Abstract
Most changes in profiles of relative abundance of Chironomus, Sergentia, Procladius, Heterotrissocladius, Micropsectra, Tanytarsus, and other taxa in dated sediment cores suggested a trend to more oligotrophy conditions at present in four dimictic lakes, but not in the single polymictic lake in which species composition was least variable temporally. Decreased numbers of chironomids and chaoborids toward the present indicated declining productivity in all five lakes. Total phosphorus concentrations in sediments declined through these periods of inferred greater oligotrophy. Most of these trends were probably not caused by anthropogenic acidification because they began up to 130 yr B.P. and they were sometimes duplicated even deeper in sediment cores. Furthermore, taxa common in acidified lakes elsewhere did not increase in these lakes. Recent acidification may have affected taxonomic composition and diversity of chironomids in the headwater lake, the only lake which showed a near-surface reduction in members of the tribe Tanytarsini, and the ratio of taxa to individuals. Chaoborus americanus occurred at high densities throughout the cores from the two upper lakes, indicating that these lakes probably never had fish communities.
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