Effects of invasive European frogbit and its two physical control methods, hand pulling and shading, on macroinvertebrates
European frogbit (Hydrocharis morsus-ranae L.) is an invasive floating plant that negatively affects native plants in freshwater ecosystems. A field mesocosm experiment was conducted in Oneida Lake, NY, USA to study the impact of European frogbit cover on macroinvertebrates and to test the effects of two physical control methods - hand pulling and shading on macroinvertebrate assemblages. Metrics including density of different types of organisms, total abundance, taxon richness and Simpson’s diversity index were compared. Both Hester-Dendy sampler and Ekman grab were used to collect surface/phytophilous and benthic macroinvertebrates.
European frogbit cover had significant positive effects on mollusc density, amphipod density, taxon richness, and Simpson’s diversity of macroinvertebrates in the Hester-Dendy surface samples. There were significantly fewer benthic worms and more chironomids (Hester-Dendy bottom samples) and possibly higher diversity of benthic macroinvertebrates (Ekman samples) in the sites with European frogbit. These data together suggest that European frogbit had positive effects on surface and phytophilous macroinvertebrates and may change population density and increase diversity of benthic macroinvertebrates. The frogbit control measures hand-pulling and shading had no significant effects on macroinvertebrates with one exception: amphipod density in surface water was significantly lower in the shading sites. This suggests potential negative impacts of shading on macroinvertebrate communities. Therefore, hand pulling is preferable to shading for controlling invasive European frogbit from the perspective of minimizing effects on the macroinvertebrate communities in the lakes.