SWAP (Surface Water Acidification Programme) surface sediment diatom data from Birks et al. (1990) and Stevenson et al. (1990). Dataset is a list with the following named elements: (spec) diatom relative abundances for 277 taxa in 167 surface samples, (pH) associated lake-water pH. Column names in spec are short, 6-character alphanumeric codes for each diatom taxon. SWAP$names contains the full names for each taxon, in the correct order).