Wetlands may remove between 70% and 90% of entering nitrogen. The estimated
mean retention of phosphorus by wetlands is 45%.
Removal of biological
oxygen demand from surface water
Biological oxygen demand (BOD) is a measure of the oxygen required for
the decomposition of organic matter and oxidation of inorganics such
as sulfide. BOD is introduced into surface water through inputs of organic
matter such as sewage effluent, surface runoff, and decomposition. If
BOD is high, low dissolved oxygen levels result. Low dissolved oxygen
levels can lead to mortality of aquatic life. Wetlands remove BOD from
surface water through decomposition of organic matter or oxidation of
inorganics. BOD removal by wetlands may approach 100%.
Removal of suspended
solids and associated pollutants from surface water
Suspended solids (such as sediment and organic matter) may enter wetlands
in runoff, as particulate litterfall, or with inflow from adjacent water
bodies. Sediment deposition in wetlands prevents a source of turbidity
from entering downstream ecosystems. Typically wetland vegetation traps
80-90% of sediment from runoff. Less than 65% of the sediment eroded
from uplands exits watersheds that contain wetlands.
Removal of metals
Certain wetlands play an important role in removing metals from other
water resources, runoff, and ground water. Wetlands remove 20% - 100%
of metals in the water, depending on the specific metal and the individual
wetland. Forested wetlands play a critical role in removing metals downstream
of urbanized areas. A study of wetands and water quality in the Twin
Cities metro area showed that lakes in watersheds with less than 18
% wetland area are predicted to have levels of lead that exceed EPA
aquatic life criteria for this metal. (Detenbeck et al 1991)
Removal of pathogens
Fecal coliform bacteria and protozoans, which are indicators of threats
to human health, enter wetlands through municipal sewage, urban stormwater,
leaking septic tanks, and agricultural runoff. Bacteria attach to suspended
solids that are then trapped by wetland vegetation. These organisms
die: after remaining outside their host organisms, through degradation
by sunlight, from the low pH of wetlands, by protozoan consumption,
and from toxins excreted from the roots of some wetland plants. In this
way wetlands have an important role in removing pathogens from surface
Wetlands act as reservoirs for the watershed. Wetlands release the water
they retain (from precipitation, surface water, and ground water) into
associated surface water and ground water. In Wisconsin watersheds composed
of 40% lakes and wetlands, spring stream outflows from the watersheds
were 140% of those in watersheds without any wetlands or lakes. Forested
wetlands, kettle lakes and prairie potholes have significant water storage
and ground water recharge.
Ground water can
be adversely affected by activities that alter wetlands. Drainage of
wetlands lowers the water table and reduces the hydraulic head providing
the force for ground water discharge. If a recharge wetland is drained,
the water resources into which ground water discharges will receive
less inflow, potentially changing the hydrology of a watershed
Wetlands help protect adjacent and downstream properties from potential
flood damage. The value of flood control by wetlands increases with:
(1) wetland area, (2) proximity of the wetland to flood waters, (3)
location of the wetland (along a river, lake, or stream), (4) amount
of flooding that would occur without the presence of the wetlands, and,
(5) lack of other upstream storage areas such as ponds, lakes, and reservoirs.
and upstream of urban areas are particularly valuable for flood protection.
The impervious surface in urban areas greatly increases the rate and
volume of runoff, thereby increasing the risk of flood damage. The drainage
of wetlands, the diversion of the Mississippi and Missouri Rivers from
their original floodplains, and the development allowed in the floodplains
over the past 100 years were partly responsible for the billions of
dollars in damage to businesses, homes, crops, and property that occurred
as a result of the Midwest flood of 1993.
Marshes located at the margin of lakes protect shorelines and streambanks
against erosion. Wetland plants hold the soil in place with their roots,
absorb wave energy, and reduce the velocity of stream or river currents.
Fish and wildlife
Diverse species of plants, insects, amphibians, reptiles, birds, fish,
and mammals depend on wetlands for food, habitat, or temporary shelter.
Although wetlands make up only about 3.5 percent of U.S. land area,
more than one third of the United States' threatened and endangered
species live only in wetlands. An additional 20% of the United States'
threatened and endangered species use or inhabit wetlands at some time
in their life.
Many of America's
bird species utilize wetlands as sources of food, water, nesting material,
or shelter. Migratory waterbirds rely on wetlands for staging areas,
resting, feeding, breeding, or nesting grounds.
Up to one-half of
North American bird species nest, feed, or rest in wetlands. As our
wetlands have been destroyed, bird populations have slowly declined.
Nearly half of all federally threatened and endangered species rely
on wetlands. A majority of fish and many species of amphibians, insects
and plants are wetland dependent.