Long-term water chemical trends in two Swedish lakes after termination of liming.

Lime treatment has been extensively used as a remedial measure against acidification of surface waters. In view of the decreasing acid deposition, the possible consequences of a termination of liming are discussed intensively in Sweden. This paper presents the results of the first study of long-term effects of termination of liming. The temporal trends in water chemistry were studied in two lakes after termination of liming, in a still-limed lake and in an unlimed reference lake. The lime treatment was intentionally stopped in order to evaluate the effects on water chemistry and biota. After the last liming, pH decreased steadily in both reacidifying lakes until annual mean values stabilised around 5.5-6.0 and 6.2-6.5 respectively. ANC and concentrations of non-marine Ca+Mg decreased after the termination of liming. The decreasing pH resulted in increasing trends of inorganic Al (Al(i)), which during recent years exceeded the lowest known effect level for fish on several occasions. This indicates that the lime treatment may have been terminated to early from an ecological perspective and with respect to the critical load of acidifying substances during the study period. However, during the same time, non-marine sulphate decreased in all lakes in the study and pH and ANC increased in the unlimed reference lake.