I absolutely love the written word. Why? Because we can have so many names for the same subject/object and it can confuse those who don’t know all the different words.

For instance, estuaries can also called be bays, sounds, sloughs, or lagoons. Though, a coastal lagoon is different from an estuary, but we’ll get to that in a later post—let’s just agree for now that common names are a blessing and a curse.

Estuary is a very broad term because it includes both fresh and brackish water ecosystems, and there are three types of estuaries: salt-wedge, partially mixed, and fully mixed. Each type of estuary is determined by how the fresh and salt water mix.

Today, we’ll just start with a broad overview of coastal estuaries.

An estuary is an area where fresh water from a large stream or river mixes with the salt water from the ocean. This mixing results in brackish water, a chaotic medium between fresh and salt water. Brackish water is too salty to be considered fresh water, but not salty enough to be sea water.

What do I mean by chaotic? The salinity of an estuary changes with every season, passing day, and cycle of the tides. For instance, at high tide the salinity will be higher than at low tide because there is more ocean water mixing with the river water. During the rainy season, the salinity will be lower because the river/stream will have a lot more fresh water from the land runoff than normal, mixing more fresh water than usual with the ocean water. This chaotic nature makes estuaries interesting ecosystems for organisms to adapt to.

Due to their long, funnel-like shape, most estuaries don’t experience a gradual rise and fall of the tides. Instead, the tides rush into the estuary with such force that they create strong currents and even wall-like waves called tidal bores. With the tide comes offshore sediments that get deposited in the estuary as mud, and with the high rate of sedimentation, this mud builds up and creates special habitats within the estuary. In the tropics, these mud areas are called mangrove swamps; everywhere else, they’re called tidal mudflats and salt marshes—but I’ll discuss these habitats in detail later.

Estuaries are crucial to both the land and the ocean.

Estuaries provide a rich source of food for waders and shorebirds. Various types of migratory birds will stop at an estuary to consume worms and crustaceans that live in the mud before continuing on their way. Estuaries also provide breeding grounds for both land and aquatic animals.

One of the greatest functions of an estuary is that it is a natural water filter, helping to ensure that the water coming from the land is as clean as possible before entering the ocean. The process of filtration is complicated and requires a lot more explanation than I have time for today.

The take-away message for this post: estuaries are highly important to the ocean and the land, and they have recorded high levels of productivity for centuries. A lot of the early settlements of man were found in estuaries, such as the Nile Delta, the Ganges, and the Yellow River valley. Even many of today’s popular coastal centers, like New York City and London, were developed on estuaries. And because they attract us, estuaries are also highly susceptible to pollution and need to be protected.
Ocean the Definitive Visual Guide made by the American Museum of Natural History

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