one (RLP8112) from its center and the other (RLP8104) from its periphery. The site at the center of the island (RLP8112) began to accumulate peat with a pH close to 7 about 3,300 years ago. About 1,100 years later the fen began to be invaded by Sphagnum mosses, which in less than three centuries transformed it into an acid bog with a pH close to 4. This change was accompanied by a lowering of the water table relative to the peat surface. The site at the margin of the present bog (RLP8104) exhibits very different depth profiles: drier, acid bog conditions alternated with wetter, near-neutral conditions, presumably as a result of island expansion and contraction in response to differing degrees of ground water upwelling. The utility of such depth-time profiles as baselines for assessing the effects of human disturbance lies in the boundaries they set. For example, if acid deposition were to lower the pH of the bog surface substantially, we should expect to see the development of moss assemblages characteristic of much more acid conditions and distinctly different from any of the assemblages observed in these peat cores over the past three millennia. Similarly, if global warming were to lower the bog water table significantly, we should expect to see moss assemblages characteristic of drier conditions and quite different from those observed at any time since peat began to accumulate.
3. By analyzing the pH preferences of individual species of diatoms and chrysophytes, paleolimnologists can calculate the past pH of a lake from the composition of diatom and/or chrysophyte remains at various dated strata in the lake's sediments (e.g., Smol et al., 1984a,b; Charles et al., 1989; Cumming and Smol, 1993; Cumming et al., 1994). The pH preferences for these species are evaluated by examining their remains in surface sediments from numerous lakes spanning a broad range of pH, and transfer functions are derived from modern data by multivariate statistical methods for extrapolating the data to past periods. Using such techniques, Cumming et al. (1994) showed that of 20 acid-sensitive lakes examined in Adirondack Park, approximately 80 percent have acidified since preindustrial times. This information refutes the contention of some that lakes in the Adirondacks are naturally acidic. Lakes that became acidic around 1900 generally were smaller, higher-elevation lakes with lower preindustrial pH values than the lakes that did not acidify or acidified more recently. Post-1970 pH trends in the lakes have been small and variable, suggesting that the lakes have been unresponsive to post-1970 declines in sulfate deposition.