Wet-Dry Cycle of Marshes
As described by the Van der Valk and Davis model, the wet-dry cycle of marshes is an integral part to wetland health. Marshes in nature typically go through wet and dry stages. With these wet and dry stages, there are continually changing seasonal water levels, which influence the amount and types of vegetation present, and in turn the amount and types of wildlife attracted to the ecosystem, as well as the ecological goods and services derived from the wetland.
Dry Marsh
_ The dry-marsh stage occurs as a
result of a drought or induced drawdown. With the
absence of water, both terrestrial and emergent vegetation have an opportunity
to reestablish by seed germination or rhizomal growth. Terrestrial
vegetation may include those plants such as willows, grasses and sedges, and
emergents include typical wetland vegetation such as phragmites, cattail and
bulrush. During this stage, some species of aquatic
invertebrates may leave the area and recolonize elsewhere; others may go into a
dormant, drought-tolerant stage, whereas the rest without drought-resistant
capabilities will expire.
Regenerating Marsh
_ As water refills the depression, the
wetland regenerates out of the dry-marsh stage. Terrestrial
vegetation is drowned out and emergent plant species begin to thrive. With standing water, submerged vegetation will begin to colonize, along
with aquatic invertebrates. The marsh moves into a state of
high productivity in both the algal and macrophyte communities, and more fauna
are attracted to the wetland. Waterfowl are attracted to areas
with an abundance of emergent vegetation as it blurs the sight lines between
territorial nesting pairs, which allows for a higher density of birds in an
area.
Degenerating Marsh
_
Standing water will eventually cause
a reduction in the amount of emergent vegetation in the basin of the marsh and
can be replaced by submerged vegetation. With the lack
of emergent vegetation, the amount of dissolved oxygen in the water will also
decrease. There is an increase in benthic invertebrates,
those that live in the sediment and are tolerant of poor water quality. At this point, waterfowl numbers are beginning to decrease because of
the lack of diversity of invertebrate as food sources under water, and lack of
cover above the water.
Lake Marsh
_ Prolonged periods of standing water
will drive a wetland into a lake-marsh stage. At this point,
there is a general decrease in plant matter; emergent vegetation is confined to
the edges of the marsh area and submerged vegetation begins to disappear from
the deeper parts of the basin. There can be an increase in
phytoplankton depending on the nutrient availability and the most common
invertebrates are those living in the benthos. Some types of
waterfowl are still attracted to the area, mostly diving and piscivorous waterbirds.
Hemi-Marsh
_
Somewhere
in between the dry and lake-marsh stages, there is an ideal state which
optimizes both vegetation and wildlife in a wetland. A hemi-marsh
condition is where the wetland characteristically has a relatively even
interspersion of open water and emergent vegetation. This
hemi-marsh state is ideal for any plan wishing to manage for wetlands.
_ENVR 4000 Sustainable Water Management 2012