LAKES AND RESERVOIRS - Water Quality Engineering in Natural Systems - page 280

Figure 7.20. Mechanical harvester. Source : Dirnberger (2005).

Figure 7.19. Grass carp. Source : Wisconsin Department of
Natural Resources (2012).

which macrophytes are able to grow depends on
water transparency and the plant species. Hydrilla,
a nuisance exotic plant in southern waters, can
grow at lower light intensities than can native
plants, limiting the effectiveness of lake deepen-
ing. Reductions in nutrient loads to control eutro-
phication can increase lake transparency,
increasing the depth at which macrophytes can
grow and countering the effectiveness of dredg-
ing to reduce macrophyte growth. Sediment
removal and tilling (e.g., rototilling using cultiva-
tion equipment) can also be used to disturb the
lake bottom, tearing out plant roots for short-
term macrophyte control. Both dredging and
tilling can have negative side effects, including
destruction of the benthic community and an
increase in turbidity and siltation.
Water-Level Drawdown. In lakes where water levels
can be controlled, lake levels can be lowered to
expose macrophytes in the littoral zone to pro-
longed drying and/or freezing. Some species of
plants are permanently damaged by these condi-
tions, killing the entire plant, including roots and
seeds, with exposures of 2-4 weeks. Other plant
species are unaffected or even increase. Water-
level drawdowns in the winter tend to be more
effective than during the summer (Jørgensen
et al., 2005).
Shading and Sediment Covers. Covers can be
placed on the water or sediment surface as a
physical barrier to plant growth or to block light.
Sediment covers made of polypropylene, fiber-
glass, or a similar material can effectively
prevent growth in small areas, such as near docks
and swimming areas, but are generally too

expensive to install over large areas. Applica-
tions of silt, sand, clay, or gravel have also been
used, but plants eventually root in them. Shading
to reduce growth rates can be provided by float-
ing sheets of polyethylene or by planting ever-
green trees along the lakeshore.
Introduction of Grass Carp. Grass carp is an exotic
fish species that feeds on macrophytes; however,
grass carp do not consume all aquatic plant species.
Generally, they avoid alligatorweed, water hya-
cinth, cattails, spatterdock, and water lily. The fish
prefer plant species that include elodea, pond-
weeds ( Potamogeton spp.), and hydrilla. A grass
carp is shown in Figure 7.19. Low stocking densi-
ties of grass carp can produce selective grazing on
the preferred plant species.
Introduction of Insects That Infest Macrophytes.
Several exotic insect species have been imported
to the United States and approved by the U.S.
Department of Agriculture for use in macrophyte
control. Each insect species grows and feeds on
only select target plant species. In particular,
insects have been used in southern waters to aid
in the control of alligatorweed and water hyacinth.
Because insect populations tend to grow more
slowly than the plants, insects work best when
used in conjunction with another plant control
technique (e.g., harvesting or herbicides).
Mechanical Harvesting. Mechanical harvesters con-
structed on low-draft barges can be used to cut
and remove rooted plants and floating water hya-
cinths. A typical mechanical harvester is shown in
Figure 7.20. Cutting rates range are typically in the
range of 0.1-0.3 ha/h (0.25-0.75 ac/h), depending
on machine size. Harvesters can effectively clear