Inorganic nitrogen retention in acid‐sensitive lakes in southern Norway and southern Ontario, Canada—a comparison of mass balance data with an empirical N retention model

摘要

In-lake retention of inorganic nitrogen species (nitrate and ammonium) was estimated from mass balances in five acid-sensitive lakes in southern Norway and eight in southern Ontario, Canada, to evaluate an empirical in-lake N retention () model. This model is included in the First-order Acidity Balance (FAB) model, which currently is used for calculation of critical acid loads and exceedances in many countries. To estimate in-lake , the FAB model uses a recommended mass transfer coefficient () of 5 m year, which mainly is derived from NO mass balances in Canadian lakes. To date, the in-lake model has not been evaluated for large parts of Europe. At the Norwegian study sites receiving the highest N deposition (>120 meq m year) the net in-lake retention of inorganic N (TIN) exceeded the corresponding terrestrial retention by a factor of 1·1–2·6. Despite differences in N loading and hydrology at the Norwegian and Canadian sites, both the mean mass transfer coefficients for NO (; 6·5 versus 5·6 m year) and TIN (; 7·9 versus 7·0 m year) were of comparable magnitude. Both mean values and ranges of suggest that the default value presently recommended for FAB model applications seems valid over a large range in N inputs and areal water loads (). However, owing to the relatively few data available for lakes with high values (15–150 m year), it is recommended that more lakes within this range be included in future studies to obtain a more precise prediction of in-lake N retention over a wide gradient. Also, when considering that the FAB model treats all inorganic N leaching from a catchment as NO, it seems reasonable to use a default value instead of just when estimating in-lake . In that case, the in-lake presently calculated by the FAB model might be slightly underestimated. Copyright © 2003 John Wiley & Sons, Ltd.