Suspended Solids in the Western Arm of Lake Superior
1984-07
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Suspended Solids in the Western Arm of Lake Superior
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1984-07
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Abstract
Suspended solids in the western arm of Lake Superior were analyzed for their distribution and composition. A grid of up to 26 stations was selectively sampled from spring mixing in May 1983 to fall overturn in October 1983. Water temperatures and turbidity (as percent light transmission) were monitored for temporal and spatial change. Suspended solids were analyzed for concentration, grain size, organic carbon, chlorophyll-a, phaeophytin, and diatom species abundances. This study provides the first substantial evidence for the existence of a persistent, well-developed nepheloid layer in the bottom 10 to 20 m of the western arm of Lake Superior. A layer of suspended particulates near the lake floor, derived from river suspended load from spring runoff, wave erosion of the Wisconsin shoreline, and resuspension of lake floor silt and clay, developed in spring during the onset of thermal stratification. The nepheloid layer was well-developed and persistent throughout the period of thermal stratification. The details of mechanisms contributing to the maintenance of the layer are uncertain. The layer may be supplemented by sediment introduced by density flows, abrasion of the thermocline against the bottom in shallow depths, and/or local resuspension. Extremely low shear velocity values (U* values typically 1.0 to 4. O x 1 o- 3 cm/sec), ca1culated from concentration gradients and low resuspension rates taken from the sediment load, suggest that very little energy is needed to maintain the layer. Comparisons of sedimentation rates and the spring runoff sediment load indicate that the nepheloid layer does not contribute a significant amount to the sediment budget for the extreme western arm of the lake. Fall overturn disrupts the bottom turbidity and sediment is more uniformly distributed throughout the water column. The destratification probably allows much of the suspended solids to move out of the study area in the extreme western arm into other portions of the lake. The median grain size of the suspended solids ranged from 3.6 to 6.8 um (8.1 to 7.2 phi) and was moderately sorted (standard deviations of .75 to 1.0 phi) and very to extremely leptokurtic (2 to 3). Particulate organic carbon was typically 29% of the total suspended solids in surface waters and 10% below the thermocline. Concentrations of particulate organic carbon ranged from 0 to 2.0 mg/l. The concentrations of chlorophyll-a and phaeophytin averaged 3.7 ug/l and 0.4 ug/l respectively. These concentrations are typical of an oligotrophic to ultra-oligotrphic lake. Common diatom species and species succession trends in this study were comparable to those of earlier studies. The enumeration of diatoms indicates a potential usefulness for certain diatom species as tracers of polluted water from the Duluth/Superior harbor into the lake.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Barbara Mary Halfman in partial fulfillment of the requirements for the degree of Master of Science, July 1984. This thesis includes a handwritten letter dated 12-6-84 at the beginning from the thesis author, Barbara Halfman, to Dr. Darby, which was tucked into the physical copy that was scanned.
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