Evidence of recovery from acidification in the macroinvertebrate assemblages of UK fresh waters: A 20-year time series

This paper deals with the 20-year (1988–2008) record of macroinvertebrate sampling from the UKʹs Acid Waters Monitoring Network. At 12 of the 22 sites a significant temporal trend in the macroinvertebrate community is now evident. Indices of acidification suggest biological recovery at five of the 11 streams sites and at five of the lakes. All 10 sites indicating biological recovery from acidification also showed an increase in acid-neutralising capacity (ANC), although a further seven sites showed chemical (ANC) recovery but no evident biological recovery. On a site-by-site basis eight (four streams and four lakes) of the 20 sites investigated had significant relationships between biotic indices of acidification and chemical measures of acidity, the latter explaining between 30% and 72% of the biotic variation. Thus, the match between chemical and biological recovery is incomplete, with biological recovery lagging improvements in chemistry, modest community changes and most sites still showing signs of acid stress. The sluggish biological recovery may be ascribed to aspects of the chemical environment that are still deleterious and/or to ecological inertia in the reassembly of an acid-sensitive fauna.