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rat method (1669 ± 748 versus 799 ± 379 g/m2 DW (x +SE, n = 12, p< 0.01)), variability of the two methods was compara- ble (116 versus 110% coefficient of variation). Thus, caution is needed in comparing our results to other researcher's data but our values indicate that the rake sampling method pro- vides accurate comparisons among our sites. After collection, plants were washed, roots were removed and above ground shoots were dried to a constant weight at 60 C (after Engel 1990). Milfoil was separated from other plants on the 3rd, 6th and 9th sampling date using the taxo- nomic key of Fassett (1957). Dry weights were determined for milfoil, and for all other plants combined, and percent- age milfoil by weight was determined. Dissolved oxygen, specific conductivity, and temperature were measured on each sampling date at 0, 1, and 2 meters depth and then averaged. Dissolved oxygen, specific conduc- tivity and temperature were measured with Yellow Springs Instruments (YSI) meters. Water clarity was measured on each sampling date with a 20.3 cm Secchi disk in open water at random locations within 5 meters of each experimental plot. To assess canopy density at the surface, we developed a method using a Secchi disk (called a plant disk when used in a weed bed). This disk was lowered into the weeds and the depth at which the plant canopy obscured the disk was mea- sured. This was done in both harvested and unharvested areas from the 3rd to the 9th week. The disk was gently low- ered owered through the plant stems by moving the disk side to side so that it did not push down surface mats, but rather slipped underneath them. Plant surface cover was measured as the ratio of Secchi disk to plant disk depth. When plant stems do not obscure the plant disk at all the ratio of plant disk depth to Secchi depth outside the weed bed (the plant canopy 04 104 t as 3 13 H p 103 L Vl a 0 m c a a 102 0 • Unharvested v Harvested R2=0.81 T r R =0.26 3 4 5 6 7 8 9 10 Weeks after harvest JULY AUGUST SEPTEMBER Figure 2. Plant biomass changes over time in harvested and unharvested plots. Harvest done on July 3, 1990. Results presented as mean ± standard error. ratio) will be near 1.0 [this ratio may be slightly different from 1.0 due to the effects macrophytes can have on phy- toplankton production (Wetzel 1983) ] . Sediment cores were collected in the 3 sub -divisions of each plot 3, 6, and 9 weeks after harvest. Cores represented a composite of the upper 15 cm of sediment. Forty grams were sub -sampled from the cores for textural analysis using a hydrometer (Klute 1986). An additional 20 grams of dried material from each core were ashed at 545 C to determine organic content (following Lillie and Barko 1990). DATA ANALYSIS Differences in plant biomass between harvested and unharvested plots were analyzed with an ANOVA for each sampling date (n= 30 for each date). Biomass data was aver- aged within each depth range (near shore, mid, and furthest from shore) and loglp transformed, to compensate for unequal sample variance because plants grow in a geometric progression. ANOVAs were also used to compare differences in plant canopy ratios for each week (n=30 for each week). Standard errors for both biomass and plant canopy ratios are determined for weekly data and reflect the variability within treatment plots, sub divisions, and between bays. The rate of biomass accumulation was determined using linear regression of the weekly loglp transformed biomass average over all sites and depth ranges (Figure 2) (n=9). The slopes were determined as: log WW2 - log Wl = Relative growth rate T2 -Tl _ with units of week -'. W2 and Wl are biomass at time 2 (T2) and time 1 (Tl) respectively. We tested the statistical difference in rates of biomass accumulation with the following model He: the slopes of the regression lines were parallel and the intercepts are differ- ent; Ha: the slopes and intercepts are different. The models were compared using an F -test (Weisberg, 1985). Bay to bay differences in lake physical/chemical characteristics were determined using Tukey's pairwise comparison of means for each bay. Pearson correlation and multiple regression tech- niques were used to look at the relationships between lake characteristics and plant biomass and percentages of milfoil. RESULTS AND DISCUSSION Regrowth after harvest Harvested plots had significantly higher relative growth rates over the remaining field season than did reference areas (p = 0.001) . Relative growth rates, determined from the slopes of the regression lines shown in Figure 2, - 0.03 week-' (p=0.001 for regression) in reference areas and 0.02 week-' (regression not significant) in harvested areas. Without the data from the 2nd week the relative growth rate in the harvested areas is 0.03 week-' (p= 0.08 for regres- sion). The anomalous increase in harvested biomass shown the 2nd week after harvest (Figure 2) may be due to an increase in fragments, which are also sampled by our method. Before this sampling date there was a storm which 58 J. Aquat. Plant Manage. 32: 1994. were