end.
Evidence regarding the significance of this removal to reservoir water
Experiments with enclosures (Landers and Lottes
quality is insufficient.
1983) and calculations from shallow Lake
Wisconsin (Carpenter
suggest that macrophyte decay is a significant factor to
state.
Carpenter (1983) also points out that macrophytes can close a positive
feedback loop (see Figure 4) that enhances sediment accumulation and thus
This occurs through stimulation of algal growth by release
macrophyte growth.
Sedimentation of dead algal cells and
of nutrients and organic matter.
Since these plants are limited
phyte tissues adds to sediment accumulation.
by light penetration, any factor that promotes a decrease in depth, as
increased sedimentation would do, will ultimately promote an increase in the
area of coverage of macrophytes in the reservoir.
Harvesting of plants may be
one factor in disrupting this positive feedback loop.
The costs of harvesting are related not only to high purchase price and
problems with efficiency, but also with machine breakdowns and the number of
Downtime may increase sharply when an undersized machine
reharvests per year.
is employed or where the equipment is heavily stressed or not operated
The British Columbia lakes were an example of high stress on the
properly.
equipment.
A typical operating year there consisted of 2,764 hr of work, of
which 44 percent was downtime (Cooke et al. 1986).
It is strongly recommended
by manufacturers that the machine purchased be of a size appropriate to the
area to be harvested, as well as to plant density.
Or, the reservoir manager
may wish to employ one of the several contract harvesting companies and, in
this way, test harvesting as a solution of that particular reservoir's weed
problems.
Costs for harvesting vary regionally and reflect differences'in the
density of plant infestations, their regrowth rate, and other factors affect-
ing operations.
Table 12 is a comparison of harvesting and herbicide costs
Literature cost values have been converted to
for the
and Florida.
1987 dollars by using changes in the consumer price index.*
In the Midwest,
expenditures for harvesting and herbicides are clearly comparable, but in
Florida, harvesting is significantly more expensive (and less effective).
Cost comparisons are difficult to make due to wide variances in reporting of
Dr. Thomas Lough,
State University,
Personal
Kent, OH.
120
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