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left many broken pieces of milfoil floating in the water col- umn. The fact that there was a rapid decline in the biomass in the 3rd week of sampling supports this interpretation. Harvested areas had lower average biomass than refer- ence areas for 6 weeks after harvest. Biomass increased in harvested areas and decreased in reference areas such that the two were no longer different 6 weeks after harvest based on weekly comparisons of harvested and reference biomass (Figure 2) [week 3 p = 0.0019, week 6 p = 0.0919, week 9 p= 0.31321. Plant canopy cover was inversely related to plant biomass (Figure 3), with plant biomass explaining 46% of the vari- ability in the plant canopy ratios. As plants regrew after har- vesting, plant canopy ratios showed a similar pattern to biomass readings, with harvested and reference canopy ratios converging to a single point at week 9 (Figure 4) . Har- vested areas had significantly higher plant canopy ratios than reference areas until the 6th week after harvesting [p=0.0022 week 3, p=0.0820 week 6, p = 0.912 week 91. Other researchers have found that harvesting reduced biomass for only 3 to 4 weeks. Thus, milfoil populations har- vested in mid-July attained pre -harvest and reference levels within 23 days after harvesting in LaRue Reservoir, Ohio (Cooke et al. 1990) and sites in Dundee and Herring Creek, Maryland had to be harvested once per month throughout the summer in order to attain effective management (Rawls 1975). However, harvesting is sometimes more effective. Mil - foil in lake Wingra, Wisconsin never returned to reference levels when harvested during July (Kimbel and Carpenter 1981). Similarly, in Lake Minnetonka, both plant canopy ratio and biomass data indicated that harvesting was an effec- tive control method for up to 6 weeks after harvesting, when harvested in early July. While harvested areas in Lake Minnetonka achieved ref- erence area biomass and canopy density 6 weeks after harvest they did not achieve a significantly higher biomass or canopy density in the following weeks. Despite a higher relative growth rate in harvested areas than in reference areas follow- ing ollowing harvest, harvested areas at no time attained higher biom- ass or canopy density than reference areas (Figures 2 and 4). The growth of milfoil is often limited by water depth (Aiken et al., 1979) which could account for harvested areas' biom- ass leveling out after regrowth. Plant biomass in relation to environmental factors The percentage of milfoil by weight in the reference areas for each site was not correlated with any lake character- istic, nor did the percentage of milfoil correlate with biomass in the control areas (r2= 0.005). This indicates that in Lake Minnetonka a high percentage of milfoil does not always coincide with high total plant biomass nor does having high total plant biomass imply that there is greater abundance of milfoil in that area than in areas with lower plant biomass. While milfoil relative abundance was not correlated with abi- otic conditions, total plant biomass was correlated (p<0.01) with water clarity and percent sediment organic matter (Table 2). There were significant differences in sediment organic matter among sites (p<0.05) (Table 1). Within the range of 0.9 • r 0.8 v O S •� U N a t 0.7 o c a a m U -a c Y o v a. 0.6 `c 0 FL v R2 = 0.46 • • • 0.5 300 400 500 600 700 800 900 Plant biomass (grams dry weight m-2) Figure 3. Plant canopy ratio in relation to plant biomass. physical variability in our plots, higher total plant biomass occurred in sediment with more organic matter. This corre- lation could be due to a preference of aquatic plants for sed- iments which are enriched with organic matter or could be caused, as Lillie and Barko (1990) suggest, by the enrich- ment of the sediments by the aquatic vegetation itself. WO 0.9 r a 0.8 t O U U O m 0.7 O L 0 c a o a� v a 0.6 G y, O CL 0 0.5 IL 0.4 0.3 • Unharvested Harvested Weeks after harvest July August September 10 Figure 4. Plant canopy ratio changes over time in harvested and unhar- vested plots. Harvest done on July 3, 1990. Results presented as mean ± stan- dard error. J. Aquat. Plant Manage. 32: 1994. 59