Estuaries and Salt MarshesAn estuary is that area where the mouth of a river meets the sea. Sea water mixes with fresh river water to form brackish water. It is characterized by tides, by opposing currents flowing outward from the river and inward from the sea, and by brackish water, which is a mixture of sea and river water. Since salt water is denser than fresh water, in an estuary, incoming sea water forms a wedge under the lighter river water. Mixing occurs in the narrow zone between the layers. Percent salt content (salinity) of an estuary is less than that of sea water but more than that of fresh water, and changes every 12 hours with the tidal cycle. Another characteristic of an estuary is the presence of sediment. River water contains fine mud and silt that has been washed off the land. As the river widens into the estuary and meets the sea water, the current slows and the particles suspended in the water settle to the bottom. The saline conditions in the estuary also cause mud and silt to settle; especially where fresh and salt water mix. In the area of the estuary closest to the ocean, mud and silt deposits can form extensive mudflats and sand banks (deltas). River runoff also carries organic and mineral matter which settle with the estuary sediment (nutrient trapping). This enriched sediment provides an important source of nutrients for microscopic one-cell animals (protozoans), which are consumed by larger organisms in the estuarine and surrounding salt marsh food chains. Decaying vegetation (microbial biomass) in the salt marshes provides an additional food source. Salt marshes are common in estuaries often forming on mudflats, and in the low coastal land of sheltered bays and inlets.
What Animals and Plants Live in Estuaries and Salt Marshes?Confined to small, transitional areas between the land and sea, estuaries and salt marshes may support fewer animal and plant species than do marine or freshwater ecosystems, but they still provide productive habitats for many fish, shellfish and water birds. A habitat is the natural home of a plant or animal. Changes to this home can affect the organisms that live there, sometimes, in harmful ways. Commercially important fish such as flounder, salmon, trout, striped bass, shad, gaspereau and smelt feed in estuaries and migrate through them to reach spawning grounds. Many birds feed and over-winter in estuaries. The nutrient-rich mudflats provide abundant food for certain species of shellfish including soft-shell clams, mussels, periwinkles, crabs and shrimp. Productive shellfisheries are often found in estuaries. Saltmarsh plants grow on mudflats that have accumulated to a height where they are exposed at low tide and submerged at high tide. Salt-tolerant plants (halophytes) such as cordgrasses (spartinas), are common in salt marshes and can withstand periodic submergence and waterlogged soil. In the Bay of Fundy, spartinas provided salt hay for early settlers.
How are Estuaries and Salt Marshes Disrupted?Estuaries and salt marshes can be disrupted by events occurring in the river, the sea or the estuary itself. These disruptions can also affect nearby coastal areas that are dependent on the estuarine ecology. Natural events can be disruptive. Storms and floods, from tidal surges or from the river, can erode mudflats and the seaward edge of salt marshes. Whereas salt water flooding can kill terrestrial plants and extend the marsh area. Flooding by fresh water can damage the marsh and erode channels. Ice buildup can scour mudflats. A thick blanket of ice can deprive surface- dwelling organisms of much needed oxygen. Salt marshes are common in estuaries often forming on mudflats and in the low coastal land of sheltered bays and inlets. Human activity can cause severe disruptions to estuaries and salt marsh habitats. Many cities and towns have been built around estuaries, which offer natural transport and shipping lanes. Saltmarsh land is flat and easy to build upon when drained or filled. Developed areas continue to be disrupted through dredging, dumping of dredge material, shipping and other port activities, development of recreational facilities and overharvesting of fish and shellfish. Estuaries may also be affected by industrial or urban development occurring upriver, by drainage projects, flood control activities and agricultural runoff. Estuaries and salt marshes can be disrupted by natural events, such as storms and floods. Human activity, including urban development and agriculture can cause severe and long-lasting damage. Pollution can affect an estuary in four major ways: 1) oxygen depletion (e.g. from sewage), 2) chemical accumulation (e.g. toxic organic compounds, petroleum products, heavy metals), spills (e.g. oil. spills) and thermal pollution (e.g. heated effluent from power plants). Just as nutrients collect in estuarine mudflats through nutrient trapping, many pollutants can also collect in the sediment. Microbial activity can decompose some pollutants, such as sewage, but others, such as heavy metals, are bio-accumulative.
Draining and DikingThe drainage of salt marshes for farmland and other developmental purposes, or to control flooding, was widespread in Nova Scotia. Channels were usually cut in the marshes to reduce the amount of surface water at high tide, while dikes were built around the perimeters of the drained land to prevent water from entering the marsh area. Harvesting salt marshes can reduce the amount of decayed vegetation entering the aquatic food chain, which, in turn, can reduce the populations of fish and shellfish in adjacent waters. About half of the 32,000 hectares (80,000 acres) of Nova Scotia salt marshes have been diked. Fortunately, most of this land is high-level marsh, as in the Bay of Fundy, and is rarely flooded. Farming these areas probably has little effect on the marine food chain. Many organisms that live in estuaries are close to their physiological limits of tolerance, making them sensitive to extra stress, such as oxygen depletion or chemical pollution. They have adapted to changing salinities, soft sediments and, muddy waters and may suffer if these conditions are altered. Single or short-duration events may harm species or major portions of the habitat, but the damage should eventually correct itself. For example, storms or a single oil spill may impact severely at first, eroding or covering mudflats and salt marshes. However; if no other disruptions occur, animal and plant communities should return to previous populations. Recovery time depends on such factors as direction of the current, rates of sedimentation, hydrocarbon breakdown, and regeneration of the grasses. Migrating and young animals will also move into the area, hastening its recovery. Long-term discharges of pollutants or recurrent pollution will have long lasting effects. Persistent chemicals, such as PCBs or heavy metals, accumulate in the food chain and are absorbed directly by invertebrates feeding on sedimentary deposits. If the sources of chemical pollution are removed, clean sediment may eventually bury the polluted layer, thus removing it from the food chain. However, the polluted sediment could be re-exposed through storms or dredging, for instance When salt marshes are burned or drained to convert them to farmland, fish and shellfish populations in the estuary and surrounding coastal areas may be reduced, creating difficulties for certain inshore fisheries. Such disruptions can be irreversible if corrective action is not taken. Urban and industrial development of the areas adjacent to estuaries and salt marshes may destroy entire plant and animal communities. The marshes and mudflats may even be eliminated. Undeveloped salt marshes; however, can coexist with industrialized areas and enhance communities.
How Can Disruptions Be Prevented?Many disruptions to estuaries and salt marshes result from industrial and urban development, or from poor land use practices in the watershed upriver. These effects may be difficult to correct, although treatment of urban and industrial waste will reduce pollution from developed areas. In the watershed, reducing soil erosion, water diversion and toxic chemical runoff will help protect habitats downriver. Care must be taken to avoid resuspending toxic materials that may be embedded in the sediment. Dredging and dumping of dredge spoil, for example, could dislodge such dormant toxic compounds. Estuaries and marshes affected by isolated and short-term events, such as storms, floods, oil spills or organic enrichment, may recover rapidly. Microbial activity in the sediment can break-down oil or organic pollution, provided the amount and duration of the pollution does not overwhelm the ecosystem. In fact, in the case of polluted mud, microbiological processes may be preferred to artificial cleanup methods. Marsh habitats can be protected by legislation aimed at controlling development and waste dumping. Preventative or corrective action to preserve estuaries and salt marshes depends on changes in land use, development and industrialization. Such planning and development falls under the jurisdiction or regulatory authorities of several federal and Nova Scotia government departments.
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