ESTUARIES ARE KNOWN by many different names: bays, coves, inlets, harbors, sounds, and lagoons. Simply put, they are places where rivers meet the sea. Estuaries are, however, far more complex than merely areas where seawater is mixed with fresh. They are also ecotones, places where different ecosystems meet and boundaries overlap: terrestrial and aquatic, freshwater and marine, inland and offshore. They are dynamic regions of transformation, high productivity and species diversity. Estuaries act as buffers and filters, protecting upland habitats from storm surges and, preventing sediments and pollutants from reaching coastal waters. Estuaries contain diverse habitats: sandy beaches, rocky shorelines, mudflats, fringing salt marshes and mangroves, and provide critical habitats for many species.
Diverse definitions of estuaries have been proposed, emphasizing various sets of attributes, delineating landward and seaward ranges differently, but there is general agreement that estuaries are semienclosed bodies of water with freshwater inputs and some open connection to the sea, allowing for dilution and tidal exchange. The proximity of barrier islands, reefs, bars or peninsulas protects them from the full force of the ocean. Several geophysical processes give rise to estuaries and, as a result, estuaries exhibit different forms. Several types of estuaries have been delineated, encompassing those of glacial origin, known as fjords, those of tectonic origin, those formed from river deltas, flooded river mouths and bar-built estuaries where barrier islands or peninsulas form from sand bars, protecting river mouths from the ocean.
Environmental conditions in an estuary may be highly variable, unpredictable and extreme. Considerable variation can exist even within a given estuary. Due to tidal fluxes, river flows, topography, and weather conditions, estuarine waters experience dramatic changes, both regular and irregular, affecting salinity, sediment load, oxygen concentrations, and temperature. Salinity fluctuates, varying from brackish to almost fresh and in some arid areas, hypersaline. Waters may be well-mixed, or strongly stratified, with water layers of different salinities and densities, called a salt wedge.
These physical extremes present physiological challenges for organisms, but species have adapted to the rigors of the estuarine environment. Estuarine productivities are among the highest in the world, due to high nutrient loads in water and sediments. Estuaries are critically important in the life histories of many species, including recreationally and commercially valuable species. Some species, particularly benthic invertebrates, live their entire lives within the bounds of the estuary. These include species of oyster, clam, and scallop. Some spend only a portion of their lives in the estuary, using them for reproduction, larval, and juvenile rearing. Other species, notably shorebirds, waders and wildfowl, utilize these areas for feeding. Salmon use estuaries as nurseries for juveniles and migrate through them as adults on their way to freshwater tributaries in which they spawn.
Humans settled around estuaries because they provided subsistence opportunities, good harbors, direct linkages between rivers and the sea, facilitating transportation and commerce. Settlements became cities and now several of the world’s largest cities are found along estuaries. Estuaries are also considered attractive places to live for recreational and aesthetic reasons. In the United States, over 50 percent of the human population lives along the coast, which constitutes less than 10 percent of the lower 48’s land. Globally, coastal density is approximately 40 percent. In either case, human populations are increasing in estuarine areas and, as a result, the ecological footprint of humans on the estuarine environment is significant and growing.
Some estuaries habitats and species have declined significantly. Eelgrass (Zostera marina), an aquatic flowering plant that provides habitat for many aquatic species, has declined dramatically, due to dredging, impacts with boat propellers, reduced water quality and clarity. More recently, dieoffs of salt marsh grass (Spartina alterniflora), an important primary producer in the highly productive foodweb of many estuaries, have been observed. The phenomenon, called Sudden Wetlands Dieback, may be caused by various factors such as pollution, drought conditions, elevated water temperatures, increasing sea levels as well as a non-local strain of Fusarium, a pathogenic soil fungus, acting alone or in combination.
Anthropogenic impacts such as pollution, habitat destruction and degradation, resource extraction, and the spread of invasive species are negatively impacting estuaries worldwide. Estuarine pollution comes in many forms and from many different sources, including both point and nonpoint source discharges. Pollutants encompass organic substances and nutrients from sewage outfalls as well as diffuse sources, which can create eutrophic conditions. Oil, synthetic organic compounds, heavy metals, radioactive substances, pathogens, large debris as well as excess heat, produced from electrical generating plants, also contribute to estuarine pollution. Upland land conversion, water diversions, dredging, beach defenses and modifications constitute some of the habitat changes impacting estuaries. Estuaries also provide sites for aquaculture and mariculture operations, with attendant releases of pollution, pathogens, and escapees. Invasive species threaten endemic species via increased predation and competition. Resource extraction occurs as both a directed and incidental activity. Subsistence, recreational and commercial harvesters take shellfish, fish, seaweeds, and various bird species from estuaries and harvest estuarine-dependent species in other areas, depleting reproductive stocks. Estuarine animals are killed by impingement and entrainment through cooling water intake during the operation of power plants.
However, the most serious threat facing the world’s estuaries may be global climate change and its attendant increases in storm frequency, intensity and sea level. Although new estuaries will form at higher elevations, the rate and magnitude of change is critical. At high rates of sea level rise, newly flooded habitats may not be able to form and retain the fringing marshes and mangrove swamps, the various bioscapes that are so important to maintaining productivity and providing habitats for the diversity of organisms that currently utilize the world’s estuaries.
Ecotones; Oceans; Rivers.
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