Biology, ecology, and management of starry stonewort (Nitellopsis obtusa; Characeae): A Red-listed Eurasian green alga invasive in North America

Nitellopsis obtusa (starry stonewort) is a green macroalga (family Characeae) native to Europe and Asia that is of conservation concern in its native range but expanding in North America. We synthesize current science on N. obtusa and identify key knowledge gaps. Nitellopsis obtusa is able to reproduce sexually or asexually via fragments and bulbils. Native populations reproduce primarily asexually; sexual fertility increases with longer growing seasons and in shallower waters. In North America, only males have been observed. Nitellopsis obtusa has been known from North America for four decades and confirmed in seven U.S. states and two Canadian provinces. It is typically associated with low-flow areas of lakes with alkaline to neutral pH and elevated conductivity. Nitellopsis obtusa has ecological benefits in its native range, contributing to food webs and water clarity. In its invaded range, N. obtusa could negatively influence native macrophytes and habitat quality, but there has been little research on impacts. There have been many efforts to control N. obtusa through physical removal or chemical treatments, but little systematic evaluation of outcomes. Substantial areas of uncertainty regarding N. obtusa include controls on reproduction, full distribution in North America, ecological impacts, and control strategies.