Modelling the effect of atmospheric deposition on shallow heathland lakes in the Netherlands: dry deposition on water — now you see it, now you donʹt

The most important nitrogen source for isolated heathland lakes in the Netherlands is the atmospheric deposition of nitrogen compounds. Calculations were performed to relate in-lake concentrations with deposition. Using a dynamic retention model, all major components deposited (NH4+, NO3− and SO42−) appeared to be retained by the lake. Simulations with the aquatic ecosystem model Aqu. Acid showed a big discrepancy between calculated and measured concentrations when using the estimated total deposition. However, using only wet deposition data, a reasonable fit with measured data was obtained. It was concluded that the dry deposition of nitrate and ammonium actually entering the lakes was far lower than the deposition estimated. The reasons for the apparent absence of dry deposition of nitrogen compounds on heathland lakes in general, and the impact on estimates of critical nitrogen deposition, are discussed.