Effects of repeated, early season, herbicide treatments of curlyleaf pondweed on native macrophyte assemblages in Minnesota lakes

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We examined the response of native aquatic macrophyte communities to spring herbicide treatments of curlyleaf pondweed (Potamogeton crispus) from 2006 through 2009. Eleven lakes were examined during our study; 8 were treated in May with endothall at 0.75–1.00 mg active ingredient per liter (ai/L) and 3 were used as untreated reference lakes. Macrophyte communities were assessed for frequency of occurrence in the littoral zone with the point intercept method in the summer after each treatment. During each survey, we collected biomass samples from 40 random locations in each lake. In the reference lakes, curlyleaf persisted at moderate to high frequencies over the 4 years, and no consistent changes in native macrophyte frequency of occurrence were seen. In most treated lakes, overall native macrophyte frequency of occurrence and species richness changed little over the 4 consecutive years of treatment, although shifts in the abundance of some species were observed. In untreated lakes, biomass varied between years, whereas in many treated lakes, biomass generally increased; however, these increases were usually not significant. The most substantial increases in biomass were attributed to single species in each treatment lake. Likewise, we observed substantial but insignificant increases of Chara sp. frequency and biomass in many treated lakes. Multiple years of treatment may be needed to see significant increases in overall native macrophyte abundance because significant changes in abundance were not observed within 4 years of treatment; however, consecutive early season, lakewide endothall treatments of curlyleaf pondweed can control curlyleaf pondweed without substantial harm to native macrophytes.