Isotope effects of O2 consumption in a deep lake as means for understanding partitioning of O2 demand among microorganisms, particles, and sediment

The isotopic ratio O-18/O-16 of dissolved O-2 in aquatic systems is affected by the preferential biological uptake of O-16 (epsilon). Studies over the past six decades reveal that during incubation experiments, the isotopic effect of microorganism respiration (epsilon(organism)) varies in the range of -18 parts per thousand to -22 parts per thousand. In contrast, natural variations in the deep-ocean O-2 concentration and delta O-18 levels show a considerably weaker effect (similar to -10 parts per thousand). The differences between these observations have been explained to result from either O-2 uptake by sediments or organic particles that, due to diffusion-limited respiration, are expected to weakly fractionate oxygen isotopes, by mixing processes or by weak fractionation at low temperatures. To gain better insight, we studied oxygen demand and delta O-18 in the deep, cold hypolimnion of Lake Stechlin between 2018 and 2021 as well as in various laboratory incubations. Our incubation results demonstrate an epsilon(organism) of about -24 parts per thousand. Simple model calculations demonstrate a sediment O-2 demand isotope effect (epsilon(SOD)) of about -8.4 parts per thousand, and a variated water-column O-2 demand isotope effect (epsilon(WOD)) which is lower than epsilon(organism), ranging from -13.9 parts per thousand in 2019 to -23 parts per thousand in 2021. Accompanying experiments indicate that the lower magnitude of epsilon(WOD) may be related to respiration at organic particles lending to a weaker fractionation effect. Thus, variations in epsilon(WOD) may reflect a changing partitioning of hypolimnion oxygen uptake between suspended particles and their containing microorganisms. Based on own incubation experiments with Daphnia carcasses, we discuss how possible changes of particle rigidity might influence epsilon(WOD).