HR: 0800h
AN: B41A-0167    [Abstracts]
TI: Silica Deposition in Two Large, Natural Riverine Lakes Sheds Light on Global Silica Cycle
AU: * Triplett, L D
EM: trip0043@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, 310 Pillsbury Dr. SE Pillsbury Hall room 108, Minneapolis, MN 55455 United States
AU: Engstrom, D R
EM: dre@umn.edu
AF: Science Museum of Minnesota, St. Croix Watershed Research Station, 16910 152nd St. N., Marine on St. Croix, MN 55047 United States
AB: Two large, natural riverine lakes in the upper Mississippi River basin offer a unique perspective on how human activities have affected the global silica cycle. Conley et al. (1993, Mar. Ecol. Prog. Stud., 101:1-2) and others have demonstrated that the eutrophication of artificial impoundments on large rivers around the world has significantly increased the sequestration of biogenic silica (as diatom frustules, etc.) in impoundment sediments, thereby decreasing silica supply to the oceans. One difficulty has been in establishing the ``natural'' (pre-dam) silica concentrations in large rivers when water quality data does not extend beyond a few decades. A mass balance approach was used to characterize the modern silica budgets of Lake Pepin and Lake St. Croix, large lakes in the Mississippi and St. Croix Rivers, respectively, in Minnesota and Wisconsin. These lakes formed as glacial meltwaters subsided around 9000 years B.P., so they provide the relevant qualities of impoundments (continuous sediment deposition, lake-like conditions for primary productivity) without the inherent disruption caused by dam building. Silica concentrations were measured in the lake inflows, outflows, lake water columns and surface sediment porewaters during 2004-2005, and the in-lake diatom productivity was calculated. In addition, these two lakes provide a valuable long-term record; our understanding of the lakes' modern silica cycles can help constrain historical values inferred, in part, from biogenic silica in the lake sediments. The timeline of twentieth century eutrophication of each lake has been established in previous paleolimnological studies, providing further perspective on the interconnection of silica and phosphorus cycles and land-use change.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
DE: 1834 Human impacts
DE: 1871 Surface water quality
SC: Biogeosciences [B]
MN: Fall Meeting 2005