Taking over the dominance of the macrophyte community by Elodea nuttallii (Planch.) H. St. John is poorly reflected in ecological status assessment results

Nuttall’s waterweed ( Elodea nuttallii ) is an aquatic plant native to North America. In Poland, the species was first identified in the early 1990s and since then it has started to spread in surface waters. We investigated one of the six lakes in Poland (Probarskie Lake), which were reported to be invaded by E. nuttallii among all the lakes monitored in the period 2005–2016. Based on our field survey data (2019 and 2020) and historical monitoring data (2011 and 2017), we explored the rate of invasion, the effects on taxonomic composition, spatial structure, and abundance of biological assemblages, and the ecological status assessment of this new incomer. We surveyed the lake for macrophytes, phytoplankton and macrozoobenthos for two subsequent years using the field protocols applied in lake monitoring in Poland. Water physicochemistry, planktonic algae and benthic invertebrates were sampled at the sites that were invaded and uninvaded by E. nuttallii , and phytoplankton and physicochemistry also in pelagic sites. Nuttall’s waterweed was identified in Probarskie Lake in 2017 for the first time, occupying about 30% of the phytolittoral area, while in 2019 and 2020, it dominated the macrophyte community, occupying over 80% of the total hydrophyte area. The appearance of the alien invader and its relatively fast taking over of the dominance in the macrophyte community were not reflected in the bioassessment results. None of the analysed biological assemblages truly mirrored the severe changes in macrophyte taxonomic composition among the sampling sites as well as over the subsequent years. As the spread of E. nuttallii is anticipated to increase in Polish waters, there is an urgent need for verification of ecological status assessment methods to improve their diagnostic capacity to capture the problem of ecological invasions.