THE EFFECTS OF CATCHMENT LIMING ON THE CHEMISTRY AND BIOLOGY OF UPLAND WELSH STREAMS - TESTING MODEL PREDICTIONS

1. The catchments of three acidified streams in mid Wales were limed i n 1987/88. Here we assess their chemical and biological response in co mparison with unmanipulated reference streams over a period of 5 years post-liming. 2. Stream chemistry was measured weekly/fortnightly betw een 1985 and 1992, while macroinvertebrates were sampled annually. Col onization by acid-sensitive taxa was assessed and trends in community structure were monitored by TWINSPAN. Real biological responses were c ompared with those predicted by an empirical model constructed using c hemical data. 3. There were marked changes in stream chemistry followi ng liming: calcium concentrations and pH values increased, while alumi nium concentrations decreased to levels similar to those in naturally circumneutral streams. These conditions have persisted since liming. 4 . Empirical models predicted that stream invertebrates would respond t o the altered stream chemistry, with the establishment of communities typical of circumneutral conditions. 5. Following liming, the taxon ri chness and abundance of acid-sensitive taxa was significantly higher i n limed compared with reference streams. Colonization by, and persiste nce of acid-sensitive taxa was patchy, however, and richness was still substantially lower than in naturally circumneutral streams. Moreover , contrary to the model predictions, there were no wholesale changes i n the structure of macroinvertebrate communities. 6. We conclude that liming has created and maintained chemical conditions suitable for mac roinvertebrate communities typical of circumneutral streams, but these chemical changes have not been matched by sustained responses among t he biota.