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I n situ enclosure experiments were performed in a highlr humic lake to examine the effects of acidification on phyto-, bacterio-and zooplankton. The acidity of three enclosures was adjusted with H 2 S 0 4 to pH 3.5, 4 and 5 and one enclosure and the lake served as controls. The diversity of plankton as well as the mean concentration of chlorophyll a, primary production and respiration of plankton decreased with increasing acidity. Bacterial density was slightly lower in the pH 3.5 enclosure than in the other enclosures and in the lake, but there were no differences in the morphological type or mean volume of the cells between different treatments. In theacidifidenclosures Cryptomonas ozwh and Chlainyclomonas spp. were the dominant phytopisnkters, while Dinobryon and Mallmnonas species seemed t o be most sensitive to acidity. Keratella cochleuris, Kellicoftia longispina and Bosminca. longispina were the most tolerant zooplankton to acidity.

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