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Water Passage Where The Tide Meets A River

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According to the Merriam-Webster dictionary, an estuary is defined as a water passage where the tide meets a river current; especially: an arm of the sea at the lower end of a river. In other words, it is where the freshwater from the river and the salty ocean water meet.
Estuaries can be looked at in five different ways. The first view is the top view. When the top view is seen, one end of the river entering a large bay and on the other side, a small barrier island separates the ocean from the bay.
The second view of an estuary is the cross-section view. With the cross-section, the body of water is vertical as well as horizontal. Starting with the intertidal zone, it is the part that is exposed and can be seen in the salt marsh or
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From looking at the estuary from this position, the salinity gradient can be seen. Looking at the length of the estuary, it is noted that as the estuary gets deeper, the salinity increases. Fresh water is less dense than salt water, so normally it can be seen in the river water flowing on top of the seawater. Then as the river water flows further into the ocean, deeper salt water mixes with it making the river water saltier. River water that flows into the ocean causes more salty water to enter into estuary causing a net inflowing current. In addition with longitudinal section, the euphotic and aphotic zones are valued. The euphotic zone is the top portion of the estuary where most of the freshwater is located. The bottom portion where most of the ocean water is found is called the aphotic zone. Organisms that can cross this water column include organisms as little as bacteria and as big as seals and dolphins.
The fourth way to look at estuaries is by a food web. By looking at the food web this aids in understanding what kind of organisms live in the estuaries. The food web with the primary producers is present. The primary producers can include any range of life from phytoplankton, benthic algae, submersed sea grasses, and mangroves. The primary producers can decompose and create their own food web called detrital food web. If the primary producers do not decomposed, they are grazed on by zooplankton.
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