Effects of invasive European frogbit and its two physical control methods on macroinvertebrates

Abstract

European frogbit (Hydrocharis morsus-ranae L.) is an invasive floating plant that negatively affects native plants in freshwater ecosystems. We conducted a field mesocosm experiment in Oneida Lake, New York (USA), to study the effect of frogbit cover on macroinvertebrates and to test the effects of 2 physical control methods, hand pulling and shading, on macroinvertebrate assemblages. We used Hester–Dendy samplers and Ekman grabs to collect surface/phytophilous and benthic macroinvertebrates. Metrics including density of different types of organisms, total abundance, taxon richness, and Simpson’s diversity index were compared between control methods. Frogbit cover had significant positive effects on mollusk density, amphipod density, taxon richness, and Simpson’s diversity in the Hester–Dendy surface samples. Sites with frogbit had significantly fewer benthic worms and more chironomids (Hester–Dendy bottom samples) and possibly higher diversity of benthic macroinvertebrates (Ekman samples). These data suggested that frogbit had positive effects on surface and phytophilous macroinvertebrates and may change population density and increase diversity of benthic macroinvertebrates. Hand pulling and shading did not affect macroinvertebrates except that amphipod density in surface water was significantly lower in shading sites. Therefore, from the perspective of minimizing effects on the macroinvertebrate communities in the lakes, hand pulling is preferable over shading for controlling frogbit.