Urbanization modifies phosphorus cycling and drives eutrophication in Lake Wilcox, a kettle lake in the Toronto metropolitan area

The watershed of Lake Wilcox (LW), a shallow kettle lake located in the Greater Toronto Area (GTA) in southern Ontario, has experienced multiple phases of land use change since the early 1900s. Land surface imperviousness increased from near-zero to about 65% due to urbanization, especially since 2000 when stormwater management infrastructure was installed. These watershed changes coincide with observable lake water quality deterioration, more notably since the 1980s. We analyzed historical water quality and sediment core data to reconstruct the impacts of urbanization on LWs water column phosphorus (P) concentrations, trophic status, and in-lake biogeochemical cycling. Direct water quality data for the period 1995-2018 include total P (TP), dissolved inorganic P (DIP), dissolved oxygen (DO) and chlorophyll-a (chl-a). The 76 cm-long sediment core taken in the lake and dated using lead-210 provides insights into changes in P accumulation rates and speciation going back to the 1920s. In addition to the depth distribution of HCl-extractable TP, selected depth intervals of the core were subjected to a sequential extraction procedure targeting the following operational P pools: redox-labile, humic-bound, organic and exchangeable P. Results show correlations between increased impervious cover and decreased DO and increased water column chl-a, TP, DIP and DIP:TP ratio in the water column. The TP burial flux, however, has decreased since the 1980s, while the total and relative concentrations of sediment organic and humic-bound P have increased. The decrease in the sediment TP accumulation rate is likely due to a sediment diversion channel built in the 1980s. Altogether, our findings suggest that urbanization has caused important changes in P cycling in LW, with more active in-lake recycling of P including a large increase in the relative contribution of internal P loading from the sediments. The enhanced P recycling efficiency is both a symptom and a driver of LWs eutrophication. It further implies that reducing external TP loading to the lake may not be sufficient to remediate the eutrophic conditions of Lake Wilcox.