| Themes > Science > Earth Sciences > Oceanography > Habitats > Estuaries > Humans & the Environment |
Throughout history, we have depended on estuaries for trade, food and shelter from the sometimes-violent open ocean water. Some types of estuaries are better suited to this than others. The Chesapeake and San Francisco Bays are home to large, economically and militarily important harbors, where many, many ships pass each year. Some estuaries are quite deep, depending on how they were formed. Others have to be dredged often because the rivers that feed them carry so much silt and sediment that the deep shipping channels slowly fill in Estuaries are home to a great many different plants and animals that we depend on for food. A very large number of commercially valuable fish return from the ocean to spawn in the protected waters of saltmarshes and mangroves. Oysters, clams and other shellfish thrive in bays and inlets, as do many species of crabs and fish. Any trash or sediment in a river naturally
ends up in an estuarine area, since the current of the river slows here
due to widening or running into the tidal force of the ocean. This
pollution can have long-term impacts on the health of the creatures and
plants that live there. Chemical contamination can linger in bottom
sediments for years and has caused many areas to be closed for fishing
until the chemical has been broken down. Excessive loads of silt and other
sediments caused by erosion can suffocate bottom-dwelling plants and
animals. Disruption of the flow of a river, due to damming and irrigation,
can cause salty water to move farther upstream than it normally does, with
devastating consequences for fresh-water species that cannot move upstream
and out of danger. Many productive commercial fishing areas depend on
phytoplankton nurtured by the seaward flow of clearer water, which in turn
nurtures the fish. This rich outward flow may have a great deal to do with
the overall health of continental shelf ecosystems. |
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