A PRELIMINARY ASSESSMENT OF NITROGEN-BASED FRESH-WATER ACIDIFICATION IN SOUTHEASTERN CANADA

Sulphate deposition is the primary cause of acidification in northeast ern North America, and new SO, emission control is being implemented. However, continuation of existing levels of N deposition may undermine the environmental benefits derived from SO, control. This likelihood has been assessed for Canadian lakes. Maximum N deposition (similar to 13 kg N ha(-1) yr(-1)) occurs in south-central Ontario and southweste rn Quebec. Regional median NO3- levels are generally low (<5 mu eg L(- 1)) suggesting that on average, N-based acidification is minor compare d to the S-based component. However, examination of the seasonal NO3- pattern at 5 intensively monitored basins reveals that 2 of them (in O ntario 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; leaching. Many lakes throughout southeastern Canada exhibit some leaching, particularly those in south -central Ontario and southwestern Quebec. While the evidence for a dep osition-acidification link appears strong, sources of N other than the atmosphere should be considered for certain anomalous cases.