BIOTIC INTERACTIONS WITHIN THE LITTORAL COMMUNITY OF SWEDISH FOREST LAKES DURING ACIDIFICATION

This paper presents an integrated response of abiotic and biotic facto rs to the acidification process in littoral communities in Swedish for est lakes. In structuring the acidified community three major forces c an be recognized. Firstly, abiotic variation, including changed water chemistry and habitat structure, has a strong impact on all trophic le vels. Low pH and increased toxicity of metals, especially aluminum, ar e ultimate causes of the extinction and succession of species during a cidification. In the acidified community, the abiotic factors affect b oth the top-down and bottom-up forces, and changes can be triggered at all trophic levels. The second force structuring the community is a b ottom-up effect caused by reduced nutrient cycling and a shift in plan t heterogeneity. This shift, mainly resulting from a shift in the CO 2 -system of the water and increased water transparency, enhances the bi omass of primary producers, e.g. Sphagnum and Juncus, and alters habit at heterogeneity in higher trophic levels. The third force structuring the acid community is atop-down effect; the loss of the fourth link i n the food chain (fish) alters the impact on the third trophic level. From being mainly regulated by predation from fish, the heterogeneity of the third level will be regulated by abiotic variation and competit ion in the acid state. This effect cascades down the food web, and the second trophic level, the herbivores, will now be preyed upon by inve rtebrates instead of fish.