The present paper is devoted to the study of the degradation of zooplankton communities in Karelian lakes polluted by the mineral waste waters of Kostamus Mill.

The species Eudiaptomus sp. and Heterocope appendiculata Sars. (Calanoida) were the most vulnerable to the action of potassium-bearing waste water. In undiluted media they died within 2-4 hours. Sida crystallina (O.F.Muller) died in 6-48 hours. The species Macrocyclops albidus (Jur) and Daphnia magna Straus displayed the highest tolerance. They survived in waste water through a month. Similar tolerance sequence has been observed in the experiments with potassium nitrate solutions. Median lethal concentration of potassium ions (mg/l) made up: 41 (34-49) for Eudiaptomus sp., 80 (69 - 93) for S. crystallina, 135 (115 - 160) for D. magna, 325 (281 - 373) for M. albidus.

The results of the toxicological tests on the least tolerance of Calanoida to the action of potassium ions have been generally supported by the on-site investigations. An emergency discharge of a great volume of waste water in the summer of 1994-1995 was followed by an increase in potassium concentration up to 42-60 mg/l hence exceeding the 34 mg/l - the critical for Calanoida level. Prior to the emergency discharge in 1992-1993 the number of Eudiaptomus made up 777-2148 specimens in cub. m whereupon in fell to 19-123 in 1994, and 0-30 in 1995. Similar abundance decrease was observed also for H. appendiculata.

Gradual increase of potassium content in the lakes results in successive exceeding of the critical levels for various plankton species, their extinction and simplification of the planktonic community structure where only the most tolerant species - cyclopoids and rotifers - survive.