Marine Biology (2+1)

Types of Estuaries

There are many definitions of an estuary, because several zcomorphological features of coastlines, such an lagoons, sloughs, fjords, and other shallow embayments are often considered estuaries. A simple definition is that as estuary is a partially enclosed coastal embayment where freshwater and seawater meet and mix. This definition implies the free communication of the sea with the freshwater source, atleast during a part of the year. This definition excludes permanently isolated coastal water impoundments, as well as such isolated brackish or saline bodies of water of the Caspian Sea, Aval Sea, and Great Salt Lake.

 Based on  the geomorphology of an estuary, the geological history of the area, and the prevailing climatic conditions, there may be different estuarine types, each displaying somewhat different physical and chemical conditions. These may be grouped into a few basic types. Perhaps the most common type of estuary is the coastal plain estuary. Coastal plain estuaries were formed at the end of the last ice age when the rising sea level invaded low-lying coastal river valleys. Estuaries such as the Chesapeake Bay and the mouths of the Delaware and Handsome rivers in the United States and the Cornwall and Devom estuaries in Great Britain, are examples of this type. Similar to this is the tectonic estuary. In this class of estuary, the sea reinvades the land due to subsidence of the land, not as a result of a rising sea level. A good example is San Francisco Bay. A third type of estuary is the semienclosed bay or lagoon. Here sand bars build up paralled to the coastline and partially cut off the waters behind them from the sea. This creates a shallow lagoon behind the sandbars, which collects the freshwater discharge from the land. The water in such lagoons varies in salinity, depending on the climatic conditions, whether or not any major river flows into the lagoon, and the extent to which the bars restrict sea water access. Such estuaries are common in north Carolina and along the Texas and Florids  only coasts in northwestern Europe (the Netherlands), and part of Australia. A final category of estuary is the Fjord. These are valleys that have been deepened by glacial action and are then invaded by the sea. They are characterized by a shallow sill at the month that greatly restricts water interchange between the deeper waters of the fjord and the sea. Often, these deeper waters are stagnant because of lack of circulation. Fjords are abundant on the coasts of Norway, Chile, Scotland, Alaska, New Zealand and British Columbia.

Estuaries may be classified in yet another way the salinity gradients are formed. In most estuaries, there is a gradient in salinity from whole seawater (33 – 37 ppt (psu) at the month to freshwater at the upper reaches. Freshwater is less dense than seawater and where the two meet, the freshwater will float on the seawater. Mixing occurs when the two come in contact, but the extent of the mixing varies with many other environmental factors, including basin shape, tide, river flow, and rainfall. In estuaries where there is substantial freshwater outflow and reduced evaporation (typical temperate zone estuaries), the fresh water moves out over the top of the Salt Water, mixing with it near the surface and reducing the salinity, leaving the deeper waters more saline. In such a situation, a cross section of the estuary shows isohalines (lines of equal salinity), which extend upstream at bottom. At any given point on the estuary, a vertical column of water has highest salinity at or near the bottom and lowest at or near  the surface. This is a positive estuary (or) salt wedge estuary. These estuaries are also called river dominated (or) stratified. Such estuaries form a continuum –from those with little mixing and very prominent salt wedges, through those with partial mixing and lesser wedges, to homogenous, marine-dominated, and mentral estuaries where either complete mixing or having an evaporation rate equal to the freshwater inflow gives similar salinities from surface to bottom at any point. Where one estuary fits in this continuous depends not only on the amount of mixing of the water masses, but also on the  region, the geometry of the estuarine basin, and the river flow. The tidal region and river flow may be further altered seasonally. In general, positive estuaries are common, while mentral estuaries are rare. Galveston Bay, Texas, and Alligator Harbour, Florida, are examples of mentral estuaries. In desert climates, where the amount of freshwater input to the estuary is small and the rate of evaporation is high, a negative (or) evaporite estuary results. In a negative estuary, the incoming salt water enters at the surface and is some what diluted by mixing with the small amount of freshwater. The high evaporation rate, however, causes this surface water to become hypersaline . Hypersaline water is denser than seawater, sinks to the bottom and moves out of the estuary as a bottom current. A salinity profile of such an estuary is the reverse of the positive estuary, with highest values at the bottom and lowest at the top. A final category is the seasonal (or) intermittent estuary These estuaries are formed in areas where there is a marked wet and dry season (Mediterranean climate). In the rainy season, they have freshwater input and are open to the ocean. In the dry season, they have no freshwater input, may become dry or stagnant, and are often cut off from the ocean by seasonal sandbars. Salinity in these  estuaries varies not spatially but temporally.