Challenges in Defining Critical Loads for Nitrogen in UK Lakes

Abstract

It is now widely recognised that the deposition of nitrogen (N) compounds can lead to both acidification and eutrophication impacts in upland lakes. While major reductions in sulphur (S) emissions and deposition in the UK have been largely matched by chemical recovery from acidification in surface waters, reductions in emissions of N compounds have not been matched by corresponding reductions in deposition. Here we explore two related issues in the use of critical loads for N in upland waters:

• 1. Identifying potential impacts of nutrient N in naturally nutrient poor systems of conservation importance and links to biodiversity, and

• 2. Problems in defining critical chemical limits with respect to reference conditions in upland lakes.

Empirical critical loads for nutrient N have been recommended to protect macrophyte communities of shallow softwater lakes in Europe. The recommended range of 5–10 kg N ha⁻¹ year⁻¹ is exceeded across most of the UK, but most oligotrophic lakes are of a different habitat type to those for which empirical critical loads have been recommended. Furthermore, while there is widespread evidence from nutrient bioassay work for N limitation of phytoplankton production in oligotrophic lakes there is little direct evidence to date of impacts on other biological groups in the UK, including macrophytes. A major problem is the lack of data on reference communities in unimpacted lakes and lack of identified “harmful ecological effects” required by the definition of critical loads. There are also fundamental differences in approach between the critical loads employed under the UNECE Gothenburg Protocol and the EU Water Framework Directive. We show that there are major challenges in application of critical loads for nutrient N which must be overcome if we are to protect designated sites of conservation interest and maintain, or allow recovery to, the good ecological status required by the EU Water Framework Directive.