Long-term effects of catchment liming on invertebrates in upland streams

1. Catchment liming to mitigate acidification causes major chemical change in freshwaters but longer-term effects are poorly understood. Using a replicated basin-scale experiment with a multiple BACI design (= before-after-control-impact), we assessed chemical and biological effects for 10 years after the catchments of three acidified Welsh streams at Llyn Brianne were limed in 1987/88. 2. Stream chemistry was measured weekly to monthly, and macroinvertebrates monitored annually, between 1985 and 1998. Biological change through time was assessed from the abundance and taxon richness of invertebrates. We paid particular attention to 18 species known to be acid-sensitive. The effects of liming were assessed by comparing chemical and biological trends among the three replicate limed streams, three acid reference streams and two naturally circumneutral streams. 3. Following single lime applications, acid-base chemistry in treated streams changed significantly. High mean pH (> 6), increased calcium (> 2.5 mg L-1) and low aluminium (< 0.1 mg L-1) persisted throughout the 10 years following liming. 4. The effects of liming on invertebrates were modest. Acid sensitive taxa increased significantly in abundance in limed streams, but only during 2 years following treatment. Significant effects on richness were more sustained, but on average added only 2-3 acid-sensitive species to the treated streams, roughly one-third of their average richness in adjacent circumneutral streams. Only the mayfly Baetis rhodani and the stonefly Brachyptera risi occurred significantly more often in limed streams after treatment than before it. 5. Despite these modest long-term effects on invertebrates, nearly 80% of the total pool of acid-sensitive species has occurred at least once in the limed streams in the 10 years since treatment. This pattern of occurrence suggests that the colonization of limed streams by acid-sensitive taxa reflects limited persistence rather than restricted dispersal. We present evidence to show that episodes of low pH continued to affect acid-sensitive taxa even after liming. We highlight the importance of extending the time-periods over which the effects of large-scale ecological experiments are assessed.