sediments into the water column, releasing nutrients and other accumulated compounds.
However, not all secondary effects of harvesting are negative. Removal of large amounts of plants
can improve the diel oxygen balance of littoral zones and rivers, particularly in shallower water
(Carpenter and Gasith 1978, Madsen et al. 1988). At this point, no studies have indicated whether
native communities respond preferentially to harvesting.
In the past, harvesting was widely touted as a mechanism to remove nutrients from lake systems.
However, ecosystem studies indicated that harvesting was not likely to significantly improve the
trophic status of a lake. For instance, harvesting all available plants in Lake Wingra, Wisconsin
removed only 16% of the nitrogen and 37% of the phosphorus net influxes into the lake; these
removals were insignificant compared to the lake's internal pools of those nutrients (Carpenter and
Adams 1976, 1978). Plant harvesting in Southern Chemung Lake, Ontario removed 20% of the
annual net phosphorus input (Wile 1975). In a more eutrophic system (Sallie Lake, Minnesota),
continuous harvesting of aquatic plants in the littoral zone during summer removed only 1.4% of
the total phosphorus input (Peterson et al. 1974). In a less eutrophic system (East Twin Lake,
Ohio), harvesting the entire littoral zone would have removed from 26% to 44% of the phosphorus
and from 92% to 100% of the nitrogen net loadings to the lake over a 5-year study period
(Conyers and Cooke 1983).
Harvesting aquatic plants is not an effective tool for reducing nutrient loads in a lake; in none of
the above scenarios was the internal nutrient pool reduced. In the best-case scenario, removing all
the plants in the lake only kept pace with the amount of external nitrogen loading and with not
quite half of the external phosphorus loading. Because no operational control program is going to
remove all plants in the littoral zone, it is unlikely that any operational harvesting program will
significantly impact the internal nutrient balance of the system.
The use of diver-operated suction harvesting (or dredging, as it is often called) is a fairly recent
technique. Called "harvesting" rather than "dredging" because, although a specialized small-scale
dredge is used, sediments are not removed from the system. Sediments are resuspended during the
operation, but using a sediment curtain mitigates these effects. Divers use this device to remove
plants from the sediment (NYSDEC and FOLA 1990). The technique can be very selective; divers
can literally choose the plants to be removed. Removal is efficient and regrowth is limited. The
system is very slow (100 m
2
per person-day; Eichler et al. 1993), and disposal of plant material
must also be resolved. However, it is an excellent method for small beds of plants or areas of
scattered clumps of plants too large for hand harvesting.
The last major mechanical management technique is rotovating, which is widely used in the
Pacific Northwest and, formerly, in British Columbia for management of Eurasian watermilfoil.
This method uses rotovator heads on submersible arms to till up the bottom sediments and to
destroy the root crowns. Rotovating is relatively rapid and can effectively control dense beds of
Eurasian watermilfoil for up to 2 years (Gibbons and Gibbons 1988). However, it spreads
Eurasian watermilfoil fragments, resuspends large amounts of sediments and nutrients, causes
high levels of turbidity, disrupts benthic communities, and is nonselective.
Physical management methods may or may not utilize large equipment but are distinguished from
mechanical techniques in the following manner: in mechanical techniques the machines act
directly upon the plants, in physical techniques the environment of the plants is manipulated,
which in turn acts upon the plants. Several physical techniques are commonly used: dredging,
drawdown, benthic barriers, shading or light attenuation, and nutrient inactivation (Table 7).