620 LIMNOLOGY
Another approach to reducing the phosphorus concentra-
tion in eutrophic lakes is to dilute the lake water with sufficient
quantities of another water source that is low in phosphorus;
algal cells will be flushed out of the lake at the same time.
When water low in phosphorus is added to the inflow, the
actual phosphorus loading will increase, but the mean phos-
phorus concentration will decrease, depending upon initial
flushing rate and inflow concentration. Concentration will
also be affected by the degree to which loss of phosphorus
to sediments decreases and counters the dilution. Lakes with
low initial flushing rates are poor candidates for this tech-
nique because in-lake concentration could increase unless the
dilution water is essentially devoid of phosphorus. Internal
phosphorus release could further complicate the effort (Cooke
et al. , 1993b; U.S. EPA, 1990).
Flushing can control algal biomass by cell washout;
however, the flushing rate must be near the cell growth rate
to be effective. Flushing rates of 10 to 15 percent of the lake
volume per day are believed to be sufficient (U.S. EPA,
1990).
There are very few documented case histories of dilu-
tion of flushing because additional water is seldom avail-
able, especially water that is low in nutrients. One successful
example is Moses Lake in eastern Washington. Low-nutrient
Columbia River water was diverted through the lake. Daily
water exchange rates of 10 to 20 percent were achieved in this
eutrophic lake. Lake transparency dramatically increased and
algal blooms dramatically decreased (Welch and Patmont,
1980).
For dilution and flushing to be successful, lake outlet
structures must be capable of handling the added discharge.
The increased volume of water released downstream could
have negative effects. Water used for dilution or flushing
must be tested to ensure that no toxics are present before the
water is introduced into the eutorphic lake.
Another in-lake restoration technique is artificial cir-
culation. This eliminates or prevents thermal stratification,
through the injection of compressed air into lake water
from a pipe or ceramic diffuser at the lake’s bottom. The
artificial circulation structure must be designed properly to
ensure an air flow of about 1/3 cubic foot per minute per acre
of lake surface; this is required to maintain oxygen within
the lake. Algal blooms may be controlled through one or
more of the following processes. First mixing of algae to
the lake’s bottom will decrease their time in full light, lead-
ing to reduced net photosynthesis. Introduction of dissolved
oxygen to the bottom of a lake may inhibit phosphorus
release from the sediments (i.e., have the same impact as
hypolimnetic aeration), hence curtailing internal phosphorus
loading. A third possible process is that rapid circulation and
contact of lake water with the air, as well as the introduction
of carbon dioxide-rich bottom water during the initial period
of mixing, can increase the water’s carbon dioxide content
and lower the pH, leading to a shift from blue-green algae
to less noxious green algae. Finally, when zooplankton are
mixed to the lake’s bottom, they are less vulnerable to plank-
tivorous fish. If more of the zooplankton survive, then they
may eat more algae (Cooke et al. , 1993b; U.S. EPA, 1990).Results of artificial circulation have been highly vari-
able. In about half the lakes where this technique has been
attempted and where temperature differences are small
between surface and bottom waters during the summer,
algal blooms have been reduced. In other cases, phospho-
rus and turbidity have increased and water transparency has
decreased (U.S. EPA, 1990).Management of Aquatic MacrophytesThe watershed BMPs outlined above for reducing the quan-
tity of algae in a lake are also effective in reducing the
quantity of aquatic macrophytes in a lake. Watershed BMPs
involve voluntary changes in behavior and are easy and
inexpensive to implement. Any reduction in nutrient load-
ing to a lake as a result of BMPs can maintain or extend
the effectiveness of in-lake methods for managing aquatic
macrophytes. The disadvantage of watershed BMPs is that
they will not result in immediate, substantial reduction in
nuisance aquatic plant growth because habitat has already
been created in the lake that supports aquatic plant growth.
Therefore, watershed BMPs usually must be combined with
other methods to reduce aquatic plant growth.
There are physical, chemical, and biological methods for
reducing aquatic plant growth in lakes and restoring balance
to the lake ecosystem so that the aquatic plants are beneficial
rather than harmful. Following is a description of examples,
principles, advantages, and disadvantages of each method,
summarized from Aquatic Plant Control (Washington State
Department of Ecology, 1994), A Citizens ’ Manual for
Developing Integrated Aquatic Vegetation Management
Plans (Gibbons, Gibbons, and Systma, 1994), and Crary
WeedRoller Pilot Project Report (Cooke, 1996).Physical Methods Physical methods of reducing the amount
of aquatic plants in a lake include hand-pulling, hand and
mechanical cutting, mechanical harvesting, bottom barriers
(sediment covers), water level drawdown, water column dyes,
rotovating, diver-operated suction dredging, and weed
rolling.
Hand-pulling aquatic plants is similar to pulling weeds
out of a garden. This method involves digging out the entire
plant with a spade or long knife and disposing of the residue
onshore. In waters deeper than three feet, hand removal can
best be accomplished by snorkelers or scuba divers carrying
collection bags for plant disposal. The technique results in
immediate clearing of the water column of nuisance plants
and is most appropriate for small-area, low-plant density
treatment, e.g., clearing pondweed from areas around docks
and beaches.
Hand and mechanical cutting differ from hand-pulling
in that plants are cut below the water surface (roots are usu-
ally not removed) with scythes, rakes or other specialized
devices that can be pulled through the weed beds by boat or
people. Rakes can be equipped with floats to allow easier
plant and fragment collection. Mechanical cutters can be
battery-operated and hand-held, portable and mounted on
boats, or specialized underwater cutters using a sickle to cutC012_002_r03.indd 620C012_002_r03.indd 620 11/18/2005 10:36:48 AM11/18/2005 10:36:48 AM