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Biodiversity - Estuaries/Mangroves/Coral Polyps
                                                                                                    December 19, 2017


Estuaries are partially enclosed bodies of water along coastlines where fresh water and salt water meet and mix. They act as a transition zone between oceans and continents.

D.W. Pritchard (1967) define estuary as “An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage.”

Types of Estuaries

a) Coastal Plain estuary: Coastal plain estuaries were formed at the end of the last ice age. As the ice melted and the waters warmed, sea level rose. The rising seas invaded low-lying coastal river valleys. These valleys are usually shallow with gentle sloping bottoms.

b) Tectonic Estuary: The earth’s crust is constantly in motion. This motion causes large cracks or faults and folds to form in the crust. Often due to folding and faulting, the land sinks or subsides. Tectonic estuaries are created when the sea fills in the “hole” or basin that was formed by the sinking land.

c) Bar-Built Estuary: Bar-built estuaries are formed when sandbars build up along the coastline. These sand bars partially cut off the waters behind them from the sea. Bar-built estuaries are usually shallow, with reduced tidal action.

Importance of estuaries

Estuaries provide us with a suite of resources, benefits, and services. Some of these can be measured in dollars and cents, others cannot. Estuaries provide places for recreational activities, scientific study, and aesthetic enjoyment. Estuaries are an irreplaceable natural resource that must be managed carefully for the mutual benefit of all who enjoy and depend on them.

Thousands of species of birds, mammals, fish, and other wildlife depend on estuarine habitats as places to live, feed, and reproduce.

 And many marine organisms, including most commercially-important species of fish, depend on estuaries at some point during their development. Because they are biologically productive, estuaries provide ideal areas for migratory birds to rest and re-fuel during their long journeys. Because many species of fish and wildlife rely on the sheltered waters of estuaries as protected spawning places, estuaries are often called the “nurseries of the sea.”

Estuaries have important commercial value and their resources provide economic benefits for tourism, fisheries, and recreational activities. The protected coastal waters of estuaries also support important public infrastructure, serving as harbors and ports vital for shipping and transportation.

Estuaries also perform other valuable services. Water draining from uplands carries sediments, nutrients, and other pollutants to estuaries. As the water flows through wetlands such as swamps and salt 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. Wetland plants and soils also act as natural buffers between the land and ocean, absorbing flood waters and dissipating storm surges. This protects upland habitat as well as valuable real estate from storm and flood damage. Salt marsh grasses and other estuarine plants also help prevent erosion and stabilize shorelines.

Threats to estuaries

Human activities within an estuary (shipping, recreation, aquaculture), or within the lands surrounding the estuary (urbanization, agriculture, logging), may alter estuarine habitats either directly (shoreline alteration, channelization, landfill) or indirectly through such problems as excessive nutrients or introduction of invasive, non-native species.

Estuaries are coming under increasing pressure from:

• Estuary margin development – population growth and coastal settlement.
• Increased demands for recreational uses – such as boating and fishing.
• Development in estuaries – such as marine farms and marinas.
• Catchment development – such as forestry and agriculture.
• Land clearance and reclamation.
• Excavation and dredging for example for boat ramps and boat channels.
• Introduction of invasive species such as Spartina.
• Resource extraction – such as fishing.
• Long term climate changes including sea-level rise.