Long‐term changes of diatoms and chemistry in headwater streams polluted by atmospheric deposition of sulphur and nitrogen compounds

SUMMARY 1. Samples for the analysis of attached diatoms and surface water chemistry were taken at thirty‐five stations in sixteen soft‐water streams in the Netherlands during autumn 1990. Most were located in pine forest and heathland catchments, fed by deep groundwater (median residence time 87 years), with no direct influence of agricultural or urban drainage water. 2. The chemical data from sixteen stations were compared with data collected in 1974 and 1981. There were no significant increases in pH (6.3 in 1974, 6–4 in 1990) or in sulphate (200 mmol m−3in 1974, 229 mmol m−3in 1990). Over the same period nitrate increased significantly from 17 to 158 mmol m−3, while alkalinity decreased significantly from 355 to 251 meq m−3. 3. The most important correlates with the distribution of diatoms, as revealed by canonical correspondence analysis, were pH, nitrate, cross‐sectional area of the stream and the Ca/(Ca + Cl) ratio. 4. Diatom assemblages of upstream stations indicated more acid conditions than those of downstream stations. Also there was considerable variation in the diatom assemblages of different habitats, 5. A diatom‐pH transfer function was developed using the weighted‐averaging method. The function was applied to comparable pairs of samples which were collected in 1974 and 1990 at ten stations. There was a significant decrease in the median diatom‐inferred pH from 6.78 in 1974 to 6.55 in 1990. 6. The diatom assemblages are diverse and contain a high proportion of filamentous diatoms, particularlyAulacoseira crenulataandA. alpigena, which are rare in Western Europe. This is a response to the low current velocity (median 5 cm−1) and the relatively constant deep groundwater discharge.