ESTUARY AND TIDES

Gwyn Michael M. PadillaKatrina Chari Estavas

Rio dela plata estuary

Yachats river Estuary Mouth

Estuaries - Where Rivers Meet the Sea

The word “estuary” is derived from the Latin word aestuarium meaning tidal inlet of the sea, which in itself is derived from the term aestus, meaning tide.

An estuary is a partially enclosed body of water where incoming seawater is mixed with fresh water coming from the land. Examples of estuaries include bays, sounds, salt marshes, mangrove forests, mud flats, swamps, inlets, and sloughs.

Estuaries are divided into four types, depending on how they are formed:

Estuary Mouth

Coastal Plain Estuaries are formed by the sea level rising and filling an existing river valley. Examples of this are the Chesapeake Bay in Maryland and the harbor in Charleston, South Carolina

 Tectonic Estuaries are caused by the folding or faulting of land surfaces. These estuaries are found along major fault lines, like the San Francisco Bay area in California.

Bar-built Estuaries form when a shallow lagoon or bay is protected from the ocean by a sand bar or barrier island. Examples of these are found along the Eastern Seaboard and the Gulf Coast of North America.

Fjords are U-shaped valleys formed by glacial action. Fjords are found in areas with long histories of glacier activity, like northern Europe, Alaska and Canada.

Life in an Estuary

River Exe Estuary

Life in an estuary is an interesting and diverse mix of land and sea creatures, and some animals.

Mammals, birds, fish, reptiles, shellfish, and plants all interact in this ecosystem to create some very complex food webs. Birds are very common in estuaries because of the abundance of fish, worms, crabs and clams.

Many marine organisms depend on estuaries at some point during their development.

Some fish only use estuaries at certain times of the year, while others use the natural protection for the laying of eggs. Most commercially valuable fish and shellfish spawn, nurse, or feed in estuaries.

Estuaries are home to an astonishing variety of plants and animals and important in the lifecycles of many more. Crabs and clams of many species call these fresh and saltwater mixing zones home, each finding the salinity (saltiness) that suits them best.

Filter feeders are found in all parts of the estuary. They are creatures that pull small bits of organic material, like plankton and larvae, from the water as it moves past them. Horseshoe crabs are one of the older inhabitants of coastal estuaries, living happily and largely unchanged for millions of years.

 

Oysters blanket the estuary floors in areas where they use foot secretions to cement themselves to the substrate. Not only are these oysters a rich food source for us, but they are also important food sources to many estuarine creatures.

Predatory snails, including Oyster Drills, whelks, sponges, especially the Boring Sponges, and fish all find oysters a tasty treat

The Black Drum has jaws powerful enough to crush adult oyster shells.

"Tides" Refers to the alternating rise and

fall of sea level within a day.

What causes the sea level to change?

It is actually the gravitational attraction of the sun and moon that cause waters of the ocean to swell and recede at different parts of the earth.

Influence of Tides Like other coastal communities,

estuaries are dramatically influenced by tides. During the day time, when the tide is out, many aquatic creatures retreat into protective postures. Clams can close their shells, worms stay underground, while other creatures sleep

A Water Filtration System

Rivers often contain lots of sediment, nutrients and pollutants. Estuaries remove sediments and nutrients before they reach the ocean. Otherwise, valuable top soil and nutrients would be flushed into the open seas where they could not be used again.

Flood & Storm Control

Estuaries act as natural buffers between the land and the ocean. Porous, resilient salt marsh soils and grasses absorb flood waters and dissipate storm surges.

Like barrier islands, they protect the mainland and people from the brunt of heavy storms. Estuaries help protect human lives, upland animals, and real estate.

The Moon Tide

The earth and the moon are two great masses that have a significant gravitational pull on each other.

 

Picture the earth with a uniform level of water all around it. The moon’s gravity pulls on the earth, and pulls the water towards it

Semidiurnal tides 

As the earth turns upon its own axis in about 24 hours, a point on the earth moves through areas with these different forces acting on it.

In one rotation (one day), a point on earth travels from an area of high tide (where there is a force pulling water outward), through an area of low tide, through an area of high tide again (the opposite pull), and through another area of low tide, before it returns to the point of origin at high tide.

The Tidal Day

The moon does not stay put, but rotates around the earth at a rate of about 12° a day, or one rotation a month.

The rotation is in the same direction as the earth’s spin, so by the time the earth has done one rotation, the moon has shifted 12° further, and it takes an extra 50 minutes for the moon to be in the same position relative to a point on the earth.

Therefore, the tidal cycle is not 24 hours long, but 24 hours and 50 minutes. Because of this, high and low tides are about 50 minutes later every day.

The Sun Tide

The tides are caused mainly by the gravitational attraction of the moon and the earth, but there is also a gravitational attraction between the earth and the sun.

The effect of the sun upon the tides is not as significant as the moon’s effects. Basically, the sun’s pull can heighten the moon’s effects or counteract them, depending on where the moon is in relation to the sun.

spring tide.

When the moon is between the sun and the earth (at new moon), the sun’s gravitational pull is in the same direction as the moon’s. During these days the high tides are higher and the low tides are lower than they'd be with just the moon’s pull alone.

neap tides When the moon is in its first quarter or

its last quarter, the sun’s gravitational pull is in perpendicular direction to that of the moon.

The sun pulls water away from the areas of high tide to the areas of low tides, resulting in lower high tides and higher low tides. These are called neap tides.

Inclination

The moon does not rotate around the earth’s equator, but follows an orbit that is inclined in relation to the earth’s axis.

Because of this, northern and southern latitudes commonly face only one high tide and one low tide in a day, called diurnal tides. The inclination of the moon changes in relation to the earth on a 19 year cycle.

Tides in estuaries

The dominant influence on the tides in estuaries is the change of water depth and estuary width as the tide propagates up the estuary.

Effects produced by sudden changes in depth, as discussed above in the context of shelf seas, occur in estuaries as well, so the phase relationship between tidal currents and high or low water can vary strongly along estuaries.

River Nith Estuary

References: http://en.wikipedia.org/wiki/Estuary http://www.mbgnet.net/salt/sandy/estua

ries.htm http://www.onr.navy.mil/focus/ocean/ha

bitats/estuaries_quiz.htm http://oceanlink.island.net/oinfo/tides/ti

des.html http://library.thinkquest.org/27728/tides

.html http://gyre.umeoce.maine.edu/physicalo

cean/Tomczak/ShelfCoast/chapter05.html

http://www.youtube.com/watch?v=-mBvBdGTkR8&feature=related