RECOVERY FROM ACIDIFICATION IN LAKE-ORVATTNET, SWEDEN

Lake Orvattnet has been monitored extensively for both chemical and bi ological variables since 1967. The lake acidified during the 1960's an d pH was mostly below 5 throughout the 1970's. Due to the acidificatio n, peat moss (Sphagnum spp.) expanded over the lake bottom and the onl y surviving fish species was perch (Perca fluviatilis), but it experie nced reproduction problems. In the mid 1980's, the Sphagnum cover coll apsed, and by 1989 it had almost disappeared. There has been a clear r ecovery of the perch population. Recovery of the lake is also recorded by diatom assemblages in the lake sediment. Diatom-inferred pH increa sed from 4.7 to 4.9. The development of measured lake-water pH is uncl ear, but acid episodes in spring have become less severe. By 1993, atm ospheric sulphate deposition had decreased by 30-40% in this area of S weden compared to the late 1960's. Lake-water sulphate concentrations have decreased by similar to 30% since the 1960's. Nitrogen deposition has increased over the last decades, but is not yet contributing to l ake acidification. No major land-use changes have occurred and changes in hydrology cannot explain the observed changes in chemistry and bio logy. We ascribe the recent recovery in the lake to reduced deposition of sulphate. In conclusion, Lake Orvattnet has begun to recover from acidification.