, Volume 11, Issue 1, pp 23-43

The role of ammonium and nitrate retention in the acidification of lakes and forested catchments

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The relative contribution of HN03 to precipitation acidity in eastern Canada has increased in recent years leading to some concern that the relative importance of NO 3 deposition in acidification of terrestrial and aquatic ecosystems may increase. To gauge the extent of this impact, annual mass balances for N0 3 and NH+ 4 were calculated for several forested catchments and lakes in Ontario. Retention of NH+ 4 (R NH4) by forested catchments was consistently high compared to retention of NO3 (R NO3) which was highly variable. Retention of inorganic nitrogen was influenced by catchment grade and areal water discharge. In lakes, the reciprocals of retention of N0 3 and NH+ 4 were linearly related to the ratio of lake mean depth to water residence time (z/τ; equal to areal water discharge), and retention did not appear to be a function of degree of acidification of the lakes. Net N consumption-based acidification of lakes, defined as the ratio of annual NH; mass to N0 3 mass consumption, was negatively correlated with /τ and N consumption-related acidification was most likely to occur when − was < 1.5 m yr−1.

If retention mechanisms are unaffected by changes in deposition, changes in deposition will still result in changes in surface water concentrations although the changes will be of similar proportions. Therefore, ‘NO 3 saturation’ should not be defined by concentrations alone, but should be defined as decreasing long-term, average NO 3 retention in streams and lakes in response to long-term increases in NO 3 deposition. Analysis o f survey data will be facilitated by grouping lakes and catchments according to similar characteristics.