A preliminary assessment of nitrogen-based fresh water acidification in southeastern Canada

Sulphate deposition is the primary cause of acidification in northeastern North America, and new SO₂ emission control is being implemented. However, continuation of existing levels of N deposition may undermine the environmental benefits derived from SO₂ control. This likelihood has been assessed for Canadian lakes. Maximum N deposition (～13 kg N ha^?1 yr^?1) occurs in south-central Ontario and southwestern Quebec. Regional median NO ₃ ^? levels are generally low (<5 μeq L^?1) suggesting that on average, N-based acidification is minor compared to the S-based component. However, examination of the seasonal NO ₃ ^? pattern at 5 intensively monitored basins reveals that 2 of them (in Ontario and Quebec) have incipient N saturation. A regional status for nitrogen-based acidification was qualitatively assessed by classifying survey data to identify cases of NO ₃ ^? leaching. Many lakes throughout southeastern Canada exhibit some leaching, particularly those in south-central Ontario and southwestern Quebec. While the evidence for a deposition-acidification link appears strong, sources of N other than the atmosphere should be considered for certain anomalous cases.