A bstract: Diversity o f the Tanytarsini chironomids (Diptera: Chironomidae) collected on the Żarnowieckie Lake (northern Poland) in the early 1980s and in 2008-2009 was compared. The analysis demonstrated a significant impoverishment o f the Żarnowieckie chironomid fauna. Within less than 30 years, the number o f Tanytarsini species dropped from 21 to 14, and the fauna became dominated by common eurytopic species Cladotanytarsus atridorsum Kieffer (more than 73% o f all individuals collected

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... 2008), C. mancus (Walker) (more than 8%), and Paratanytarsus inopertus (Walker) (more than 9%). The four species rare in Poland [ Tanytarsus niger Andersen, Rheotanytarsus muscicola Thienemann, Stempellinella brevis (Edwards), Paratanytarsus tenellulus (Goetghebuer)], recorded in the area in the 1980s, were absent in 2008-2009. The reduction in the Tanytarsini diversity is interpreted as an effect of unstable environmental regime of the lake following its artificial modification in 1983 to serve the needs o f the "Żarnowiec" pumped storage power station, and interventions related to the planned construction o f a nuclear power plant on the lake.