An estuary is a coastal ecosystem, typically consisting of a body of brackish water that is supplied by inflows of fresh water from rivers and streams that mix with the ocean's salty water. Estuaries provide a natural habitat between ocean waters and fresh waters. The convergence of both ocean salty water and river water supplies high levels of nutrients into this ecosystem, turning estuaries into one of the most fertile ecosystems in the world; however, the estuary ecosystem is in danger due to natural and anthropogenic stresses.
Estuaries allow the development of a wide variety of plants and animals due to the rich nutrient soil that abounds throughout the whole ecosystem. The estuary ecosystem supports complex food webs, where plants serve as primary foods for many animals, which in turn are the prey of bigger animals. In addition to providing dynamic food webs for birds, fish, reptiles and other animals, estuaries also provide the habitat for a great number of commercial fish and marine mammals. Estuaries consist of a number of animal and plant habitats that include rocky shores, coral reefs, mud flats, barrier beaches, submerged aquatic plants, mangrove forests and salt marshes, among others.
Apart from supporting a wide range of animals and plants, estuaries are natural filters, maintaining entire communities around them free of pollution. The vegetation and marsh grass in estuaries help filter pollutants and sediments often emptied into rivers and streams in upland regions. During heavy storms, the vegetation in estuaries prevents the flooding of inland areas. Plants also help prevent erosion along the coastline. Estuaries absorb water stemming from high tide events or during storm surges, preventing it from reaching upland areas.
Animals in estuaries
Because of the abundance of food, the biodiversity in estuaries is very rich. Common animals include: shore and sea birds, fish, including salmon and trout, flounder, eels, stripped bass, mud snails, oysters, seahorses, turtles, crabs, lobsters, clams and other shellfish, marine worms, raccoons, opossums, skunks, lots of reptiles and migratory birds, such as the Canada goose and black-tailed god-wit. Some birds develop long legs and toes that permit them to walk over the mud with ease. All of these animal form dynamic food webs in the estuary ecosystem.
Plants in estuaries
Bottom-dwelling algae, marsh plants and eel grass are at the base of the food web in estuaries. When these types of plants die, they serve as food for other organisms, such as protozoa, which in turn are eaten by small invertebrates. Seagrass is a type of plant capable of living submerged in sea water. Mangrove forests grow in shrublands and can withstand floods from sea water, but their roots need to absorb oxygen. Sea rush is a type of plant that is abundant in marshes and which forms dense shrublands. Another plant common to marshes is the ribbonwood shrub.
Anthropogenic damage to the estuary ecosystem includes coastal development, pollution, sewage, overfishing and the introduction of non-native species into the habitat. Estuaries are often the dumps of materials from industrial, agricultural and domestic waste. Contaminant waste products can remain in the estuary for long periods of time due to slow degradation, causing damage to the fauna and flora in the ecosystem. When an estuary is affected by man-made disturbances, it usually takes long periods of time to recover, or never recovers at all. Overfishing has depleted some animal species, affecting food webs in estuaries.
Estuaries are subjected to natural disturbances, such as tidal currents, waves, winds and ice, due to their fragility. When natural habitats such as estuaries are disturbed by nature, they usually undergo long periods of recovery. Storm surges and hurricanes are known to have caused damage to the estuary ecosystem, changing its entire physical structure due to flooding and erosion. Such destructive events may take significant amounts of time to recover.
Estuaries are some of the most biologically productive ecosystems in the world. Animals and plants rely on estuaries for habitat, breeding and feeding. Human beings also rely on estuaries for recreation, food and jobs. It is estimated that more than fifty percent of the world's population lives on the coastline close to estuaries. According to oceanservice.noaa.gov, 32 of the world's largest cities are situated on or near estuaries. This has led to human activities that have placed estuaries under severe damage, making them among one of the most threatened ecosystems in the world.