Phytoplankton Regime Shift Toward Filamentous Cyanobacteria Following Nutrient Reductions in a Hypereutrophic Lake

ABSTRACT Phytoplankton community responses to nutrient‐management interventions in agricultural catchments remain poorly understood, particularly with respect to functional group transitions. Petit‐lac‐Saint‐François, a hypereutrophic northern temperate lake, underwent agricultural nutrient‐mitigation measures beginning in 2011, providing an opportunity to examine long‐term ecological change. This study assessed phytoplankton community dynamics over a 10‐year period (2011–2020) to determine how nutrient reductions influenced taxonomic composition, cyanobacterial functional groups and ecological regime shifts. Seasonal and annual phytoplankton biovolume, class‐level composition and cyanobacterial functional traits were analysed, and Sequential t ‐test Analysis of Regime Shifts was applied to identify transitions. Nutrient fractions (TKN, TN, NH 4 + , PP, DOP, DP, TP) and stoichiometric ratios (TN:TP) were evaluated relative to ecological transition dates (August 2014 and August 2017). Cyanophyceae dominated the community (88.6% of annual biovolume), with strong seasonal contrasts between summer (91.6%) and winter (8.6%). Although overall class composition remained stable, substantial restructuring occurred within cyanobacteria. Nitrogen‐fixing‐capable taxa increased sharply, with regime shifts in their proportional abundance from 0.28 to 0.71 (August 2017) and then to 0.89 (June 2018). Species richness increased by 50.8% after August 2014, while evenness declined by 26.0%. Colonial Microcystis (42.8% pre‐2015) was replaced by filamentous Dolichospermum (45.2% post‐2015). Dissolved nutrient fractions (DP, DOP) declined during the initial community reorganisation (2014), whereas particulate fractions (PP, TP) and TN:TP ratios shifted during the later morphological transition (2017). Phytoplankton communities responded rapidly to nutrient‐management interventions, particularly through functional reorganisation within cyanobacteria. However, the persistence of cyanobacterial dominance and the rise of nitrogen‐fixing filamentous taxa indicate that full lake restoration will require measures beyond nutrient reduction alone. This study provides a rare long‐term assessment of how nutrient‐management actions influence cyanobacterial functional structure in a hypereutrophic lake. By linking specific nutrient fractions to distinct ecological transition points, it clarifies how dissolved and particulate nutrients are associated with different phases of community change. These results refine the understanding of cyanobacterial responses to shifting nutrient regimes and offer broadly applicable insights for managing eutrophic lakes under agricultural pressure.