Laboratory evaluation of copper-based algaecides for control of the invasive macroalga starry stonewort (Nitellopsis obtusa)

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 below-sediment 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.