Isotope effects of O 2 consumption in a deep lake as means for understanding partitioning of O 2 demand among microorganisms, particles, and sediment

The isotopic ratio 18 O/ 16 O of dissolved O 2 in aquatic systems is affected by the preferential biological uptake of 16 O ( 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 O 2 concentration and δ 18 O levels show a considerably weaker effect (~ −10‰). 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 δ 18 O 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 O 2 demand isotope effect ( ε SOD ) of about −8.4‰, and a variated water‐column O 2 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 .