Paleolimnology of McNearney lake: an acidic lake in northern Michigan |
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| Authors: | Robert B Cook Russell G Kreis Jr John C Kingston Keith E Camburn Stephen A Norton Myron J Mitchell Brian Fry Linda C K Shane |
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| Institution: | (1) Environmental Sciences Division, Oak Ridge National Laboratory, 37831-6036 Oak Ridge, TN, USA;(2) Department of Geology, University of Minnesota, 55812 Duluth, MN, USA;(3) Present address: Large Lakes Research Station, U.S. Environmental Protection Agency, 48138 Grosse Ile, MI, USA;(4) Department of Biology, Queen's University, K7L 3N6 Kingston, Ontario, Canada;(5) Present address: 163 Park Fairfax Dr., 28208 Charlotte, NC, USA;(6) Department of Geological Sciences, University of Maine, 04469 Orono, ME, USA;(7) Department of Environmental and Forest Biology, SUNY College of Environmental Science and Forestry, 13210 Syracuse, NY, USA;(8) Marine Biological Laboratory, Ecosystem Center, 02543 Woods Hole, MA, USA;(9) Limnological Research Center, University of Minnesota, 55455 Minneapolis, MN, USA |
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| Abstract: | McNearney Lake is an acidic (pH=4.4) lake in the Upper Peninsula of Michigan with low acid neutralizing capacity (ANC=-38  eq L-1) and high SO
inf4
sup2-
and aluminium concentrations. Oligotrophy is indicated by high Secchi transparency and by low chlorophyll a, total phosphorus, and total nitrogen concentrations. The lake water is currently acidic because base cations are supplied to the lake water at a low rate and because SO
inf4
sup2-
from atmospheric deposition was not appreciably retained by the lake sediments or watershed and was present in the water column.This interdisciplinary paleolimnological study indicates that McNearney Lake is naturally acidic and has been so since at least 4000 years B.P., as determined from inferred-pH techniques based on contemporary diatom-pH relationships. Predicted pH values ranged from 4.7 to 5.0 over the 4000-year stratigraphy. Considerable shifts in species composition and abundance were observed in diatom stratigraphy, but present-day distributions indicate that all abundant taxa most frequently occur under acidic conditions, suggesting that factors other than pH are responsible for the shifts. The diatom-inferred pH technique as applied to McNearney Lake has too large an uncertainly and is not sensitive enough to determine the subtle recent changes in lakewater pH expected from changes in atmospheric deposition because: (1) McNearney Lake has the lowest pH in the contemporary diatom data set in the region and confidence intervals for pH predictions increase at the extremes of regressions; (2) other factors in addition to pH may be responsible for the diatom species distribution in the lake and in the entire northern Great Lakes region; (3) McNearney Lake has a well-buffered pH as a consequence of its low pH and high aluminium concentrations and is not expected to exhibit a large pH change as a result of changes in atmospheric deposition; and (4) atmospheric deposition in the region is modest and would not cause a pH shift large enough to be discernable in McNearney Lake.Elevated atmospheric deposition is indicated in recent sediments by Pb, V, and polycyclic aromatic hydrocarbon accumulation rates and to a lesser extent by those of Cu and Zn; however, these accumulation rates are substantially lower than those observed for acidified lakes in the northeastern United States. Although atmospheric loadings of materials associated with fossil fuel combustion have recently increased to McNearney Lake and apparently are continuing, the present study of the diatom subfossil record does not indicate a distinct, recent acidification (pH decrease).Order of the first two authors is alphabetical |
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| Keywords: | paleolimnology diatoms diatom-inferred pH trace metals sulfur lakewater chemistry seepage lake |
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