in Egypt (WCD, 2000). Furthermore, increases in salinity from lessened freshwater flows can allow marine predators to invade, lowering recruitment rates (WCD, 2000). The overall effects of these changes can be significant; in the Zambezi Delta, dam-related changes cause an estimated $10 million annual loss to the shrimp fishery (WCD, 2000).
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Wetland soils are saturated long enough during the growing season to create an anaerobic (low oxygen) state in the soil horizon (the layers of soil found as you dig a hole) . The wetland soil becomes so saturated with water that it cannot hold much, if any, oxygen.
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Rainwater runs off and brings exposed soil particles toward larger bodies of water. In water, sediment may either settle to the bottom or remain suspended in the water column. Settled sediments may destroys the spawning grounds for fish and may suffocate fish eggs. Sediments may also smother macro-invertebrate benthos (bottom dwellers)---an important source of food for fish. Suspended sediments also affect aquatic organisms. Sediment makes water more opaque so the water temperature increases. It can also abrade fish gills and make feeding difficult for fish that rely heavily on sight to find food.
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From an economic perspective, about 75% of the nation's commercially important species of marine fish and shellfish, and 80 to 90% of recreationally important species, are dependent on shallow inshore waters, such as bays, estuaries, and rivers flowing to the sea, for their survival (Vymazal, 2007). However, the importance of wetlands is perhaps best shown by example:
New OrleansThe coastal area of New Orleans discovered the importance of wetlands firsthand through Hurricane Katrina. Every 2.7 miles of marshland reduces a hurricane surge tide by a foot, dispersing the storm's power. Simply put, had Katrina struck in 1945 instead of 2005, the surge that reached New Orleans would have been as much as 5 to 10 feet less than it was. These marshes, as well as the barrier islands, were created by the sediment-rich flood waters of the Mississippi River deposited over thousands of years. But modern levees have prevented this natural flooding and the existing wetlands, starved for new sediments and nutrients, have eroded and "subsided;" they just washed away. Every ten months, even without hurricanes, an area of Louisiana land equal to Manhattan in size is covered by water---50 acres per day, a football field every 30 minutes (Tidwell, 2005).
Three Gorges Dam in China In China, the Yangtze River branches out into a broad estuary that stretches 655 kilometers into the East China Sea, and forms one of the largest continental shelves in the world. Over half of the Three Gorges Dam in China In China, the Yangtze River branches out into a broad estuary that stretches 655 kilometers into the East China Sea, and forms one of the largest continental shelves in the world. Over half of the Yangtze's annual sediment load is deposited in the estuary. The health of the estuary depends on the delivery of this sediment because a significant relationship exists between intertidal wetland growth rate and riverine sediment supply. Yet, due to the Three Gorges project and other dams, the sediment accumulation rate in all reservoirs on the river has increased from close to zero in 1950 to more than 850 * 106 tons per year in 2003. This is causing erosion of the wetland habitat there, which provides nurseries for fish and resting areas for migratory birds and is considered one of the world's most important wetland ecosystems. There is also concern about the impact the project will have on biological diversity. The baiji dolphin, the ancient river sturgeon, and the finless porpoise depend on the Yangtze for their survival. The population of Siberian cranes in Poyang Lake will also be affected by the dam (Cleveland 2007).
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