622 LIMNOLOGY
In addition to controlling aquatic plant biomass, drawing
down the water level makes it possible to use several other
lake restoration or improvement procedures. For example,
water level drawdown can be used for fish management to
repair structures such as docks or dams, to facilitate local-
ized dredging or bottom barrier placement, or to remove
stumps or debris. This technique can result in compaction
of certain types of sediments, such as mucky substrates and
thus improve shoreline use. Drawdown can reduce nearshore
vegetation, thereby reducing potential inputs of nutrients to
the water from seasonal dieback of aquatic plants. Drawdown
can also be used to attract waterfowl by enhancing growth of
emergent plants such as cattails and bulrushes.
A disadvantage of water level drawdown is that it is not
species-selective; hence beneficial plants may be removed
along with nuisance plants. Wetlands adjacent to the lake can
be exposed with adverse impacts on their plant and asso-
ciated animal communities. Prolonged freezing and drying
can kill benthic infauna that are important food sources for
planktivorous fish. Lowering the water level in a lake may
result in decreased levels of dissolved oxygen, with result-
ing negative impacts on fish and other aquatic biota. During
the period of drawdown, recreational use of the lake may be
limited or unavailable.
A sixth physical plant-control method is the application
of dark-colored dyes to reduce the amount of light reaching
the submersed plants, thereby shading the plants from the
sunlight needed for photosynthesis. Several commercial dye
products are available; they impart a blue color to the water.
Best results are obtained when the dye is used early in the
growing season.
Advantages of using water column dyes for aquatic
plant control are that no special equipment is needed for
application (the dye can be poured into the water by hand
from shoreline or boat) and the dyes are non-toxic to aquatic
organisms, livestock, and humans. However, there are sev-
eral drawbacks. Water column dyes can be used only in
shallow water bodies with no outlet and are less effective
when aquatic plant growth is within two feet of the lake sur-
face. Repeat dye treatments may be necessary throughout
the plant-growing season. Water column dyes should not be
applied to lakes used for drinking water.
Another method for reducing aquatic plant biomass in a
lake is rotovation (bottom derooting or underwater bottom till-
age). Rotovators use underwater rototiller-like blades to uproot
aquatic plants. The rotating blades turn seven to nine inches
deep into the lake bottom to dislodge plant roots. Plants and
roots may then be removed from the water using a weed rake
attachment to the rototiller head, a harvester or manual col-
lection. This technique is most suitable for use in larger lakes
because of the larger size and high costs of the equipment.
Rotovation can be used year-round but is most effective
in the winter and early spring when plants have died back.
Depending on plant density and sediment type, two to three
acres per day can be rotovated. Rotovation is particularly
effective in controlling Eurasian watermilfoil and can pro-
duce a high level of milfoil control for two to three growing
seasons.Advantages of rotovation are that it can remove the
entire plant, can decrease density of undesirable plants, and
may stimulate the growth of some desirable native plants.
Rotovated areas in the state of Washington and the province
of British Columbia have shown increases in species diver-
sity of native plants, with resulting benefits to fisheries. Fish
are not removed through rototilling as they are by mechani-
cal harvesters. Since rotovating takes place during winter
and early spring, there is no interference with peak summer-
time lake recreational activities.
On the other hand, rotovation is limited to areas with
few bottom obstructions and should not be used where water
intakes are located. Bottom sediments are disturbed which
can result in short-term impacts on water quality and ben-
thic infauna. Plant nutrients or toxic chemicals in the sedi-
ments may be released into the water. Since rotovation is
not species-selective, beneficial plants may inadvertently
be removed. Rotovation may also interfere with fish spawn-
ing or migration. Some rotovators are difficult to maneuver
around docks and in shallow water. Plant fragmentation
resulting from rotovation may increase the spread of inva-
sive weeds like milfoil.
Another physical method for controlling aquatic plant
growth in lakes is diver-operated suction dredging. This
method was used in the late 1970s in British Columbia as an
improvement to hand removal of sparse colonies of Eurasian
watermilfoil. Scuba divers operate portable dredges with
suction heads to uproot and remove individual plants from
the lake sediment. After the divers physically remove the
plants with sharp tools, the plant/sediment slurry is then suc-
tioned up and carried back to a barge through hoses oper-
ated by the diver. Plants parts are sieved out on the barge
and retained for later off-site disposal. The water-sediment
slurry can be discharged back to the water or piped off-site
for upland disposal.
Efficiency of plant removal is dependent on sediment
condition, density of aquatic plants, and underwater visibil-
ity. Diver-operated suction dredging is best used for localized
infestations of low plant density where plant fragmenta-
tion must be minimized. Therefore, this technique has great
potential for milfoil control and can remove 85 to 97 percent
of milfoil from a lake.
Advantages of diver-operated suction dredging are that
it is species-selective and site-specific. Disruption of sedi-
ments and plant fragmentation are both minimized. The
method can be used to cover areas larger than practicable
for hand pulling or cutting, and can be conducted in tight
places or around obstacles that would preclude use of larger
machinery.
Drawbacks to this method are that it is labor-intensive
and expensive. The usefulness of this method may be reduced
in dense plant beds. Returning the water-sediment slurry
directly to the lake may result in some loss of plant fragments.
If the dredged slurry is disposed of upland, more specialized
equipment and materials are required and the process is much
more costly. Short-term environmental impacts include local-
ized increases in turbidity and release of nutrients and other
contaminants from the sediments. Some sediments, benthicC012_002_r03.indd 622C012_002_r03.indd 622 11/18/2005 10:36:48 AM11/18/2005 10:36:48 AM