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 ¹⁸O/¹⁶O of dissolved O₂ in aquatic systems is affected by the preferential biological uptake of ¹⁶O (ε). Studies over the past six decades reveal that during incubation experiments, the isotopic effect of microorganism respiration (ε_organism) varies in the range of −18‰ to −22‰. In contrast, natural variations in the deep-ocean O₂ concentration and δ¹⁸O levels show a considerably weaker effect (~ −10‰). The differences between these observations have been explained to result from either O₂ 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 δ¹⁸O in the deep, cold hypolimnion of Lake Stechlin between 2018 and 2021 as well as in various laboratory incubations. Our incubation results demonstrate an ε_organism of about −24‰. Simple model calculations demonstrate a sediment O₂ demand isotope effect (ε_SOD) of about −8.4‰, and a variated water-column O₂ demand isotope effect (ε_WOD) which is lower than ε_organism, ranging from −13.9‰ in 2019 to −23‰ in 2021. Accompanying experiments indicate that the lower magnitude of ε_WOD may be related to respiration at organic particles lending to a weaker fractionation effect. Thus, variations in ε_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 ε_WOD.