Coastal Lagoons

Lagoons are a special, rather rare class of restricted coastal bays that are separated from the ocean by an efficient barrier that blocks any tidal influx, and they do not have significant freshwater influx from the mainland. Water enters lagoons mainly from rainfall and occasional storm wash-over. Evaporation from the lagoon causes their waters to have elevated salinity and distinctive environments and biota.

most lagoons are elongate parallel to the coast and separated from the ocean by a barrier island or in some cases by a reef. They are most common in dry or near-desert climates, since freshwater runoff needs to be very limited to maintain lagoon conditions. Lagoons are therefore common along coasts including the Persian Gulf, North Africa, southeast Africa, Australia, Texas, mexico, and southern Brazil.

many lagoons show large seasonal changes in salinity, with nearly fresh conditions during rainy seasons and extremely salty conditions as the waters evaporate and even dry up in the dry seasons. Normal marine and estuarine organisms cannot tolerate such wide variations in salinity, so typically large numbers of a relatively few specialized species of organisms are found in lagoons. some species of fish, such as the killifish, can regulate the salinity in their bodies to match that of the outside waters, so they are well suited for the lagoon environment. Certain species of gastropods (snails) are also very tolerant to variations in salinity, and are found in large quantities in some lagoons.

Plate Tectonics Climate

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6 Infobase Publishing

0% Vegetation 100%

6 Infobase Publishing

Block diagram of types of dunes including (A) barchan, (B) transverse, (C) barchanoid, (D) linear, (E) parabolic, and (F) star. Graph (triangular) illustrates which types of dunes form under different conditions of wind, sand supply, and vegetative cover.

As the water in lagoons evaporates in summer months, it deposits chemical sediments known as evaporates and carbonates. These typically include a sequence of minerals from aragonite to calcite to gypsum to halite. Many ancient lagoon environments are recognized by the presence of this repeating sequence of evaporate and carbonate minerals in the rock record.

Lagoons are not significantly influenced by waves or tides, and are dominated by effects of the wind. Winds can induce circulation in lagoons or even waves during windstorms. small wind tides in lagoons may transport more water to one side of the lagoon, and deposit fine-grained sediments on this one side as the waters retreat when the wind dies out. During large ocean storms tidal surges may overtop the barrier to the lagoon, bringing a surge of seawa-ter and sediment into the lagoon. During storms, and during the daily sea breeze cycle, sand from the beach and coastal dunes can be transported into lagoons. This can be a major contributor to sediment accumulation in some lagoons, and in some examples sand dunes from the beach are moving landward into lagoons, migrating over lagoonal sediments and vegetation.

sediments deposited in lagoons include layers of chemical sediments that precipitated from the water as it evaporated, leaving behind the elements initially dissolved in the water as sedimentary layers. These sediments are most commonly fine-grained, clay-sized calcite and aragonite, and micrite, a form of carbonate mud. Many lagoons are covered by mucky micrite layers that have green slimy microorganisms known as cyanobacteria, or blue-green algae, growing along the edges of the lagoon in the mud and forming matlike pads surrounding the central, water-filled part of the lagoon. Many times these mudflats and algal pads are dried out and cracked by the sun, forming thin flakes that can be blown around by the wind. Lagoons also have sediments such as sand grains carried by the wind, and the skeletal and other remains of the organisms that lived in the lagoon. sand washed into lagoons from storms often forms small, fan-shaped bodies known as wash-over fans that cover parts of the lagoon on its seaward side.

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