Environmental Engineering Reference
In-Depth Information
lished but uptake and storage of nutrients by microbes growing on the
straw is a possible explanation. Finally, alterations of the food web (top-
down control) have been advocated to control algal blooms. This method
will be discussed thoroughly in Chapter 19.
Macrophyte Removal
One symptom in eutrophication of shallow lakes is excessive growth
of macrophyte vegetation (Chambers et al., 1999). Some macrophyte
growth is a healthy part of aquatic ecosystems; the plants provide habitat
for other desirable species (e.g., fish) and stands of macrophytes can pre-
vent unwanted sediment suspension (Bachmann et al., 1999). However,
macrophytes can interfere with recreation, clog water flow structures, lead
to low O 2 conditions, and cause taste and odor problems. Thus, removal
of macrophytes is desirable at times. Physical, chemical, or biological con-
trols can remove macrophytes (Table 17.3).
Physical control methods include direct harvesting, sealing aquatic sed-
iments with plastic to prevent establishment of rooted macrophytes, shad-
ing, and alternation of water level (dry down; Wade, 1990). The lack of
ability to withstand freezing is used to control macrophytes in some tem-
perate zone reservoirs. Water levels can be drawn down during freezing
weather, killing some species (Murphy and Pieterse, 1990).
Chemical control methods require application of herbicide. Preferable
herbicide properties include a limited lifetime in the water, toxicity pri-
marily to target plants, and no bioconcentration in the food web. In gen-
eral, physical methods are more expensive than chemical methods (Mur-
phy and Barrett, 1990). Both methods often require repeated use because
the macrophytes can recolonize the area.
Biological control methods include use of fungi, insects, and herbivo-
rous fish. Selective organisms are the most desirable agents of biological
Methods for Controlling Macrophytes a
TABLE 17.3
Method
Explanation
Positive aspects
Negative aspects
Physical
Dredge or cut out
Removal rapid; high
Costly; ineffective in large
macrophytes
control on area treated
systems; needs to be repeated
Chemical
Apply herbicides
Removal rapid; moderate
May affect nontarget species;
control on area treated
needs repeated application;
biomagnification of toxins
Sealing sediment
Seal sediments with plastic Also keeps phosphate from
Expensive, not effective in large
film to stop macrophyte
reentering lake, thus
systems; plastic lake bottom
establishment
inhibiting phytoplankton
may be aesthetically
blooms
unpleasant
Biological control
Find organisms that
Inexpensive, lasting control
May eat desirable species; may
specifically graze
become pest species; may be
macrophytes
impossible to eradicate after
introduction
a After Cooke et al. (1993). Whole-lake control of nutrients (Table 17.2) can be useful in controlling excessive
macrophytes.
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