Slowinski, S., Radosavljevic, J., Shafii, M., Rezanezhad, F., Wiklund, J., Parsons, C., Thomas, K., Hall, R. I., Van Cappellen, P., Ippolito, I. (2022) Changes in Lake Phosphorus Loading and Cycling Associated with the Transition of Agricultural to Urban Land Use in a Watershed of the Greater Toronto Area. In Fall Meeting 2022. AGU. https://agu.confex.com/agu/fm22/meetingapp.cgi/Paper/1159068

Land use changes modify the flows of materials, including nutrients, from watersheds into receiving water bodies. Agricultural intensification and urbanization can cause massive changes in the loadings of nutrients exported to lake ecosystems. Here, we use a sediment core dated back to circa 1920 to reconstruct the trajectories of phosphorus (P) loading, in-lake P (re)cycling, and related biogeochemical changes in Lake Wilcox (Ontario, Canada) whose initially forested watershed was converted to agricultural use in the early 1900s and subsequently underwent urban development since the 1960s. The watershed now comprises 60% urban land cover and has become integrated into the sprawling greater Toronto area. Agricultural expansion in the watershed was accompanied by a 2.5-times increase in the bulk sediment accumulation rate whereas urbanization saw a marked drop in the sediment accumulation rate to values below those observed at the start of the agricultural period. The latter drop is attributed to the growing impermeable land cover and the implementation of stormwater sediment retention ponds. Although the concentration of total P (TP) in the sediment core increased with urbanization, the TP accumulation rate decreased by around 60% compared to its peak value during the agricultural period. The chlorophyll-a accumulation rate also peaked during the agricultural period and decreased as the watershed became more urbanized. Reconstructed TP budgets for the lake indicate that both the whole-lake TP retention and in-lake TP recycling efficiencies have increased with urbanization. These findings are consistent with our previous work on water chemistry trends in Lake Wilcox since 2000, which concludes that worsening symptoms of eutrophication, including enhanced in-lake P recycling and internal loading, are driven by salinization that strengthens the lake’s water column stratification rather than by increased external P loads. These results highlight the large differences in the impact of agricultural versus urban land use on the lake’s P budget and cycling, and on other aspects of the lake’s biogeochemistry.

https://agu.confex.com/agu/fm22/meetingapp.cgi/Paper/1159068