Spatiotemporal changes of atmospheric depositions over 43 years in south-central Ontario

Atmospheric deposition significantly influences nutrient and chemical inputs to ecosystems, playing a crucial role in water quality and environmental management. This study utilizes long-term monitoring data spanning 43 years to evaluate changes in deposition for Dorset (a representative rural/remote area) and 27 years for Simcoe (representing a mixed-use landscape including agricultural, urban, natural and industrial areas) located along a north-south transect in south-central Ontario, east of Georgian Bay. Analysis focused on the annual deposition trends of 21 chemicals in Dorset and 12 chemicals in Simcoe, comparing changes within and between the two regions. Many consistent patterns emerged in both regions, revealing decreasing trends in nitrogen, chloride, sulphate and calcium deposition, and no significant trend in total phosphorus deposition. Metals and silica deposition, which were only measured at Dorset, also significantly declined. Notably, the magnitude and rate of these trends varied between Dorset and Simcoe. Simcoe exhibited higher deposition levels and more rapid/larger trends, attributed to its proximity to urbanization, agriculture, shipping, and industries. The varying contribution of chemical deposition to catchment export was evaluated for a small stream close to the deposition study site in Dorset, based on a ratio of export to deposition. This comparative analysis provides valuable insights into the spatiotemporal changes in atmospheric deposition, contributing to a comprehensive understanding of regional biogeochemical dynamics. • Depositions of most chemicals declined at south-central Ontario over 43 years. • Depositions differed between Dorset (rural area) and Simcoe (urban area). • Contribution of deposition to catchment export varied among chemicals.