Plant community impacts and environmental associations of the invasive macroalga Nitellopsis obtusa (starry stonewort)

Abstract

Nitellopsis obtusa (starry stonewort, family Characeae, hereafter Nitellopsis) is a large, characean macroalga that is non-native in North America and was first found in Minnesota in 2015. Nitellopsis is the first invasive macroalga in the Midwest and arguably the only charophyte considered invasive globally. This novelty has led to high uncertainty of its ecological threat and limited information on best management practices. Nitellopsis can spread quickly and grow to dominate littoral zones, generating substantial concern regarding the potential ecological threats that it poses lake ecosystems. I addressed key knowledge gaps on the ecology of Nitellopsis by: 1) evaluating impacts of its invasion on native plant communities, and 2) investigating the influence of fine-scale environmental conditions on its growth. To explore these aspects of Nitellopsis ecology, I conducted field work using two observational methods – including space-for-time-substitution sampling in 2018 and time sequence sampling from 2017 - 2020. I found that the cover and richness of the native macrophyte community declined as Nitellopsis cover increased, and that increases in Nitellopsis were further associated with changes to species and functional group composition. While a diverse array of native macrophytes were negatively affected by Nitellopsis, the taxa most sensitive to its invasion were Chara spp. (characean algae that are native relatives of Nitellopsis) and Najas guadalupensis (southern naiad), both of which are similar to Nitellopsis in having a “carpet-forming” habit. The ability of Nitellopsis to invade occupied habitat is indicative of a highly competitive invader. Multi-year observations of Nitellopsis invasion revealed that abundance patterns varied between lakes and between years within lakes. Importantly, locations or periods of lower abundance may provide opportunities for native plants to recover from invasion effects. Abundance of Nitellopsis was strongly correlated with its impacts for all native responses evaluated, which was expected and motivated the exploration of Nitellopsis’ environmental associations. However, the investigation into environmental conditions associated with nuisance growth yielded little explanatory power, except for slight declines in cover of Nitellopsis under increasing total nitrogen in sediments. The lack of explanatory power could be due to study limitations or may be indicative of broad environmental tolerances of Nitellopsis. The apparent ability of Nitellopsis to reach peak growth under a variety of conditions in Minnesota lakes elevates its threat as an invader. Overall, this study revealed that Nitellopsis is an aggressive, high-impact invader and spread prevention and management should be prioritized for this species across its invaded range in North America.