Freshwater Umbrella Research

CLAG national freshwater survey

April 1990 — March 1994

Project summary

The main aims of this research project were:

  • to produce critical load and critical load exceedance maps for the UK on a 10 km x 10 km grid square basis for use in UK SO2 emission abatement policy at both national and international levels;
  • to assess problems and weaknesses of the models used to generate freshwater critical loads;
  • to relate critical loads to the chemistry and biology of acidified streams and lakes.

The critical load maps are based on the chemical analysis of a single sample from the most sensitive standing water (or headwater stream in some cases) in each 10 x 10 km grid square in the UK. The critical load class for the whole square is then that of this single sample. This deliberately aims to represent a worst case scenario and no account is taken, at this stage, of within-square variation in water quality.

The study uses three different but complementary critical loads models:

To produce exceedance maps the critical load for each square has been subtracted from the 1986-88 sulphur deposition data. Again most areas of exceedance are in line with expectation, although some regions have unexpectedly high values, either because of artificially low critical loads (e.g. in the north and west of Scotland) or because of artificially high estimates of S deposition.

By using projected reduction in sulphur deposition for the year 2005, future exceedance (target) maps can be produced to assess the potential success of the government abatement strategy. The maps show that critical loads are met for many sites, but some exceedances still remain, indicating that reductions in emissions need to be much greater than 60% for all sites to have deposition levels below the critical load.

The most important aspect of critical loads research is the relationship between water chemistry and freshwater biology. Because the Henriksen model uses preset ANC values it is necessary to relate the probability of occurrence of various freshwater organisms to ANC values. Analyses of existing datasets for diatoms, invertebrates and fish populations shows that this approach is valid, but that key species of conservation interest may only be adequately protected at ANC levels substantially greater than 0.

The target load maps for 2005 assume that streams and lakes will respond to a decrease in S deposition. Detailed studies in Galloway, where S deposition has already decreased by 40% shows that, at least for these sites, acidification is a reversible process. However, MAGIC model predictions show that recovery at some sites will be slow and that critical chemistry (i.e., positive alkalinity) will not be achieved as indicated by the empirical models because of time-dependence factors.

It is useful to be able to assess whether the biological response to decreased sulphur deposition for any site conforms to expectations, both in terms of speed of recolonisation and in terms of species composition and abundance. Because the pre-acidification biology of most acidified sites is not recorded this information can only be gained by the analysis of sediment cores, by searching for modern analogue sites in areas of low acid deposition, and by the improvement of biological models for softwater ecosystems. An analogue matching method is presented and exemplified that allows modern analogue sites to be identified. The use of this method is restricted at the moment by the small number of suitable clean reference sites. Future work should be directed towards the identification and study of such sites.

The exceedance maps produced here, do not take into account the additional acidic load on freshwater ecosystems from N deposition. Preliminary analyses of the range and geographical distribution of nitrate values in the chemistry dataset indicate that exceedances with respect to S deposition may underestimate true exceedances for many sites. Further work on N deposition, nitrate in surface waters and its effect on freshwater biology is urgently required.

Selected publications
  • Battarbee R.W. (1992) Critical loads and acid deposition for UK freshwaters. An interim report to the DoE from the Critical Loads Advisory Group (Freshwaters sub-group). ECRC Research Paper No. 5, ECRC, University College London, London, UK 115pp.
  • Battarbee R.W. (1993) Critical loads and acid deposition for UK freshwaters (Summary). A report to the Department of the Environment from the Critical Loads Advisory Group (Freshwaters sub-group). ECRC Research Paper No. 10, ECRC, University College London, London, UK 28pp.
  • Battarbee R.W. (1994) Critical loads and acid deposition for UK freshwaters. A report of the Critical Loads Advisory Group (Freshwaters sub-group). ECRC Research Paper No. 11, ECRC, University College London, London, UK 139pp.