Hypolimnetic oxidation rates in Lake Superior: Role of dissolved organic material on the lake's carbon budget

McManus, James, Erik A. Heinen, Matthew M. Baehr

Limnol. Oceanogr., 48(4), 2003, 1624-1632 | DOI: 10.4319/lo.2003.48.4.1624

ABSTRACT: On average, the water column of Lake Superior is undersaturated with respect to dissolved oxygen and supersaturated with respect to carbon dioxide during the summer-stratified period. On the basis of temporal changes in water column dissolved oxygen, we calculate rates of oxygen consumption that range from 0.19 to 0.75 mmol m-3 d-1. These rates are a factor of 5-10 times larger than can be supported by the particulate carbon settling rates and benthic oxygen consumption rates. In addition, on the basis of the limited information available, dissolved allochthonous carbon inputs are insufficient to account for the calculated rates of carbon oxidation. Rates of nitrate and total CO2 (sigma.uc.gif - 58 BytesCO2) production are 0.019 ± 0.012 and 0.13 ± 0.06 mmol m-3 d-1, respectively, and are consistent with the oxidation of a dissolved organic component that is similar in composition (C :N ratio) to the settling particulate material. Previously published estimates of total primary production were smaller but similar in magnitude to our integrated water column respiration rates. We interpret the observed imbalance between particulate carbon delivered to the deep lake and the calculated rate of carbon oxidation to be the result of the decomposition of dissolved organic carbon that appears to have both an autochthonous and an allochthonous component.

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