Brackish water is a fairly salty mixture of freshwater and sea water. It is
unique in numerous ways and is a life giving ecosystem. To understand what
brackish water is, a background should be known about its sources.
First of all, there is freshwater. Out of all the water on the planet, only three
percent of it is fresh, and only one fiftieth of one percent is readily available.
Freshwater is not pure in that it contains minerals and other particles. There are
numerous plants and animals that depend on freshwater for their lives. Humans are
one of them. Seventy percent of the human body is made up of freshwater (744
Headlam).
Next, there is saltwater, or seawater. Seawater consists of fifty-five percent
chlorine, and thirty-one percent sodium (Groliers). It makes up approximately
ninety-seven and two tenths percent of the total volume of the world's water, and
covers more than seventy percent of the earth's surface (Groliers). Sea water
doesn't just contain hydrogen, oxygen, sodium, and chlorine; it also contains every
naturally occurring element. Although seawater has a fairly constant ratio of major
elements, salinity and seawater can fluctuate. Normally, the salinity is thirty-four to
thirty-seven parts per thousand (ppt.), but on a particularly rainy morning, the
salinity
may decrease to something as low as thirty-two ppt. (Stuller 29).
The mixing of freshwater and sea water forms a third type of water, known as
brackish water. Brackish water can be found in a variety of mixing zones such as
river deltas, freshwater title marshes, estuaries, fjords, and in the middle of the
ocean
(Stuller 30). To begin with, freshwater traveling towards the sea carries suspended
particles. As the particles make contact with saltwater, an electrochemical reaction
called flocculation takes place. The clay with a positive charge, and the sodium
chloride with a negative charge, combine and form a heavier particle that descends
to the bottom of the mixing zone and creates a layer of mud (Stuller 28). This is one
of the reasons that a mixing zone is a thriving ecosystem. A salt wedge estuary is
one of these transitional zones. The water closest to the surface has a lower salinity
content than the water nearest to the estuary's floor because the water is not
thoroughly mixed and saltwater has a higher density than freshwater (Stuller 31).
Another zone is a fully mixed zone which is apparent in fjords. Also, there are zones
in the middle of the ocean called either submarine springs, or seeps. These are
where freshwater from under the ground seeps through the ocean floor. There is
also a parallel situation, in which salt water can seep into a freshwater ecosystem,
but this can be fatal to many animals (Stuller 31). Transitional zones are homes for
all kinds of marine life, such as manatees in Charlotte Harbor.
The most prevalent mixing zones are estuaries. Estuaries are drowned
costal river valleys where salt and freshwater are present to form brackish water
(Groliers). The circulation in estuaries is stratified with river water flowing above
sea
water with some vertical mixing taking place (Groliers). As indicated earlier,
sediments accumulate at the upper reaches of estuaries because of flocculation.
This in turn spikes diatom growth, diatoms are eaten by possum shrimp, and
possum shrimp are eaten by striped bass. Some other common fish, present in
brackish water ecosystems are mullet and silversides (Groliers). Thus, the food
cycle continues.
Brackish water has some very interesting qualities. It represents unity of two
very diverse ecosystems. The mixing of seawater and freshwater is extremely
beneficent at the right time in the right place, but in the past it may have been
harmful. Scientists speculate that an ice age may have been triggered by freshwater
occupying a location where saltwater was supposed to have been. There is a deep
ocean current of very salty water beneath the gulf stream travelling from the tropics
to the North Atlantic. As it reaches the North, it disturbs surface water, the salty
water rises and discharges heat, and then it cycles back to the tropics. What could
have triggered an ice age, a surge of freshwater (Stuller 33).
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