Diatoms,lake acidification and the Surface Water Acidification Programme (SWAP): a review

The Surface Water Acidification Programme (SWAP) was set up as collaborative research project involving scientists from Norway, Sweden and the UK. Its aim was to evaluate the factors responsible for fish decline in acid streams and lakes. A substantial sub-project was concerned with the palaeolimnological evidence for acidification and its causes. The central technique used was diatom analysis. In order to harmonise methodology between the seven diatomists from four laboratories in three countries a programme of taxonomic quality control was organised, involving slide exchanges, ‘blind’ counting, and regular workshops. In addition a calibration data-set of surface sediment diatoms and water chemistry from 170 lakes was constructed and archived on DISCO, the UCL diatom database. This data-set was used to generate diatom-chemistry transfer functions for pH, DOC and total Al using a weighted averaging technique. Application of the pH transfer function to sediment cores from a range of lakes demonstrated a dose-response relationship between lake sensitivity to acidification (as represented by mean Ca²⁺ values) and acid deposition (g S m ^-2 yr ^-1), indicating the overwhelming importance of acid deposition as the cause of lake acidification.