Declining water clarity driven primarily by increases in dissolved organic carbon (DOC; i.e. ‘brownification’) has been broadly reported across the Northern Hemisphere. In Ontario, Canada, increasing lake DOC concentrations have been reported regionally, however no broad spatial and temporal water clarity trends for Ontario lakes currently exist. Here, we use data from two provincial monitoring programs to analyze historical (1962–1984), contemporary (2008–2023), and long-term (1962–2023) changes in water clarity, measured as DOC concentration and Secchi depth, across 684 Ontario lakes that were sampled two to six times over the study period. Our findings suggest that water clarity in Ontario lakes has declined since 1962, with the greatest declines in water clarity occurring in the contemporary time period. Water clarity declines were greatest in oligotrophic lakes, where DOC concentrations were also shown to have increased over time. Our findings highlight the importance of regional, temporal, and trophic contexts when considering the magnitude and direction of water clarity changes in Ontario lakes.
Ross et al. (Tue,) studied this question.