Investigating the distribution of Nitellopsis obtusa in Ontario Lakes and its role as an ecosystem engineer

Abstract

Nitellopsis obtusa is a non-native macrophyte introduced into the Laurentian Great Lakes in 1974. Over the last decade, increasing reports emerged for N. obtusa within lakes in the Great Lakes Basin. Despite N. obtusa being in North America since the 1970s, little is known about the ecosystem- and community-level impacts upon establishment. My thesis aimed to elucidate these potential effects by focusing on both within-lake and regional scale N. obtusa populations. I conducted a comprehensive investigation into ecosystem impacts of N. obtusa across 12 sites in Lake Scugog, Ontario, Canada, from spring to fall over four years (2016-2019). Upon N. obtusa establishment, diversity across lower aquatic food web (LAFW) communities significantly (p-value < 0.05, multiple linear regression) decreased. Despite community diversity decreased across the LAFW, Microcystis spp. blooms emerged where they were previously unreported. To assess drivers of bloom development, a structural equation model demonstrated that N. obtusa had a direct faciliatory (p-value < 0.05) role in Microcystis spp. blooms. In addition, N. obtusa biomass was a moderately strong negative predictor of near-bed oxygen concentration (R2 = 0.59, p-value < 0.001), which likely promoted internal loading of phosphorus. Supporting this conclusion was a strong predictive model showing that N. obtusa biomass explained 90% of pore water soluble reactive phosphorus concentrations (R2 = 0.90, p-value < 0.001). During a narrow sampling window (3 weeks) in 2019, I sampled 60 lakes across a geological transition zone in Ontario, Canada. Calcium is for N. obtusa, thus a strong response related to calcium availability was expected. Nitellopsis obtusa was documented at 37 sites in 19 of the study lakes. Interestingly, an ecological niche model demonstrated that depth and the cations potassium, magnesium, and sodium were better predictors of N. obtusa presence than calcium. Strong positive associations between N. obtusa and native Characeae members were revealed through general linear latent variable modelling, in addition to strong negative associations with other macrophyte taxa such as Myriophyllum spicatum. In combination, these findings indicate that N. obtusa may be an ecosystem engineer in invaded lakes by affecting key lake elements and altering community composition throughout the LAFW community.