Critical loads of acidity to surface waters: An empirical diatom-based palaeolimnological model

Attempts to reverse surface water acidification over large areas of Europe and North America require major reductions in the emission of sulfur dioxide from fossil-fuel combustion plants. The second sulfur protocol recently signed in Oslo (June 1994) is based on a 'critical loads' approach(1) to emission reduction. In this approach sulfur dioxide reductions are calculated according to the need to reduce acid deposition at sensitive acidified sites to below a threshold, or critical load, where ecological 'damage' should not occur (1,2). Using our large dataset of inferred acidification profiles from dated lake sediment cores throughout the UK (3- 5), we have developed an empirical model based on a dose-response function that can be used to set critical load values for a site from a knowledge of the ratio of Ca⁺⁺ of the water (sensitivity) to modelled S deposition (loading) at the site. This diatom model is most suited to setting the baseline critical load for a site as, in theory, it identifies the first point of biological change in the acidification of an aquatic ecosystem and, unlike the steady-state water chemistry model, it is not dependent on the fixing of a threshold ANC value for a waterbody. We compare the use of both models in the generation of critical-load maps for the UK.