Cyanobacterial blooms in Ontario, Canada: continued increase in reports through the 21st century

Abstract Favot EJ, Holeton C, DeSellas AM, Paterson AM. 2023. Cyanobacterial blooms in Ontario, Canada: continued increase in reports through the 21st century. Lake Reserv Manage. 39:1–20. The Ontario Ministry of the Environment, Conservation and Parks samples algal composition in response to public reports of suspected algal blooms, which have been tracked since 1994. In a previous analysis, Winter et al. noted a significant increase in the number of reports of confirmed algal blooms dominated by cyanobacteria from 1994 to 2009. Here, we determined that this increasing trend in the yearly number of confirmed cyanobacterial bloom reports (CCBRs) has persisted in Ontario over the intervening decade, to 2019. More than half of CCBRs were from waterbodies on the Precambrian Shield, in the Georgian Bay (5E) ecoregion, known for cottaging and water-based tourism. Data from the Ontario Lake Partner Program (LPP) was used to investigate total phosphorus (TP) concentrations in waterbodies with CCBRs. Approximately 44% of the waterbodies with a CCBR (mean TP 12.99 µg/L, n = 135) had average spring TP concentrations less than 10 µg/L, compared to 64% for LPP waterbodies with no reported or confirmed cyanobacterial blooms (mean TP 9.79 µg/L, n = 918). The most common taxon of cyanobacteria dominating bloom samples in inland waterbodies was Dolichospermum, followed by Aphanizomenon in waterbodies on the Precambrian Shield, and Microcystis in the Mixedwood Plains ecozone in southern Ontario. While an increase in public awareness cannot be ruled out in contributing to the rise in CCBRs across Ontario, the high proportion of cyanobacterial blooms occurring in oligotrophic waterbodies suggests that there may be a link to climate warming, rendering conditions more favorable for these blooms to occur.