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TABLE 2. PEARSON CORRELATIONS OF ENVIRONMENTAL CHARACTERISTICS AND
<br />ABOVE GROUND PLANT BIOMASS.
<br />GOND Gravel Sand Silt Clay PCOM SECCHI
<br />GOND 1
<br />Gravel -0.90*** 1
<br />Sand 0.85*** -0.81*** 1
<br />Silt ns ns -0.61*
<br />Clay -0.74** 0.64* -0.90*** 0.57* 1
<br />PCOM ns ns ns 0.69** ns 1
<br />SECCHI ns ns ns 0.64** ns 0.84*** 1
<br />Biomass ns ns ns ns ns 0.84*** 0.74**
<br />COND is specific conductivity (micro mhos/cm).
<br />Gravel, Sand, Silt, Clay and PCOM (percent organic matter) are their
<br />respective percentages in the sediments.
<br />SECCHI is Secchi depth (meters).
<br />Biomass is biomass in reference plots.
<br />Significant correlations are marked: (***) p<0.001, (**) p<0.01, (*)P<0.05,
<br />and (ns)non significant.
<br />There was also wide range of water clarity among the 5
<br />sites studied. Three of the 5 bays showed differences in Sec -
<br />chi depth readings (p < 0.05) (Table 1) . The positive rela-
<br />tionship between Secchi depth transparency and biomass
<br />suggests that plants in Lake Minnetonka are light limited.
<br />Smith et al. (1991) report that clear deep water can reduce
<br />milfoil matting on the surface, which could account for
<br />greater success by other plant species, and therefore a higher
<br />total plant biomass. However, this did not appear to be the
<br />case in Lake Minnetonka. Although milfoil grows to the sur-
<br />face and forms mats which can inhibit the growth of other
<br />aquatic plants (Aiken et al. 1979), the area which showed the
<br />highest biomass was also the area which had the most surface
<br />matting. In Lake Minnetonka, it is possible that higher water
<br />clarity leads to higher biomass because the native species we
<br />found, such as Potamogeton spp., and Ceratophyllum demersum
<br />are able to grow well, despite the presence of milfoil.
<br />In addition to individual correlations, multiple regression
<br />techniques were used to determine the best predictors of
<br />plant biomass. The models in table 3 were the best predictors
<br />of total biomass with r2 > 0.81, and tvalues < 0.05 for the
<br />regression coefficients and intercepts. However, because per-
<br />cent organic matter and percent silt, and conductivity and
<br />percent sand are autocorrelated (Weisberg 1985) (Table 2),
<br />model 2 represents the best predictor of biomass. Percent
<br />clay and percent organic matter together explain 81% of the
<br />variability in biomass in Lake Minnetonka aquatic plants
<br />(Table 3) . Percentage of clay varied from 1.7% to 10.1 %; and
<br />was negatively correlated with biomass.
<br />These findings may be useful in identifying areas likely to
<br />produce high total plant biomass. Information on sediment
<br />TABLE 3. BEST SUBSET REGRESSIONS FOR CONTROL AREA BIOMASS.
<br />Model R' Model
<br />1 0.84 300 - 27(Silt) +278(PCOM)
<br />2 0.81 431- 68 (Clay) +229 (PCOM)
<br />0.89 3507+43(Sand)-22(COND) +1127 (Secchi)
<br />Silt is percent silt in sediments, PCOM is percent organic matter in sedi-
<br />ments, Clay is percent clay in sediments, Sand is percent sand in sediments,
<br />COND is specific conductivity in the water column (micro mhos/cm), Sec -
<br />chi is Secchi depth (m) outside the weed beds.
<br />texture and water clarity for many Minnesota lakes is avail-
<br />able from the Minnesota Department of Natural Resources.
<br />Along with other factors, such as proximity to lakes already
<br />infested with milfoil, those lakes which are likely to produce
<br />high total plant biomass could be prioritized for monitoring
<br />for new milfoil infestations. Milfoil infestations can then be
<br />stopped first in areas where they are likely to result in high
<br />biomass plant beds.
<br />ACKNOWLEDGMENTS
<br />Thanks to: The Freshwater Foundation for financial sup-
<br />port for this work; the Lake Minnetonka Conservation Dis-
<br />trict for the use of a boat and harvesters;. Norm Parus and
<br />Rob Merila from the LMCD for coordinating the use of the
<br />boat and the harvester; Leigh Vanderklein, and the reviewers
<br />for thoughtful comments and useful editorial assistance;
<br />Shad Oneel, Jenna Carlson, Toby McAdams, Meg Thomas,
<br />Tom Leigh, and Ann Robertson for assistance with field sam-
<br />pling and lab processing. Smith Diving donated the use of
<br />SCUBA equipment. The Minnesota Department of Natural
<br />Resources provided information on Lake Minnetonka. Also
<br />supported in part by the Minnesota Experiment Station and
<br />the McIntire -Stennis Cooperative Forestry Act under projects
<br />42-25 and 42-38. Paper number 21305 Scientific Journal
<br />Series, Minnesota Experiment Station.
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