Estuaries

An estuary is a body of water partially surrounded by land, where fresh water from a river mixes with ocean water. They may form under a number of circumstances; at drowned river mouths; within steep glacially eroded fjords; within barrier islands or a barrier spit parallel to the coast; or within coastal indentations formed by faulting or local subsidence. Characteristics of estuaries vary greatly, determined by the shape of the estuary, the volume of river flow at the head of the estuary, and the range of tides at the estuaries' mouth. The mixing of waters of different densities, the rise and fall of tides, and the flow of the incoming river all combine with the local characteristics of wind, temperature, precipitation and currents to result in a complex pattern of water circulation. It is these water circulation patterns that greatly determine the biological productivity and diversity of local estuary ecosystems.

In general, primary productivity in estuaries is often extraordinarily high because of the availability of nutrients, the diversity of organisms, strong sunlight, protection from waves, and the large number of niches created by the complex water circulation pattern. Decomposition of fast-growing, salt-tolerant plants provides the raw material for the large and complex food webs and rapid nutrient turnover characteristic of these communities. Marine animals frequently use estuaries as nurseries, where juveniles take advantage of the abundant food during their first few weeks of life. Many commercially exploited fish species utilize estuaries as juvenile feeding grounds during part of their life cycle, including many pelagic exploited species. Additionally, some plants growing in shallow temperate estuaries have the ability to remove inorganic nitrogen compounds and metals from polluted waters running from the land, providing another essential ecosystem service for humans.

However, development is destroying the very ecosystems capable of helping to clean the water entering them. Estuaries continue to be irresistibly attractive to developers. In areas of high population density, estuaries are routinely dredged to provide harbours, marinas, and recreation resources and are filled to make space for homes and agricultural land. On a global scale, more than one-half of estuaries and other wetlands have already been lost. Global warming and the subsequent sea-level rise additionally threaten the remaining estuaries, with few uninhabited places left for land, sea or airborne animals to migrate.