Many dams, even those far from the ocean, have a significant impact on the marine environment. Blocking a river’s flow into the ocean can change the shape of a coastline, block fish migrations, and alter marine food webs. Thus it’s important for marine biology students and coastal managers to keep an eye on what goes on inland. All ecosystems are interconnected; that dams affect the ocean is one example.
Dams Affect Ocean Coastlines - Leading to Coastal Erosion
As marine biology students should know, coastlines are constantly being worn away by ocean waves and runoff. However, rivers that run to the shore deposit sediments they picked up inland. As dams block rivers to create reservoirs, inland sediments can no longer travel out to replenish coastlines. This can lead to the loss of land.
For example, the Nile River once carried 124 million tons of sediment each year out to sea and deposited 10 million tons of sediment on its floodplain, according to the 2005 environmental textbook Encyclopedia of Coastal Science edited by Maurice Schwartz. Now, however, the Aswan High Dam prevents those sediments from reaching the floodplain – leading to the delta coastline eroding by 125-175 meters per year.
Dams Change What Flows into the Ocean
Rivers not only carry dirt to floodplains, they also carry important nutrients out to the sea. As marine biology students know, estuaries are places where freshwater meets the sea, and they are rich ecosystems supporting many of the world’s fisheries. That rivers carry freshwater into the ocean is also important to note. As rivers are blocked from flowing into the sea, the salinity of estuaries increase. The creatures living in the estuaries, adapted to a certain salt level, may suffer from the change.
Dams Affect Ocean Ecosystems and Biodiversity
Dams affect oceans by blocking the flow of nutrients and freshwater into the sea. As Schwartz notes in his environment textbook, increased salinity has affected the growth of phytoplankton in the past. A dam in the US caused phytoplankton biomass to drop to less than 20% of normal in San Francisco Bay. As an important producer this drop affected many animals higher up on the food chain. Shwartz goes on to note that dams along the Volta River in Ghana led to the collapse of a once healthy clam industry and a decade after the construction of the Aswan Dam only 17 of 47 commercial fish species were still being harvested.
Dams Affect the Migrations of Ocean Fish
Interfering with salmon migrations is perhaps one of the most well known consequences of dam building. Salmon reproduce and are born in freshwater, but they spend the growing phase of their lives out at sea. Salmon however are not the only creatures that travel between freshwater and the open ocean. Certain species of eel and striped mullet also travel between the two, points out Michel Larnier of the UN’s Food and Agriculture Organization in “Environmental Issues, Dams and Fish Migration”. Unfortunately dams block the migrations of these species thus interfering with their life cycles. Although many dams now include places for fish to pass through, they vary in effectiveness.
Inland Actions Have Ocean Consequences
It is important to remember that human actions often have far reaching effects. Although many human structures may be far from marine environments they do affect the ocean. Dams affect the ocean in a variety of ways that may not be obvious to the casual observer. However, by changing a river’s flow into the ocean, dams can have serious consequences for marine life.
 |
 |
 |
 |
