Decadal shifts in algal composition and related physicochemical variables in Alberta’s seasonally ice-covered lakes, with a particular focus on taxa that produces harmful algal blooms

Seasonally ice-covered lakes are undergoing rapid transformations due to climate change and human activities. One notable consequence observed in recent decades is the increasing frequency and intensity of harmful algal blooms (HABs), which affect aquatic food webs, ecosystem processes, and human health. This study examined a 35-year dataset from three central Alberta lakes (four study basins) representing different trophic conditions (mesotrophic to hyper-eutrophic) and mixing regimes (stratified to polymictic) to investigate how long-term changes in water temperature and nutrient dynamics have influenced algal community composition and HAB-forming cyanobacteria. Two inter-decadal periods were compared: 1986–1995 (Group 1) and 2010 onward (Group 2). Significant shifts in the community composition of major algal taxonomic groups were observed between the two decadal periods, including a pronounced decline in diatom relative abundance across all three thermally stratified basins and significant increases in cyanobacteria in two thermally stratified eutrophic basins. Declining diatom relative abundance in all three thermally stratified basins (both eutrophic and mesotrophic) was positively correlated with increasing mixed layer temperature, while increasing cyanobacterial relative abundance was most strongly associated with nutrient enrichment, particularly total Kjeldahl nitrogen concentrations. In contrast, the polymictic basin exhibited a significant increase in diatom relative abundance and a corresponding decline in cyanobacteria, highlighting a contrasting ecological response to environmental change. Among HAB-forming taxa, N2-fixing genera, particularly Aphanizomenon, increased in relative abundance in Group 2, compared to Group 1 across all study basins, suggesting a competitive advantage under conditions of nutrient enrichment and elevated water temperatures. Overall, our results demonstrate that ecological responses to climate warming and nutrient enrichment in seasonally ice-covered lakes are strongly mediated by lake-specific physical and trophic characteristics. In particular, differences in mixing regime were associated with contrasting trajectories of algal community reorganization and HAB-forming cyanobacterial assemblages.