Environmental factors influencing biomass and bulbil production of Nitellopsis obtusa in Lake Koronis, Stearns County, Minnesota

Abstract

Starry stonewort [Nitellopsis obtusa (Desvaux) J. Groves] is a green macroalga in the family Characeae that is native to Eurasia. In the United States it has invaded many waterbodies across the northern tier from New York to Minnesota. In Minnesota, starry stonewort often grows later into the growing (September to December) season than most native species; however, the environmental factors that influence this life history trait are not well understood. Starry stonewort was harvested every 3 wk from May to November in 2020 and 2021 from four locations in Lake Koronis, Stearns County, MN. Light, water pH, and water temperature were also collected to correlate these variables to starry stonewort growth. Rhizoid biomass had weak negative correlations with temperature (rs ¼ 0.19) and pH (rs ¼ 0.14). Bulbil biomass had weak negative correlations with temperature (rs ¼ 0.24), pH (rs ¼ 0.31), and light transmittance (rs ¼ 0.11). Bulbil density had weak negative correlations with temperature (rs ¼ 0.23), pH (rs ¼ 0.29), and light transmittance (rs ¼ 0.10). Bulbil production was highest between October and November of each year. Bulbil biomass and density was lower in 2020 (4.6 g m2 and 1,229 bulbils m2) thanin2021(14.7gm2 and 5,211 bulbils m2). The average annual bulbil density was 1,537 bulbils m2 and ranged from 0 to 157,000 bulbils m2. The ability of starry stonewort to grow in dense mats and produce large quantities of bulbils contributes to the difficulty of controlling infestations. Bulbils are a method of spatial and temporal distribution that can allow for recolonization of previously treated areas.