On the calculation of lake metabolic rates: Diel O₂ and ^18/16O technique

Metabolic transformations have a major impact on the development of primary producers in aquatic systems and thus affect the dynamics of the entire aquatic food web. Furthermore, metabolic transformations contribute to the carbon budget and thereby influence CO₂ emissions from aquatic systems. Several techniques have been developed that aim at an easy assessment of metabolic rates over long time periods or in many systems. The ^18/16O technique, which utilizes the isotopic fractionation between ¹⁸O and ¹⁶O isotopes due to metabolic transformations, is receiving increasing popularity in studies comparing the metabolism in many different lakes and served as basis for the conclusions that production increases with increasing atmospheric CO₂ and that surprisingly little terrestrial carbon is recycled in lakes of the arid circumpolar landscape. However, we demonstrate here that the steady state assumptions underlying the ^18/16O technique cause large uncertainties in the estimated metabolic rates. This conclusion is based on a sensitivity analysis using a numerical model of dissolved oxygen, DO, and of dissolved ¹⁸O, ¹⁸O_DO, but is also confirmed by published metabolic rates estimated from the ^18/16O and the diel O₂ techniques. Metabolic rates obtained from the ^18/16O technique appear unsuited for correlation analyses between lakes but may provide reasonable estimates in systems with low and long-term stable production. In addition we illustrate that the combination of few ¹⁸O measurements with the diel O₂ technique and an inverse fitting procedure can improve estimates of metabolic rates and in particular of respiration rates.