Reappearance of highly acid-sensitive invertebrates after liming of an alpine lake ecosystem

The amphipod Gammarus lacustris was earlier a main food item of brown trout in Lake Svartavatnet at the Hardangervidda mountain plateau in South Norwa y. In the middle of the 1980's, G. lacustris disappeared from the trout die t due to increased acidification. In order to preserve a unique genetic var iant of brown trout living in the area, a liming programme was initiated in 1994. During the first years after liming, G. lacustris was absent both in fish stomachs and in lake littoral samples. In 1999, it reappeared in brow n trout stomach samples together with two other strongly sensitive species, the tadpole shrimp Lepidurus arcticus and the freshwater gastropod Lymnaea peregra. Data from monitoring indicate that the water chemical conditions of L. Svartavatnet are still close to the critical limits of these animals. They have probably survived in small refuges of acceptable water quality, either in areas of inflowing groundwater or in the littoral, below the more acidic surface layer. The fact that these sensitive animals have not yet b een found in benthic samples emphasise fish diet as an important tool in ea rly registration of the presence/absence of invertebrates with low abundanc e or patchy distribution.