In a 1951 experiment, the two basins of acidic, dark water Peter-Paul Lake, Michigan, U.S.A., were separated by an earthen dam. Peter Lake was limed and Paul Lake was not. Since approximately 1920, reconstructed fossil diatom communities of the two basins were similar immediately before 1951 and different after 1951. Communities of both basins were altered in 1951, indicating concurrent nonliming factors were active. Since 1951 the Peter Lake diatom community has gone through a series of species successions apparently influenced by liming. Most recently, the Peter Lake community had nearly the same community structure as just prior to the manipulation, whereas the Paul Lake community stabilized soon after 1951 and retained its altered structure. The two communities were different before the 1951 experiment, and the use of Paul Lake as a reference was questioned. Postmanipulation diatom inferred pH was significantly higher in Peter Lake only when compared to premanipulation values in the same basin.
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