Human Impacts
and Management of Wetlands
The
concept of wetland management has evolved from the destruction of wetlands as
the primary management tool to the preservation and restoration of wetlands as
it is practiced now.
Early history of wetland management
Wetland
drainage has been the primary management technique applied to wetlands in North
American since colonial times. The
common wisdom, supported by the professions of engineering, public health, and
agriculture, has said that wetlands have more value when they are drained.
Wetland alteration
One
model of wetland alteration (Fig.17-1) assumes three main factors influencing
wetland ecosystems: water level,
nutrient status, and natural disturbances.
Modification
of any of these directly can lead to a modification of the other two
indirectly.
Most
common wetland conversions:
A.
Draining, dredging, filling
The
most common reason for wetland loss is conversion to agriculture. This happened early in the
Most
rapid recent conversion has been the elimination of the bottomland forest along
the Mississippi River floodplain. It has
been leveed for flood protection and converted to row crops. Soybean crops that
could be planted late (June or July), and miss the annual floods, were a boon
to this effort.
On
the coasts, conversion to urban and industrial development has caused the
greatest wetland conversion.
B.
Hydrologic modifications
1. Flood control
Canals
have generally been constructed to convey water out of wetlands quickly, rather
than as the natural sheet flow. Levees
have been constructed to keep flood waters confined to river channels rather
than distribute themselves across wetlands as they do in a natural system.
2. Navigation and transportation
Navigation
canals are larger and deeper than drainage canals. They have been constructed primarily to carry
long-range barge traffic
3. Industrial activity
Many
canals are dredged to provide access to sites for oil or gas wells, refinery
sub-units, to lay pipelines directly into without backfilling, or for other
kinds of development.
Highway
construction causes an alteration in the hydrology, elimination of sediment nourishment,
direct loss of habitat for the right-of-way, loss of habitat in adjacent areas
that are dredged to provide fill.
The
least intrusive highway is one on pilings, because it will cause minimal
disruption of water flow, which carries with it nutrients, fresh or salty
water, sediment, detrital export, etc.
Highways on pilings are very expensive.
D.
Peat mining
Northern
countries have most of the world’s peat (half is in
E.
Mineral and water extraction
Phosphate
mining in central Florida. Surface
mining of coal in Appalachia. Water withdrawal around
F.
Water pollution
Agricultural
eutrophication can lead to simplification of natural systems and spread of
invasive, weedy species. Ag runoff into
Kesterson NWR in
Wetland management by objective
A set
of common goals for wetland management might include:
1.
Maintain water quality
2.
Reduce erosion
3.
Protect from floods
4.
Process airborne pollutants
5. Provide a buffer between wetlands and
adjacent urban or industrial uses.
6. Maintain a diverse community of wetland
plants and a diverse gene pool.
7.
Provide aesthetic and psychological support.
8.
Produce wildlife.
9.
Control insect populations.
10.
Provide habitats for fish and other organisms
11.
Produce food, fiber and fodder.
12.
Expedite scientific enquiry.
Wildlife
enhancement
Best
management practices enhance the natural processes of the wetland
involved. This would involve maintaining
natural hydrology, native vegetation, local and migratory wildlife habitats.
Marsh management
Water level manipulation is a very
common management technique in flat landscapes where there is much water. It is described extensively in the literature
for the north-central U.S. and south-central Canada (prairie potholes), and for
coastal Louisiana. It is essentially
forbidden by governmental policy in
Managing the (animal) managers
Some animals are drawn to marsh
environments and have an impact.
Included are beaver, muskrat, nutria and geese. They have all had a long-term relationship
with wetland ecosystems, and the systems may have evolved under the pressures
of their impacts. If constraints on
their number are totally gone now, then some population control may be
necessary.
Agriculture
and aquaculture
Agriculture
that actually works with the natural cycles of coastal or inland wetlands has
been practiced for millennia. Examples are
double cropping of rice fields for rice, then crawfish or finfish. In
Current
practice is to dredge natural systems and create an industrial environment for
the production of shrimp or other marketable seafood.
Water
quality enhancement
Natural
wetlands may be managed for water quality improvement. See lecture on the last class day for
elaboration of this process.
Flood
control and groundwater recharge
Wetlands
may, in different ways and with different efficiencies, impound water and
recharge aquifers or abate flood damage.
Managing wetlands in changing
climate
Coastal
wetlands
With
predicted sea level rise of 0.5 to 2 m per century for the next century, many
coastal wetlands will be lost. Accretion
rates are high, however, in many coastal wetlands (they must have sediment
inputs or will disappear), so the question is whether the normal accretion
rates will be adequate. For some systems
they will, for others they will not.
Inland
wetlands
Melting
of northern permafrost tundras will probably have the greatest effects. Decreases in precipitation and increased
aridity will cause many depressional wetlands to either no longer be wet, or to
become intermittent wetlands.