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Nitellopsis obtusa ([Desvaux] J. Groves [1919]; Characeae), known as starry stonewort, is an invasive macroalga in Laurentian Great Lakes states and provinces in North America. Because of its potential negative impacts on native ecosystems and recreation, N. obtusa has become a high-priority target for management. However, there is a critical lack of foundational information on the efficacy of different algaecides, and concentrations thereof, for N. obtusa control. Additionally, control of N. obtusa bulbils--asexual reproductive structures that are the main pathway for the establishment of new plants--has proven difficult. We tested the efficacy of six commonly used copper-based algaecides, at a series of copper concentrations up to the maximum labeled rate, on N. obtusa thalli (photosynthetic aboveground tissues) and bulbils in controlled laboratory experiments. Bulbils were placed above and below sediment in separate experiments to evaluate whether sediment acted as a barrier to treatment. At 14 days after treatment (DAT), there were significant reductions in thalli biomass (34% and 40%) for two algaecides at the highest concentration evaluated (1.0 mg Cu L-1) and significant thalli discoloration at 0.75 and 1.0 mg Cu L-1 for four algaecides. There were no significant negative effects on N. obtusa thalli biomass or discoloration at lower concentrations of any product. For belowsediment bulbils, none of the algaecides reduced N. obtusa viability compared to untreated controls by 56 DAT, and viability was significantly greater than in controls for three different algaecides at 0.25 mg Cu L-1. For above-sediment bulbils, there was low sprouting across all algaecide treatments and untreated controls, indicating inhospitable growing conditions. These findings provide a baseline for improvement of chemical treatment options for N. obtusa, provide guidance for future research on N. obtusa control, and underscore the challenges in achieving sustained N. obtusa control. [ABSTRACT FROM AUTHOR]

Potamogeton crispus (curlyleaf pondweed) and Myriophyllum spicatum (Eurasian watermilfoil) are widely thought to competitively displace native macrophytes in North America. However, their perceived competitive superiority has not been comprehensively evaluated. Coexistence theory suggests that invader displacement of native species through competitive exclusion is most likely where high niche overlap results in competition for limiting resources. Thus, evaluation of niche similarity can serve as a starting point for predicting the likelihood of invaders having direct competitive impacts on resident species. Across two environmental gradients structuring macrophyte communities—water depth and light availability—both P. crispus and M. spicatum are thought to occupy broad niches. For a third dimension, phenology, the annual growth cycle of M. spicatum is typical of other species, whereas the winter-ephemeral phenology of P. crispus may impart greater niche differentiation and thus lower risk of native species being competitively excluded. Using an unprecedented dataset comprising 3404 plant surveys from Minnesota collected using a common protocol, we modeled niches of 34 species using a probabilistic niche framework. Across each niche dimension, P. crispus had lower overlap with native species than did M. spicatum; this was driven in particular by its distinct phenology. These results suggest that patterns of dominance seen in P. crispus and M. spicatum have likely arisen through different mechanisms, and that direct competition with native species is less likely for P. crispus than M. spicatum. This research highlights the utility of fine-scale, abundance-based niche models for predicting invader impacts. [ABSTRACT FROM AUTHOR]