Estuaries are semi-enclosed bodies of water connecting a river (or rivers) to the sea. The marine water from the sea mixes with the river water in addition to the run off from surrounding land and the atmosphere, making this an area of many different habitats.
Because water flow through rivers can vary with seasons, there may be times when estuaries are fully marine or fully freshwater areas and often a mixture of both. There are a variety of different estuary types.
The five main structural classification of estuaries in Australia include:
- Drowned River Valleys
- Barrier Estuaries
- Saline Coastal Lagoons
- River Estuaries
- Coastal Embayments
Drowned River Valleys
Drowned river valleys can be found in south-eastern Australia and are the largest type of estuary. The structure of drowned river valleys is narrow width with steep sides and deep channels. They experience moderate tidal range and minimal wave action. Salinity generally declines upstream, and sediment particles are moved downstream.
Barrier Estuary
If currents aren’t strong enough, a build up of silt and sand near the entrance to the sea will occur. This can slow down tidal currents and result in a barrier forming across the entrance. This type of estuary then becomes known as a barrier estuary. Barrier estuaries are characteristically shallow in depth, have a narrow inlet to sea and poor water clarity.
There is usually a decrease in salinity at the entrance and water level doesn’t change much. Barrier estuaries can be further subdivided into seasonally closed barrier estuaries and permanently open barrier estuaries, ones that are closed during dry months and those that are open throughout the whole year, respectively.
Saline Coastal Lagoons
If the entrance becomes closed for long periods of time, then estuaries become saline coastal lagoons. Eventually river input increases the water level and dilutes the salt content, until either river water or seawater overflows the bank and creates a channel to the sea once again. In very hot climates, saline coastal lagoons can become salt-pans due to evaporation of water.
River Estuary
River estuaries are a result of rivers in areas of high rainfall and low tidal range, mixing offshore with the sea due to the high velocity of which the river water runs.
Coastal Embayments
Lastly, coastal embayments, have a very diluted salinity, so much so that they closely resemble the habitats in sheltered coastal bays that lack freshwater flow. Thus these embayments are usually home to predominantly marine species.
Estuarine Conditions
Due to changing water flow patterns, estuaries are one of the most unstable habitats, and they can change from mostly marine to mostly freshwater during seasonal changes, and in drowned river valleys for example, the fluctuations can occur daily, so species need to have adaptations to deal with these changes.
In most estuaries, the number of marine species is normally greater then the estuarine species numbers, with fewer freshwater species. The are many factors that can influence salinity in an estuary, including temperature, gases and the depth and width of the estuary as well as tidal changes. Tides and the depth of the estuary affect the temperature.
During the day the sun warms the water and in the evening the water from the sea and rivers cools it. In deeper estuaries, the bottom is cool and the sun warms only the top layer.
The oxygen level in estuaries can vary also, depending on tidal effects and biological activity. If it is highly stratified, the activity at lower levels can deplete oxygen levels and if there is minimal mixing of water, oxygen wont be replenished as well.
Due to the unstable nature of estuaries, animals and plants have various adaptations to deal with these harsh changes.
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