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1. The diatom Didymosphenia geminata has emerged in recent years as a globally invasive species. Although considered native to North America, reports of nuisance blooms have increased over the last decade. 2. Previously, we determined that D. geminata was ubiquitous in two major headwaters of the South Saskatchewan River Basin (SSRB), Alberta, Canada, but found it only bloomed at certain sites, including those immediately downstream from dam outfalls. To evaluate the role of dams in the abundance and blooming of D. geminata, we compared sites just below dams to unregulated upstream reference sites in six dammed rivers of the SSRB. 3. There was a high degree of seasonal variability in D. geminata abundance among sites, but statistical analyses showed a significant propensity for the diatom to have higher cell densities and an increased frequency of blooms at dam sites. 4. Important predictor variables of D. geminata abundance included dam presence, water clarity and total phosphorus concentration. When data from dam sites were analysed, a multiple regression model using mean discharge and pH as independent predictors explained 73% of the variation in D. geminata cell density. 5. Analysis of 3 years of data from one study river (Red Deer River) revealed consistently higher D. geminata cell densities at the dam site compared to the upstream reference. This analysis also showed that average cell density fluctuated by orders of magnitude from year-to-year. 6. Due to the ecological and aesthetic concerns regarding the global spread and blooming of D. geminata, we recommend that dams be viewed as key candidates for mitigating blooms in flow regulated systems. [ABSTRACT FROM AUTHOR]

• Nitellopsis obtusa was documented at 19 of the 60 lakes surveyed. • Shallow sites, that were closer to boat launches had higher densities of N. obtusa. • Cations other than calcium have a strong influence on N. obtusa distribution. Nitellopsis obtusa (also known as Starry Stonewort) is a Eurasian charophyte first introduced into the Laurentian Great Lakes in the 1970s. Over the last decade, there have been increasing reports of N. obtusa invasion of inland lakes in the Great Lakes basin. To improve our understanding of N. obtusa 's habitat preferences, we sampled 60 lakes across a geological transition zone in Ontario, Canada. Targeting this geological transition zone allowed us to study an array of lakes representing broad nutrient and water-hardness gradients within the same geographic and climatic region. Calcium is an important macronutrient for N. obtusa , thus it was anticipated that the presence of N. obtusa would respond strongly to calcium availability. Nitellopsis obtusa was documented at 37 sites in 19 of the 60 lakes surveyed. Lakes with N. obtusa had significantly higher cation (calcium, potassium, sodium, and magnesium) concentrations than lakes without N. obtusa (general linear mixed model with binomial error, p < 0.05). An ecological niche model based on a boosted regression tree (BRT) identified habitat features most associated with N. obtusa occurrence. Interestingly, the BRT model demonstrated that depth and the cations potassium, magnesium, and sodium were better predictors of N. obtusa presence than calcium. Although N. obtusa was found within a constrained range of calcium (15 – 57.9 mg L−1 Ca), it appears that other environmental parameters have a stronger influence on N. obtusa distribution in Ontario lakes. [ABSTRACT FROM AUTHOR]