Assessment of Critical Loads of Acidity and Their Exceedances for European Lakes

Abstract

Lake acidification in northern Europe provided some of the key impetus for the development of the critical loads approach during the 1980s. While major reductions in acidic deposition have been achieved during the last 20 years, through the application of this approach, regions with continued acidification and critical load exceedance persist around Europe. This chapter describes regional applications of the First-order Acidity Balance (FAB) model in five European countries, highlighting national approaches to lake surveys and regional representation, and how the model has been adapted in each of these countries. We discuss the implications of interpreting critical load exceedances, and provide an overall synthesis of freshwater exceedance in Europe using common European deposition data. Despite uncertainties within the FAB model, such as the parameterisation of nitrogen immobilisation and denitrification, a coherent picture of the spatial extent of acidification within European lakes is evident. The ongoing failure to meet critical loads by 2020 demonstrates that lake acidification is still a current, not a historical, problem in Europe, and under current legislation many lakes will remain more acidic than their pre-industrial reference condition.

Conclusions

National scale applications of the FAB model provide maps of critical load exceedance which are highly consistent with areas of known acidification, as verified by independent chemical and biological monitoring studies, despite simplifications and uncertainties associated with steady-state models and critical limits (Chap. 2). Furthermore, many studies do show a strong relationship between increasing surface water ANC or pH and biological recovery (e.g. Hesthagen et al. 2011; Johnson and Angeler 2010; Kernan et al. 2010; Posch et al. 2012). In Finland, recovery in recruitment of perch populations matches well with reduced exceedance of critical loads (Posch et al. 2012). At long-term monitoring sites in the UK Acid Waters Monitoring Network, the re-appearance of acid-sensitive macrophytes and invertebrates has been linked to improvements in ANC with evidence that the critical ANC value of 20 µeq l⁻¹ does indeed represent an important ecological threshold for some species (Kernan et al. 2010). Similar results were found in a Norwegian lake study where the achievement of critical load and ANC > 20 µeq⁻¹ coincided with significant recovery in brown trout and invertebrate species (­Hesthagen et al. 2011). Swedish studies comparing acidified but recovering lakes with unimpacted ­reference lakes showed a movement of impacted phytoplankton and invertebrate assemblages towards those of reference lakes as pH of acidified lakes increased over 20 years (Johnson and Angeler 2010).

Hence the continued exceedance of FAB critical loads beyond 2020 in all five countries included here is a cause for concern as planned emission reductions under the recently revised Gothenburg Protocol do not go far enough. Critical load ­models like FAB evidently still have a role to play in shaping emissions policy for the ­protection of aquatic ecosystems. However, as emissions reductions become ever more expensive to achieve, the areas of uncertainty will face increasing ­scrutiny. The role of chemical and biological monitoring programmes will be critical in providing the supporting data against which model performance can be evaluated.