Evaluation of lake-wide, early-season herbicide treatments for controlling invasive curlyleaf pondweed (Potamogeton crispus) in Minnesota Lakes.

Abstract

Submersed aquatic plants play an important role in freshwater systems, affecting nutrient dynamics, trophic interactions, biological assemblages, and fish productivity. However, waters infested with non-native invasive aquatic plants often experience severe impairment of ecological and recreational quality due to excessive plant growth. Curlyleaf pondweed (Potamogeton crispus L.), one such exotic submersed aquatic plant, has become a widespread nuisance in temperate regions of North America. Curlyleaf’s early-season growth, propensity to form dense surface mats, and ability to out-compete native aquatic plants allow it to degrade the ecological and recreational quality of lakes. Consequently, there has been a great deal of interest in adopting lake-wide management strategies that can reduce the negative impacts of curlyleaf and provide some degree of long-term control. We collaborated with the Minnesota Department of Natural Resources in 2006, 2007, and 2008 to evaluate lake-wide, early-season herbicide treatments for curlyleaf management. Six curlyleaf-infested lakes were treated with herbicide (endothall or fluridone) for at least three consecutive years. Three additional lakes with established curlyleaf infestations were selected to serve as untreated reference lakes during the same period. For all study lakes, we annually assessed the frequency and biomass of curlyleaf in May and June, documented the production of new curlyleaf turions (reproductive buds) on standing plants, and tracked changes in the abundance and viability of turions in lake sediments. Previous studies have shown that biomass is a key metric for evaluating aquatic plant management projects. We used a boat-based rake method for collecting biomass samples rather than the standard diver (SCUBA) quadrat method because it allowed us to collect a greater number of samples in each lake and provided a higher degree of safety than the diver quadrat method. However, this boat-based rake method had not been thoroughly evaluated to determine whether it produced biomass estimates that were comparable to the diver quadrat method. Consequently, we conducted a separate study to compare the vertical rake sampling method to the diver quadrat method. Results of this study showed that biomass estimates from rake samples were comparable to diver quadrat samples for most individual plant taxa. However, the rake method produced substantially higher estimates than the quadrat method when sampling in dense stands of aquatic plants, particularly in areas dominated by coontail (Ceratophyllum demersum L.). Although rake estimates of plant biomass were significantly less precise than quadrat estimates, we determined that the rake method allowed us to collect a sufficiently greater number of samples to offset the method’s lower precision. Consequently, we concluded that the biomass data we collected were precise enough to make meaningful relative comparisons in our study lakes. After three to four consecutive years of herbicide treatment, curlyleaf frequency, biomass, turion production, and sediment turion abundance were all reduced and were all significantly lower in treated lakes than in untreated reference lakes. However, viable turions remained in lake sediments after three consecutive years of treatment. These results suggest that serial lake-wide, early-season herbicide treatments can effectively decrease the negative impacts of curlyleaf infestation and reduce the abundance of curlyleaf turions in lake sediments, but ongoing management will likely be required to maintain long-term control of curlyleaf in infested lakes.