Wetlands:
Marshes are flat areas level with or just above the local water table.
This relationship might be due to runoff, ponded water resources
such as lakes, or proximity to the coast and sea level. Marshes
may be salt, fresh or brackish, which is a mix of salt and fresh
water. Most coastal marshes are either salt or brackish. The image
at left is a of a salt marsh cord grass from the Florida Everglades.
Levels in salinity (concentration of salts) show this degree of
variation due to runoff. Fresh water flowing out from groundwater,
surface flow and discharges from human use all act to dilute the
salts in the system. Marshes also act to trap sediments flowing
in runoff; as such they also act as filters for heavy metals and
other pollutants that collect in flowing water.
Estuaries are interfaces between rivers and the ocean, bodies
of moving water that are generally enclosed on three sides and protected
from the ocean currents and waves. They are flushed by tides, which
mixes the salt water and fresh. Estuaries Some of the larger ones,
such as Chesapeake Bay (which produced the blue crabs shown at right),
are believed to have been formed previous river channels that have
been drowned by rising sea level with the end of the last ice age.
Estuaries, due to their gradient of salt to fresh water, are very
important to species that have portions of their life history spent
in both types of environment. Animals such as crabs, molluscs, shrimp
and fish spend either portions or the majority of their lives within
estuaries. Predators also inhabit the area to take advantage of
the abundance of prey.
Click here
to download a fact sheet about the San Francisco Bay Estuary Field
Station.
 Mangroves
are a woody plant that are able to grow in brackish conditions. An
especially important ability of these plants it their ability to survive
in anoxic soil conditions, or soil that is depleted of oxygen. Oxygen
becomes depleted in these systems by respiration that occurs during
the decomposition process by bacteria. This ability to live in anoxic
conditions allows mangroves to live in areas that are at or slightly
below sea level that have large amounts of biotic (living) activity.
Mangroves act to stabilize the sediment and serve as a nursery for
many economically important species of fish, crabs and shrimp.
Florida
Everglades Marsh and Mangrove Status: A USGS online poster
describing research on mass change of marsh and mangroves in a study
area of the Everglades
Wetlands
Photo Gallery: This gallery covers everything from geology
to plant species and gives information on current research
Florida
Wetlands: A USGS fact sheet about research in the Florida
Coastal Wetlands
South
American Nutria: A USGS site about an introduced beaver-like
rodent and its effects on the Blackwater National Wildlife Refuge,
MD
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Marshes:
