Sediment Oxygen Demand of the Big Eau Pleine Reservoir, Marathon County, Wisconsin
Hammermeister, Dean E.
University of Wisconsin-Stevens Point, College of Natural Resources
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Sediment oxygen demand (SOD) measurements were determined in 1975, 1976, and 1977 in the Big Eau Pleine Reservoir, Marathon County, Wisconsin. The reservoir is eutrophic and subject to periodic winter kill of fish. This research provided data to estimate the temperature corrected reservoir SOD. A calibrated computer model utilized SOD to compute the concentration of dissolved oxygen in the reservoir. SOD measurements were accomplished using an In Situ technique in conjunction with undisturbed sediment cores in the laboratory. Sediment physical and chemical characteristics were measured to ascertain their relationship to SOD. The seasonal pattern of SOD found is primarily because of temperature influence and to a lesser degree, seasonal primary production. The range of SOD measurements was 0.04 (February, 1976) - 1.80 (August, 1977) gO2.m^-2.day^-1. The mean In Situ Situ SOD at 20°c for summer 1976 and 1977 was 1.01 and 1.11 gO2.m^-2.day^-1, respectively. The mean winter 1976 In Situ SOD and 1977 core SOD corrected to In Situ values at 2°C were 0.080 and 0.111 gO2.m^-2.day^-1, respectively. The greater SOD occurring in winter 1977 is attributed to the greater algal biomass present.Annually, core abiotic SOD represented 46% of the total core SOD. A good relationship between.!!!. Situ and core techniques (r = 0.983, n=l3) allowed temperature effects on SOD to be quantified. The relative standard deviation of duplicate In Situ and replicate core SOD measurements were 14% (n=6) and 19% (n=24), respectively. The mean reservoir SOD values (20°c) calculated for October - April and May - September study periods are 0.476 and 0.803 gO2.m^-2.day^-1, respectively, and are significantly different (p<0.01). These values can be temperature corrected as required. A sediment map based on% silt + clay and% organic matter content was developed to enable SOD data to be weighted to a mean reservoir value. A good relationship (r= 0.538) between % silt + clay,% organic matter and SOD for the period October - April allows weighting to be possible. For the period May - September, the mean of all summer SOD measurements corrected to 20°c should be used in the computer model. The relationships of SOD and physico-chemical characteristics measured were greatest when the 0.0-0.5 cm sediment layer was analyzed. SOD can be predicted by multiple regression analysis of% silt+ clay (p<0.05), % volatile solids and % iron (r^ 2 = 0.752). Oxygen depletion in the reservoir during winter is attributed to resuspension of anaerobic sediment. SOD of undisturbed sediment is not sufficient to cause winter kill when the reservoir volume is great. Adopting a drawdown practice that minimizes sediment resuspension and retains 20-30% of reservoir volume will help alleviate problems associated with low dissolved oxygen concentrations.