Paleolimnology of a Cape Cod Kettle Pond: Diatoms and Reconstructed pH

Changes in pH over the past 12 000 yr in Duck Pond in the Cape Cod National Seashore were reconstructed using a diatom—pH transfer function. The reconstructed pH suggested that the pond has been acid for its entire history with a mean reconstructed pH of 5.3 ± 0.3 (standard error of the predictive equation ± 0.5). There was a brief period during the late—glacial (at ≈11 500 BP) of higher pH of ≈6, possibly caused by increased windiness and erosion (inferred from concurrent pollen and sediment changes indicative of an open spruce—Hudsonia parkland), increased leaching of cations from outwash sands of the drainage basin, and/or increased instability of the water levels during lake development. The reconstructed pH history indicates that Duck Pond has become more acid recently (with a mean pH of 5.1 ± 0.1 for the past 150 yr) although the pH has varied both up and down throughout the Holocene with acidity as low as the present at other times in the past 12 000 yr. The significance of these changes in reconstructed pH is problematic because the pH variations lower in the core remain within the standard error of the equation and primarily reflect a long—term naturally acid ecosystem. This finding illustrates the need for more local and regional long—term lake pH histories so that naturally acid lakes can be distinguished from anthropogenically degraded lakes and ecologically sound management decisions can be made. In the pre—European—settlement levels of the core, correlations between charcoal and alkaline Fragilaria diatoms suggest that forest fires in the drainage basin may have affected pond pH. Correlations between charcoal and acid diatoms in post—European—settlement sediments suggest an effect on pond pH from recent increase in windborne industrial acids. Negative correlation between diploxyon Pinus species and the reconstructed pH and positive correlation between haploxylon Pinus and the alkaliphilic diatoms suggest that vegetation changes, which also reflect the climate and fire history of the region, have also affected pH of the pond.