Invasive Nitellopsis obtusa (starry stonewort) has distinct late-season phenology compared to native and other invasive macrophytes in Minnesota, USA

Understanding the phenology of the invasive macroalga Nitellopsis obtusa (starry stonewort) is critical for evaluating and predicting its impacts on native macrophytes and timing control efforts. We tracked N. obtusa phenology in two Minnesota (USA) lakes from 2017 to 2018. We measured N. obtusa biomass and counted bulbils (asexual reproductive structures) throughout the growing season and in winter, under ice. Additionally, we compared phenology of N. obtusa to other invasive and native macrophytes using a long-term, Minnesota statewide dataset. We found consistent late-season peaks in N. obtusa biomass in both lakes, and this extended growth distinguished it from common native and invasive macrophytes in the region. Phenological asynchrony of N. obtusa relative to native macrophytes indicates use of a distinct temporal niche that may contribute to its invasion success. Bulbil counts were lowest early to mid-summer and increased through fall; however, high variability in this relationship warrants more intensive sampling to clarify seasonal trends. Late-season growth of N. obtusa poses challenges for timing control efforts, as early-season treatments may be limited to avoid harming native species. Treatment during peak biomass, before peak bulbil abundance, may improve control.