Effects of repeated early season herbicide treatments of curlyleaf pondweed on native macrophyte assemblages in Minnesota Lakes.

Abstract

Non-native invasive species have the potential to cause various problems in small isolated ecosystems, these constraints are exemplified in the small, shallow, lakes of Minnesota. Curlyleaf pondweed (Potamogeton crispus L.) is one of the common occurring and most influential non-native invasive species in Minnesota. Characterized by its early spring growth and early summer senescence, curlyleaf has the ability to negatively affect native macrophyte growth by forming dense monotypic canopies in late spring and releasing nutrients in early summer. Because of the negative impact on natural ecosystems along with the problems curlyleaf can cause for people, there is serious interest in the management of curlyleaf. One management practice is the early season application of herbicides, where curlyleaf is targeted before it is capable of producing turions (propagules) and native plants are unlikely to be affected. We examined the response of native plant communities to spring herbicide treatments of curlyleaf pondweed from 2006 through 2009. Thirteen lakes were examined during our study; ten were treated with herbicide and three were used as nontreatment reference lakes. Plant communities were assessed in the littoral zone with the point intercept method in early spring (before treatment), late spring (after treatment) and mid summer (peak native plant growth). For each survey, approximately 40 random biomass samples were taken in each study lake to estimate plant biomass. To determine changes in native plant frequency and biomass throughout the course of consecutive annual treatments, we compared differences between treated and untreated lakes within years for August surveys and compared August surveys between years for treated or untreated lakes. Additionally, we examined inter-seasonal changes, comparing differences within and between treated or untreated lakes from May to June and June to August in every year. In the reference lakes, curlyleaf persisted at moderate to high frequencies over the four years, and no consistent changes in native plant frequencies were seen. Herbicide treatments proved effective for controlling curlyleaf, which decreased in frequency within 1 month following treatment for each year of the study. The total frequency of occurrence of native plants in August did not decrease in most of the treatment lakes between years. Native macrophyte species richness also showed little change with continued treatment, although shifts in abundance of some species were observed. August native plant biomass increased between 2006 and 2009 in most treatment lakes, whereas native biomass varied in untreated lakes. Much of the change in biomass was attributed to a single species in most treatment lakes. We observed increases of Chara spp. frequency and biomass in most treated lakes. However, multiple years of treatment may be needed to obtain increases in native plant abundance as the largest increases occurred after 3 years of treatment. Early-season lake-wide herbicidal treatments of curlyleaf pondweed can reduce curlyleaf occurrence and density without major harm to native plants. Significant inter-seasonal changes in frequency and biomass were observed in both treated and untreated lakes although differences were not observed between treated and untreated lakes. Native species richness increased from May to June and June to August in both treated and untreated lakes. The frequency of native plants increased between May and June but less so between June and August in both treated and untreated lakes. Conversely, native plant biomass increased from June to August more so than from May to June for both treated and untreated lakes. These findings suggest that early season growth primarily manifests in distribution and colonization, while late season growth results in increased biomass. Similarly, many plant species follow the same patterns as the overall native plants, although other plant species may have different frequency and biomass regimes throughout a single season. Despite the differences in native plant species, we did not see any major inter-seasonal detriment caused by the presence of curlyleaf pondweed in untreated lakes, or positive influence of herbicide treatment in treated lakes. These findings suggest that increases in native plant growth may occur only after several annual treatments.