LAUSR

More than one-third of non-native freshwater organisms are known to cause ecological impacts, but there is still a knowledge gap on how impacts are related to non-native plant abundance. We investigated the relationship between coverage of three aquatic nonnative invasive species Hydrocotyle ranunculoides, Ludwigia grandiflora and Myriophyllum aquaticum and phytoplankton and zooplankton abundances. We hypothesized that uninvaded ponds, with a diverse submerged macrophyte community, would have higher plankton biodiversity compared to invaded ponds. In addition, we predicted that invasive plant cover would have a negative impact on plankton biodiversity. We sampled the phytoplankton and zooplankton communities of 18 ponds in Belgium, concurrently with 11 local environmental variables including cover of the invasive species. Our results showed that uninvaded ponds, with a high abundance of submerged vegetation, were associated with indicators of a clear water state and several zooplankton taxa. Variation partitioning showed that both environmental variables and invasive species cover explained zooplankton community variation. We observed a negative relationship between invasive species cover and zooplankton density as expected. In particular, large cladocerans and littoral taxa were absent when the cover of invasive species was high. This is possibly due to low dissolved oxygen concentrations, physical obstruction and a putative allelopathic property of the invasive plants. Invasive species cover was not significantly correlated with phytoplankton biovolume, although some evidence indicated that the biovolume of functional groups (FG) reflected the differences in habitat between invaded and uninvaded ponds. Reynolds functional Y group (Cryptomonas spp.) shows a tolerance to low light availability and was the dominant FG in the invaded ponds. Our study provides evidence that it is necessary to include cover of an invasive species to disentangle patterns of impact.