Habitat change and alteration of plant and invertebrate communities in waterbodies dominated by the invasive alien macrophyte Lemna minuta Kunth

The free-floating American duckweed, Lemna minuta, is an invasive species now widespread in Europe. Yet, its impact on freshwater ecosystems has been poorly investigated. In this study, the effects of the presence of this invasive duckweed on water quality, and aquatic plant and invertebrate communities were evaluated in sites in Central Italy. Water chemical and physical factors and community descriptors were analyzed to identify these effects. Surveys were carried out across 17 paired aquatic sites. Site pairs were similar in microclimate, hydrogeology and water quality, but differed in relation to the presence/absence of L. minuta floating mats. In sites with mats, light and dissolved oxygen in water were negatively correlated with increasing mat coverage and thickness. The limited light and hypoxic conditions under mats inhibited plant growth and had a selective impact on the invertebrate community. Sites with L. minuta had aquatic communities with a lower plant taxa richness and a contrasting composition, compared with those in sites without. At sites with mats some plants were unaffected, but the majority of plant taxa documented at sites without Lemna were no present at sites with Lemna or were very rare (macroalgae, submerged rhizophytes). As for invertebrates, hypoxic-tolerant taxa dominated under mats (Ostracoda, Copepoda, Isopoda), whilst those more sensitive to oxygen depletion, or obligate herbivores, or those with a winged stage or swimming on water surface, were rare or absent (Ephemeroptera, Amphipoda, Chironomus, Notonecta). Lemna minuta mats presence was associated with alterations in the underlying aquatic ecosystem, severely threatening the conservation of these habitats. Active management strategies, including spread-prevention techniques, or mechanical removal combined with biological control, are required to conserve these habitats.