Responses of four lakes to post-1970 changes in acid deposition, afforestation and liming are examined by using water quality measurements and palaeolimnological analysis. pH and non-marine sulphate concentrations at an undisturbed site approximately parallel trends in precipitation and indicate that lake water quality has improved since the late-1970s as atmospheric S emissions have declined. Carbonaceous particle contamination of the lake also declines in this period but diatom analysis shows that the ecological response to these changes are as yet small. However, at a similar but recently afforested site, major changes in sedimentary diatoms have occurred and we argue that fertilizer leaching is the cause. At the two limed sites the diatom response is proportional to liming intensity but at neither site has the pre-acidification diatom flora been re-established.