We had a terrific group of people in our recent course in the Bahamas
Estuaries come in all shapes and sizes, each unique to their location and climate. Bays, sounds, marshes, swamps, inlets, and sloughs are all examples of estuaries. An estuary is hard to define because there is no one definition to encompass all the diverse geomorphologic settings. A classic definition is that “an estuary is a semi-enclosed coastal body of water which has a free connection to the open sea and within which sea water is measurable diluted with fresh water derived from land drainage”(Alongi). However this definition does not include tides, water bodies in arid, semi arid and wet tropical areas (Alongi). So more or less an estuary is an enclosed region at the mouth of a river where freshwater from land drainage mixes with saline water from a tidal sea (Barnes). An estuarine system can be divided into three regions: (a) a tidal river zone, (b) a mixing zone, (c) a near shore turbid zone (Alongi). A tidal river zone is a fluvial zone characterized by lack of ocean salinity but subject to tidal rise and falloff sea level (Alongi). A mixing zone is characterized by water mass mixing and existence of strong gradients of physical, chemical and biotic quantities reaching from the tidal zone. A near shore turbid zone is between the mixing zone and the seaward edge of a tidal plume. There are four common types of estuaries. The first type is sometimes referred to as “coastal plain estuaries” and often contains larges areas of mudflat and salt marsh (Little). A typical coastal plain estuary the mixing of fresh water and salt water is not complete; the estuary is said to be stratified (Barnes). Some examples include Chesapeake Bay and in England the Severn. The second type is the fjord. They were formed by rising sea level but began as glaciated valleys. Fjords are usually deep with very steep rocky sides (Little). A good example can be found in Norway or on the Pacific Coast of Canada. The third type is the bar-built or barrier built estuary. These occur at river mouths where sand and shingle has accumulated off shore to form islands (Little). The fourth type is known as a tectonic estuary. It is a miscellaneous collection of types formed from faults or folding of the Earth’s crust (Ray). Tectonic estuaries often have excess freshwater flow. Estuaries are sediment traps. Patterns of sedimentation vary with overall estuarine shape and with distribution of salinity. Microtidal estuaries are those with a tidal range of less than 2 m (Little). Microtidal estuaries are formed when waves are active in moving sediment and are usually the bar built type. Mesotidal estuaries have a tidal range between 2-4 m. Tidal currents and waves are both active in moving sediment so there is a possibility of sand bars at the estuary mouth. Macrotidal estuaries have a tidal range higher than 4m. They tend to have a wide mouth that contain long thin mobile bodies of sand rather than ebb and flood deltas.
Estuaries include many types of habitat –mud, sand, salt marsh and mangroves. Estuarine systems contain soft sediments and rocky shores and other hard surfaces. Many different habitat types are found in and around estuaries, including shallow open waters, freshwater and salt marshes, sandy beaches, mud and sand flats, rocky shores, oyster reefs, mangrove forests, river deltas, tidal pools, sea grass and kelp beds, and wooded swamps (EPA). Estuarine soft sediments lack a substratum with long-term stability because of tidal currents and sever wave action (Little). They may also experience sever erosive forces. Rocky shores at the mouths of estuaries have communities very similar to those nearby open coasts (Little).
Estuaries are said to be the most highly productive ecosystems in the world along with near shore waters. Estuaries are regarded as amongst the most fertile natural areas of the world, being up to 20 times more productive than the open sea (Barnes). They are highly productive because phytoplankton and other organisms maximize light and space availability; tidal energy and circulation, abundant nutrients and conservation, retention and efficient recycling of nutrients (Little). Early studies show that the marine species die out with distance up-estuary, and that they are replaced by freshwater species towards the riverine end (Little). However they have now found that it depends on the individual characteristic’s of the estuary. Some characteristics include salinity, temperature, etc. They are home to shore birds, fish, crabs and lobsters, marine mammals, clams and other shellfish, marine worms, sea birds, and reptiles.
Estuaries are important to us because of its high productivity, which allows us to fish, mostly for bivalve mollusks (Barnes). We also fish for oysters, herring, salmon, and flatfish. Estuaries are easy places to farm in enclosures. Estuaries also serve as a sewer and a main drain. Most sewage and waste reaches estuaries. Estuaries further provide a safe anchorage for working vessels. However these uses do not come by themselves they bring baggage. Many of our uses for the estuary create pollutants for example the boats can lead to oil spills or dredging to maintain or deepen the channels (Barnes). Detergents reduce aeration at the water surface and damage cell membranes. Oil smothers organisms, which causes them to suffocate and die. Chemical wastes of all sorts enter estuaries ranging from lethal cyanides to salts, which are almost inert. Some pollutants maybe selective in that some organisms are more tolerant than others. Humans contribute to habitat destruction because we build on, cut through, dredge our or bury entire marine communities. Marine habitats are being destroyed because of port and harbor development, industrial facilities, tourist facilities, mariculture development and expansion of urban areas. By the 1970’s the United States already lost roughly 50% its original coastal wetlands (Thorne-Miller). Estuaries along with other marine environments and the marine organisms that live there have to fight with polluted water. Pollutants enter the water from the land and from the atmosphere. It is estimated that 25% of the nitrogen-containing pollutants entering Chesapeake Bay are from airborne sources. Another type of pollution is marine debris, again caused by humans, plastics, garbage, and all sorts of things plague marine life and marine environments.
Estuaries are also important because they are critical in the survival of many species. Many species depend on estuaries to live, feed and reproduce. Estuaries also act as a filter. Water draining from the uplands carries sediments, nutrients, and other pollutants. As the water flows through marshes, much of the sediments and pollutants are filtered out. This filtration process creates cleaner and clearer water, which benefits both people and marine life(EPA). Other benefits of the estuaries include recreation, scientific knowledge, and education.
Estuaries need to be protected because human activity is creating an unbalance in nature. People are dredging channels, waters are being polluted, shorelines are being reconstructed, etc. Stresses caused by overuse of resources have resulted in unsafe drinking water, beach and shellfish bed closings, harmful algal blooms, unproductive fisheries, loss of habitat, fish kills, and wildlife, and a host of other human health and natural resource problems.
Alongi, Daniel. Coastal Ecosystem Processes. Boston: CRC Press, 1998.
Barnes R.S.K. The Coastline. New York: Wiley and Sons, 1977.
Little, Colin. The Biology of Soft Shores and Estuaries. Oxford: Oxford University Press, 2000.
Miller, Boyce Thorne and John Catena. The Living Ocean: Understanding and protecting Marine biodiversity. Washington D.C: Island Press, 1991.
Ray, G. and Jerry McCormick-Ray. Coastal-Marine Conservation: Science and Policy. Oxford: Blackwell Publishing, 2004.
U.S. Environmental Protection Agency http://www.epa.gov/owow/estuaries/about1.htm Last Updated March 16th 2004.
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