Mobile Bay Essay, Research Paper
An estuary is where the river meets the sea. Life in the estuary is depending upon
salinity, which decreases from open ocean to the mouths of the inflowing rivers. As salinity
declines, the assortment of life declines, because most estuarine organisms are marine. But they
are high rich ecosystems, accounting for one-half the living matter of the world?s oceans. This
high productivity is a result of a nutrient trap formed by the interactions of tides and inflowing
rivers. Fresh water, lighter in weight than salt water, flows into the estuary on top of a wedge of
inflowing seawater, producing a countercurrent. Nutrients circulate between the upper and lower
layers; strong winds and circulating currents increase the vertical mixing. The countercurrent,
flowing against the waters that move toward the ocean, holds in the estuaries nutrients and
plankton. Nutrients in the seawater of the estuary are taken up by mudflats and salt marshes,
recirculated among the vegetation and associated animal life, and carried back to the estuary by
tidal waters. The sheltering inlets and abundant nutrients of estuaries make them nursery
grounds for a big number of birds, amphibians, and fish. Yet estuaries and salt marshes are being
destroyed continually by pollution, oil spills, and dredging and filling for industrial and
residential growth.
There are three types of estuaries. Vertically homogenous estuaries have a greater tidal
current and lesser river current. Partially mixed estuaries have an equal tidal current and equal
river current. Salt wedge estuaries have a lesser tidal current but has a greater river current.
Most estuaries face similar environmental problems and challenges. Some are over
enrichment of nutrients, pathogens contamination, toxic chemicals, alteration of freshwater
inflow, loss of habitat, declines in fish and wildlife, and introduction of new species. People
think that humans are the only reason that the number of estuaries are going down, but it is not.
There are several problems with nature that are causing problems in estuaries.
The National Estuary Program (NEP) has been trying to fix these problems. Although
environmental results are sometimes slow in coming, signs of improving environmental
conditions are already emerging from the NEP. The 28 National Estuary Programs are also
showing success in finding useful plans to manage their estuaries, securing and leveraging funds,
and improving public education and citizen participation through outreach efforts.
One problem in estuaries is the over enrichment of nutrients. Nitrogen and phosphorus
are necessary for growth of plants and animals and support a healthy aquatic ecosystem. But if
there is too much, they can contribute to fish disease, red or brown tide, algae blooms, and low
dissolved oxygen. The dissolved oxygen is 5 or 6 parts per million in healthy water. But
sometimes a condition called hypoxia occurs, and the dissolved oxygen is less than 2 parts per
million. Sources of nutrients include sewage treatment plant discharges, storm water runoff from
lawns, faulty or leaking septic systems, sediment in runoff, animal wastes, atmospheric
deposition from power plants or vehicles, and groundwater discharges. The excessive nutrients
stimulate the growth of algae. As the algae die, they decay and lower the amount of oxygen in the
water. The algae also prevent sunlight form entering the water. Fish and shellfish are deprived of
oxygen, and underwater sea grasses are deprived of light and die. Animals that used to live on
sea grasses for food or shelter will now leave the area or die. In addition to that, too much algae
growth may result in brown and red tides which have been linked to fish killed, manatee deaths
and negative impacts to scallops.
Another problem in estuaries are pathogens. Pathogens are disease causing
such as viruses, bacteria, and parasites. They can cause a health threat to swimmers, surfers,
divers, and seafood consumers. Sources of pathogens are urban and agricultural runoff, waste
from boats, faulty or leaky septic systems, sewage treatment plant discharges, combined sewer
overflows, recreational vehicles or campers, illegal sewer connections, and waste from pets or
wildlife.
Toxic substances, like metals and pesticides are a concern in the estuarine environment.
These substances enter waterways through storm drains. They also come in through industrial
discharges, runoff from lawns, sewage treatment plants, and from atmospheric deposition.
Bottom dwelling organisms are exposed to the chemicals and may cause a risk to human health if
we consume it. As a result there may be fishery and shellfish bed closures, and consumption
advisories. Factories have dumped DDP, PCBs, and mercury into the estuaries. Now they have
banned the use of some of these toxic chemicals.
Another major problem is habitat loss and degradation. The same areas that often attract
human development also provide essential food, cover, migratory corridors, breeding / nursery
areas for coastal and marine organisms. These habitats also perform other important functions
such as water quality, flood protection, and water storage. Ecosystems can be degraded through
loss of habitat, such as the conversion of a sea grass bed to a dredged material island or through a
change of degradation in structure, function, or composition. Threats to a habitat include
conversion of open land and forest for commercial development and agriculture, forestry,
highway construction, diking, dredging, filling, and damming. All their activities may cause
increases in the runoff of sediments, nutrients, and chemicals. Excess nutrients such as nitrogen
can lead to algal blooms that deplete oxygen and block sun light, killing submerged aquatic
vegetation.
The introduction of a new species is another problem in estuaries. It may result in
unexpected ecological, economic, and social impacts to the environment. New species have
contributed to the termination of some native populations and drastically reduced others, altering
the food web. Over population of some introduced herbivore species has resulted in overgrazing
of wetland vegetation and the result is degradation and loss of marsh. Other impacts are the
changing of watertables, modification of nutrient cycles or soil fertility, increased erosion,
interference with navigation, agricultural irrigation, fishing, recreational boating, beach use, and
possible introduction of pathogens.
Freshwater is an increasing limited resource in many areas of the country. Changes in
natural freshwater inflow can have significant impacts on the health and distribution of plants
and wildlife. Too much or too little freshwater can affect fish spawning, shellfish survival, bird
nesting, seed development, and other activities of fish and wildlife. In addition to changing
salinity levels, inflow provides nutrients and sediments that are important for overall productivity
of the estuary.
The last major problem is the decline in fish and wildlife population. The distribution
and abundance of esturine fish and wildlife depends on factors such as light, turbidity, nutrient
availability, temperature, salinity, habitat, and food availability. Declines have resulted from
fragmentation and loss of habitats and ecosystems, pollution, decreased water quality,
overexploitation of resources, and introduced species.
The National Estuary Program hasn?t found solutions for most of these problems but have
been trying hard. They will spend $400,000 on Mobile Bay to try to protect its health.
BIBLIOGRAPHY
1. Phillips, Robert S. ?Estuaries? Funk and Wagnalls New Encyclopedia. 1982.
2. www.EPA.gov