The non-native charophyte Nitellopsis obtusa (starry stonewort) influences shifts in macrophyte diversity and community structure in lakes across a geologically heterogeneous landscape

Nitellopsis obtusa is a non-native charophyte introduced to the Laurentian Great Lakes in the 1970s. Over the last decade, there have been increased reports of N. obtusa invasion of inland lakes in the Great Lakes basin. Typically, the star-shaped bulbil associated with N. obtusa can aid in identification; however, bulbils can only be found in established communities. To evaluate the impacts of N. obtusa on macrophyte communities in invaded ecosystems, we sampled 60 lakes across a large geologically diverse area of south-central Ontario. Interestingly, all populations of N. obtusa detected had bulbils present, thus we conclude that N. obtusa has likely been present in inland lakes within the Great Lakes basin well beyond the last decade. High N. obtusa abundance was associated with significantly lower macrophyte diversity (p value < 0.05). Redundancy analysis revealed that increased developed and agricultural land-use, in addition to the cations: sodium, potassium, magnesium, and calcium, positively co-varied with macrophyte communities with detectable N. obtusa presence. A generalized linear latent variable model (gllvm) demonstrated that N. obtusa had significant (p value < 0.05) positive co-occurrences with several native Characeae species. The gllvm model also revealed that N. obtusa had significant (p value < 0.05) negative co-occurrences with most other macrophytes, including the non-native Myriophyllum spicatum. Although N. obtusa was only found in a third of our study lakes, our results infer that increased expansion and establishment of N. obtusa throughout the region may result in significant changes to macrophyte communities, posing a threat to native fish habitat.