Causes and implications of changes in phosphorus delivery to Precambrian Shield lakes during the critical spring melt period

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Phosphorus (P) levels in softwater Precambrian Shield lakes in Ontario have declined over the past 3 decades, and are now approaching concentrations that could cause shifts in phytoplankton community composition and structure and impact fishery resources. These declines in lakewater P levels have occurred despite general increases in human activity (e.g., cottage use) in the watersheds of many lakes. Declines in lake P levels are associated with decreases in P delivery to lakes from their terrestrial watersheds, and P concentrations and export have declined most dramatically during the spring snow melt period. As spring snow melt accounts for over 50% of the annual flux of water and elements in Canada, changes in the spring delivery of nutrients are of critical importance for lake budgets. Furthermore, climate change projections for eastern Canada include winter warming, which may further impact nutrient delivery from watersheds to lakes. The objectives of this research are to (a) identify the cause(s) of declines in spring P export from watersheds to lakes on the Precambrian Shield, and (b) determine the impact of changes in P delivery to lakes on internal cycling and phytoplankton dynamics during the critical spring period. We will address these objectives by conducting detailed measurements of P delivery to watersheds (from the atmosphere) and lakes (from stream flow) at calibrated watersheds on the Precambrian Shield in south-central Ontario, that have been monitored by the Ontario Ministry of Environment for > 30 years. Detailed abiotic and biotic measurements within watersheds and lakes together with high-frequency monitoring of surface water quality and quantity will allow us to identify the controls on P export and cycling and evaluate the response of lakes to future changes in P delivery.