Seasonal interplay of discharge and phosphorus concentration: Year-round areal loads to western basin Lake Ontario from urban and urbanizing tributaries

The export of phosphorus from watersheds to large temperate lakes is strongly linked to the seasonality of discharge from landscapes. Urbanization in watersheds alters flow dynamics and provides novel point and diffuse sources of nutrients, creating “fast landscapes” that augment nutrient export to receiving waters. Here, a combination of hydrologic event-based composite and baseflow grab sampling regimes were used at 13 Lake Ontario tributaries along a ∼ 140-km stretch of the Canadian shoreline from 2018 to 2023 to investigate seasonality in discharge, phosphorus, and suspended solid concentrations, and areal loads across watersheds that span a gradient of urbanization for southern Ontario. Rivers within watersheds with a higher degree of urbanization had elevated dissolved phosphorus (DP) concentrations (highest in warm seasons), areal discharge, and areal loads of DP, particulate phosphorus (PP), and suspended solids (SS) across all seasons. At the mixed-use and rural watershed rivers, export of DP, PP, and SS were chemodynamic with discharge, highest in spring and winter and relatively lower during summer and autumn months. The highly urbanized rivers likely have the greatest impact on Lake Ontario’s nearshore water quality as they provide a relatively consistent supply of phosphorus to the nearshore zone.