Contrasting long-term trends of chloride levels in remote and human-disturbed lakes in south-central Ontario, Canada

Abstract Yao H, Paterson AM, James AL, McConnell C, Field T, Ingram R, Zhang D, Arnott SE, Higgins SN. 2020. Contrasting long-term trends of chloride levels in remote and human-disturbed lakes in south-central Ontrario, Canada. Lake Reserv Manage. XX:XXX–XXX. We examined a 41 yr data record from 4 monitored lakes in south-central Ontario, Canada, to demonstrate and explain long-term changes in chloride (Cl) loading and in-lake concentrations in relation to the degree of watershed disturbance. A mass balance calculation for each lake was used to determine ungauged shoreline loads. Correlations of lake Cl concentrations with each of 3 major contributing sources (inputs from inflowing streams, atmospheric deposition, and ungauged shoreline areas) revealed the contribution of each source to Cl trends at each lake. Areal loads (Cl load per year per unit area) showed the relative intensity of the contributions. Two lakes (Red Chalk, Plastic) with relatively undisturbed watersheds showed declining trends in atmospheric deposition, stream loads, lake export, and lake Cl concentrations, whereas 2 lakes (Harp, Dickie) with disturbed watersheds showed increased stream loads, lake export, and lake concentrations, despite long-term declines in atmospheric deposition. This contrasting trend was related to increased Cl concentrations in inflowing streams, which were caused by increased residential development and road salt applications. The ungauged shoreline loads in the disturbed watersheds made a contribution as high as 80% to total Cl loads. As increasing Cl concentrations have implications for aquatic biota, even at the relatively low concentrations observed in the study lakes, these findings highlight the need for improved management of road salt applications. Our results confirm that even relatively remote lakes within the Canadian Shield are potentially at risk from watershed disturbances that are disrupting the natural loading of Cl to inland waters.