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J. Aquat. Plant Manage. 32: 56-60
<br />Effects of Harvesting on Plant Communities
<br />Dominated by Eurasian Watermilfoil in Lake
<br />Minnetonka, MN
<br />WENDY CROWELL' 2, N. TROELSTRUP JR', L. QUEEN', AND J. PERRY
<br />ABSTRACT
<br />Extensive mechanical harvesting has been used in 5,746
<br />hectare Lake Minnetonka, Minnesota since 1989 to control
<br />populations of Eurasian watermilfoil (Myriophyllum spicatum).
<br />Approximately 47% of the 544 infested hectares were har-
<br />vested during the summer of 1990. We measured effects of
<br />one series of harvests in five separate locations in Lake Min-
<br />netonka. Plant relative growth rates were greater (p = 0.001)
<br />in 54 m2 harvested plots than in adjacent reference plots.
<br />The increased growth rate did not result in harvested areas
<br />having greater canopy density or higher total shoot biomass
<br />than adjacent reference areas. Harvesting reduced total
<br />shoot biomass and plant abundance at the water surface for
<br />up to 6 weeks following harvest. Eurasian watermilfoil was
<br />the dominant plant in all areas, although its presence in an
<br />area was not correlated with high total shoot biomass in that
<br />area. Total shoot biomass was positively correlated with both
<br />water clarity and percentage of sediment organic matter and
<br />negatively correlated with the percentage of clay in the sedi-
<br />ments.
<br />Key words: macrophytes, Myriophyllum spicatum, manage-
<br />ment, growth rate.
<br />INTRODUCTION
<br />Eurasian watermilfoil (Myriophyllum spicatum L.), hereaf-
<br />ter called milfoil, impairs use of water resources in many
<br />parts of the United States and Canada (Aiken et al.1979).
<br />Problems associated with milfoil include degradation of
<br />beaches (Verhalen et al. 1985), interference with boat
<br />launching (personal observation) and decreased recre-
<br />ational opportunities (Schloesser and Manny 1984). The
<br />negative effects of milfoil infestation are most evident when
<br />high biomass and matting on the surface occur (e.g. shading
<br />of other plant species [Aiken et al. 1979], limited boat access
<br />[Painter 1988] and reduced recreational activities such as
<br />fishing, swimming, and water skiing [Rawls 1975]) .
<br />Researchers in Ohio reported that milfoil grows back to
<br />reference levels within 1 month of harvesting (Cooke et
<br />a1.1990). This suggests that harvesting causes an increase in
<br />' Research Assistant, Research Associate, Research Associate, and Associ-
<br />ate Professor respectively, Department of Forest Resources, College of Natu-
<br />ral Resources, University of Minnesota, 115 Green Hall, 1530 North
<br />Cleveland Avenue, St. Paul, MN, 55108
<br />2 Aquatic Biologist, Minnesota Department of Natural Resources, 500
<br />Lafayette Rd., St. Paul, MN, 55155. Received for publication February 15,
<br />1993 and in revised form June 13, 1994.
<br />growth rate of the plant. The aim of our study was to test the
<br />following hypotheses: i) Harvesting stimulates milfoil growth
<br />rate; and ii) An increased growth rate after harvesting even-
<br />tually results in a greater amount of milfoil in the harvested
<br />areas than in the unharvested areas (i.e.,more biomass or
<br />plant stems at the surface). We also wished to determine how
<br />long the effects of harvesting lasted (i.e. how long after har-
<br />vesting does milfoil biomass or canopy density become equal
<br />in harvested and reference areas) .
<br />We also studied the correlation between physical charac-
<br />teristics of the lake basin and water column, plant biomass
<br />and percentage of milfoil. As of August 1993, there were 63
<br />waterbodies in Minnesota with confirmed milfoil infestations
<br />(Minnesota Department of Natural Resources, unpublished
<br />data). Because most of the 12,000 lakes in Minnesota (Baker
<br />and Swain 1989) have yet to be infested with milfoil, these
<br />correlations can be used to help design milfoil management
<br />programs by identifying some conditions conducive to high
<br />plant biomass (a nuisance state).
<br />MATERIALS AND METHODS
<br />Study Site
<br />Milfoil was first found in Minnesota in 1987 in Excelsior
<br />Bay of Lake Minnetonka3 (Figure 1) . Since that time it has
<br />spread both within Lake Minnetonka and to other lakes in
<br />Minnesota. The Lake Minnetonka Conservation District
<br />(LMCD) is responsible for most milfoil management activi-
<br />ties in Lake Minnetonka. The LMCD has been using custom
<br />designed mechanical harvesters which cut a path 4.9 meters
<br />wide when fully extended to manage milfoil since the sum-
<br />mer of 1989.
<br />Paired harvested and unharvested plots were located in 5
<br />of the bays of Lake Minnetonka (Figure 1) . Unharvested
<br />plots provided reference conditions for biomass accumula-
<br />tion and canopy density at the surface. Plots were established
<br />near Shady Island in Phelps Bay (145 hectares), near Hard -
<br />scrabble Point in West Upper Lake (360 hectares), in Crystal
<br />Bay (336 hectares), in North Arm (132 hectares), and in
<br />Maxwell Bay (120 hectares) (Figure 1) (Smith et al. 1991,
<br />Crowell unpublished data). The sampling locations showed
<br />distinct differences in water clarity and sediment organic
<br />matter among the bays (Table 1).
<br />'From a 1990 memo to Howard Krosch from Dan Swanson on Eurasian
<br />Watermilfoil. Minnesota Department of Natural Resources, St. Paul, MN.
<br />56 J. Aquat. Plant Manage. 32: 1994.
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