Environment

What Are The Differences Between A Lagoon And An Estuary?

Although both are partially enclosed bodies of water, there are significant differences between a lagoon and an estuary.

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Estuaries and lagoons share several similarities, and they are often confused because both of them are water bodies, and they are typically coastal features. An estuary refers to a coastal body of water which is partially enclosed having a river or a stream, or several of them flowing into an estuary while at the same time having a free link to the ocean or the sea. They are generally a transition zone between the river environment and ocean environment, and therefore they experience the influence of both the river and the ocean.

On the other hand, a lagoon refers to a shallow body of water which is alienated by a reef or barrier from the larger body of water, and they are common coastal landforms in different regions around the world. There are two types of lagoons, and they include atoll lagoons and the coastal lagoons. The coastal lagoons are often confused with estuaries. An atoll lagoon is normally a circular coral reef or a string of coral islands which surround a lagoon. Atoll lagoons are always much deeper than coastal lagoons and occasionally could have depths of about 65 feet deep. The word estuary has its origins from Latin, and it is derived from the word aestuarium, which has the meaning of a tidal inlet of the sea. It also has the same meaning as the word aestus, which means a tide. The word lagoon has its origin from the Italian language from the word Laguna, which means a shallow body of water, while in Latin the word Lacuna means a pool, gap, or hollow.

Difference Between A Lagoon And An Estuary

There is a significant difference between a lagoon and an estuary regarding their depth. Lagoons are typically shallow, especially the coastal lagoons, whereas estuaries are deeper compared to lagoons and have depths not exceeding 65 feet. Lagoons have a depth which hardly exceeds a few feet, and as a result of the shallowness, they are highly susceptible to variation in wind, evaporation, and precipitation. As a result, a lagoon could have a huge variation in salinity and temperatures. Usually, there is a tidal exchange in a lagoon of freshwater inflows which affects the salinity of the water. Lagoons, therefore, could have water varying from freshwater to brackish water and hypersaline water. Another major difference among the two features is the flow dynamics of the water. With estuaries, the water flows faster, and they are strong, whereas in the lagoons they are relatively shallow, and the water flow is sluggish. In estuaries, there is a unique pattern of flow of water that circulates in the estuary. The lighter water which is less dense flows out of the estuary into the sea or the ocean from the surface while the heavier or dense water, which is saline, flows to the estuary from the ocean or sea near the floor.

Flushing Time

The time it takes for fresh water to exchange with new fresh water in an estuary is known as flushing time. Flushing time in a lagoon vary depending on the type of the lagoon in question. It would take significantly longer for a choked lagoon and takes less time in a restricted lagoon, while it is fast in a leaky lagoon. On the other hand, flushing time in an estuary varies significantly, and it depends on the tide and the inflow of fresh water from upstream.Estuaries would typically have three different zones; the first zone is the point where the river meets the salt water, and in this region, there is more fresh water compared to saltwater. The second zone is nearer to the sea where there is almost an equal amount of freshwater and saltwater, and the third zone is the region where the waters flow into the ocean, and they mainly saline at this zone.

Classification

Lagoons can be classified into three different categories, and they are choked lagoons, restricted lagoons, and leaky lagoons. Choked lagoons typically have a narrow channel towards the sea, and they are formed in regions where waves have high energy. The narrow channels always limit the entry of tides and prevent vigorous mixing of water. In arid regions where high evaporations are experienced, these types of lagoons become permanently or temporarily hypersaline. Examples of choked lagoons include Lagoa dos Patos in Brazil, Lake Songkhla in Thailand, Rekawa and Munde lagoons in Sri Lanka.

Restricted lagoons have several channels to the sea typically more than one which temporarily restricts water exchange. However, in reality, there is a good exchange of water resulting in a net transport of water to the sea. Wind plays a major role in restricted lagoons because surface currents can develop, resulting in mixing of water. Some of the restricted lagoons include Uppar Lagoon which is found in the Eastern part of Sri Lanka, and another one is the Laguna de Terminos found in Mexico.

Leaky lagoons are characterized with wider channels to the sea without any hindrance in water interchange and typically experiences fast water Currents. Leaky lagoons are on the opposite end of the spectrum from choked lagoons. An example of a leaky Lagoon is the Mississippi Sound in the US. Estuaries, on the other hand, could be classified in different ways based on geomorphology, water balance, hydrodynamics, and water stratification.

Where Are Lagoons And Estuaries Found?

Coastal lagoons are found in low-lying coastal areas in different parts around the world, and almost all continents have lagoons except Antarctica. It is estimated that lagoons are found along 13% of all the coastlines in the world, and they are the most expensive in Africa’s coastal areas where they extend for almost 18% of the coastline in the continent. The extent of the coastline occupied by lagoons in other continents is as follows: 17.6% are in North America, 12.22% are in South America, 13.8% are in Asia, 11.4% are in Australia, and 5.3% are in Europe. The longest coastal lagoons in the world are found in the Atlantic and the gulf coast of the US, and it covers the distance of 1,739.8 miles long. Estuaries, on the other hand, are found whenever rivers or streams flow into the sea or ocean. Estuaries are regions with heavy human settlement, and 22 out of the 32 major cities in different parts of the world are located on estuaries.

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