Incongruent changes in water quality over 20 years reveals different roles for land-use and climate across the Lake Scugog watershed

Lake Scugog is a large headwater reservoir located in southern Ontario that serves as a major drinking water source across its watershed and beyond. Due to its role as a sizable feeder system to the nationally significant Trent-Severn Waterway (TSW), and status as a provincially significant inland fishery, a watershed-scale water-quality-monitoring program was established in 2004. We assessed 20 years of the irregular time-series with general additive models to identify significant periods of change in water quality parameters, as well as finite differences for tributary and in-lake sites. We observed contrasting total phosphorus (TP) trends between tributary and in-lake locations, where there was no significant (p-value > 0.05) change in tributary TP trends, but significant (p-value < 0.05) increasing TP trends at lake sites. This shows an apparent incongruence of phosphorus-concentration trends flowing into Lake Scugog with in-lake phosphorus trends, and warrants further study. Although in-lake total nitrogen (TN) and nitrate significantly declined over the study period, they were not strongly associated with environmental drivers in contrast to Total Khejdahl nitrogen, which had a strong association with total precipitation. These findings highlight notable trends and shifts in water quality in the Lake Scugog watershed over a time-frame that captured growing urbanization and climate impacts. As a sentinel ecosystem for the Kawartha Lakes and TSW, our results serve as an early-warning of how water quality may change for other inland lakes in Ontario within the context of climate change and increasing land-use pressures.