The structure of the mid-summer planktonic rotifer communities of 101 Adirondack lakes ranging in pH from 4.0 to 7.3 were characterized in relation to acidity and selected water quality parameters. More than 70 rotifer species were identified from collections in 1982 and 1984. None of the species collected could be considered acidobiontic or alkalibiontic.

Keratella taurocephala was the most commonly collected rotifer, occurring in 94 of the study lakes. It was abundant throughout the range of pH investigated but was particularly dominant in acidic waters, averaging \> 85 \% of the rotifers collected from waters of pH \< 5.0.

Rotifer community structure can be related to three groups of water quality parameters. Community parameters (richness and diversity) are most highly correlated with parameters indicative of acidity status. Rotifer abundance correlates with trophic state indicators, i.e. chlorophyll a and total phosphorus, over the full range of pH investigated. However, in acidic lakes, rotifer abundance is related to true color and DOC, indicators of humic influences.

The rotifer communities of the Adirondacks can be classified into four broad types: 1) A diverse, productive community of the more alkaline lakes, generally with ≈ 13 species, and dominated by Conochilus unicornis, Kellicottia bostoniensis, Kellicottia longispina, and Polyarthra major; 2) Relatively diverse communities of productive, highly colored acid lakes, with ≈ 8 species, and often with very large populations (\> 200 · 1-1) dominated by K. bostoniensis and K. taurocephala; 3) Depauperate (\< 4 species) communities of clear water acid lakes with generally low density populations dominated by K. taurocephala (\> 90 \% of rotifers in each sample); and 4) Extremely depauperate (2–3 species) acid lake communities associated with small lakes with high flushing rates dominated by C. unicornis.