Effect of Ambient Oxygen Concentration on Measurements of Sediment Oxygen Consumption

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Abstract
The disparity between hypolimnetic oxygen depletion in situ (HOD; independent of the ambient oxygen concentration), used to calculate sediment oxygen consumption (SOC) in one lake in southern Quebec, and measurements of SOC (dependent on the oxygen concentration) was minimal when experimental factors were controlled. Using gas-tight methods, SOC at 7–5 mg∙L−1 decreased by 75% when the oxygen concentration in respirometers fell to 3–1 mg∙L−1. With continuous mixing, the comparable decrease in SOC was 50%. Finally, the decrease was less than 30% and obvious only below 2 mg∙L−1 when the oxygen concentration was held constant in continuous flow respirometers. SOC results using the last procedure were within 10% of the mean SOC calculated indirectly from hypolimnetic oxygen budgets for the two previous years. Thus, mixing and a constant or slowly changing ambient oxygen concentration are required for realistic SOC measurements. In another experiment, oxygen uptake in water overlying the sediment accounted for a large and variable portion of the SOC, often more than 50%. Therefore, it may be impossible to separate the influence of the water and sediment on the HOD.