Lake Metabolism: Comparison of Lake Metabolic Rates Estimated from a Diel CO(2-) and the Common Diel O(2-)Technique

Lake metabolism is a key factor for the understanding of turnover of energy and of organic and inorganic matter in lake ecosystems. Long-term time series on metabolic rates are commonly estimated from diel changes in dissolved oxygen. Here we present long-term data on metabolic rates based on diel changes in total dissolved inorganic carbon (DIC) utilizing an open-water diel CO(2)-technique. Metabolic rates estimated with this technique and the traditional diel O(2)-technique agree well in alkaline Lake Illmensee (pH of ~8.5), although the diel changes in molar CO(2) concentrations are much smaller than those of the molar O(2) concentrations. The open-water diel CO(2)- and diel O(2)-techniques provide independent measures of lake metabolic rates that differ in their sensitivity to transport processes. Hence, the combination of both techniques can help to constrain uncertainties arising from assumptions on vertical fluxes due to gas exchange and turbulent diffusion. This is particularly important for estimates of lake respiration rates because these are much more sensitive to assumptions on gradients in vertical fluxes of O(2) or DIC than estimates of lake gross primary production. Our data suggest that it can be advantageous to estimate respiration rates assuming negligible gradients in vertical fluxes rather than including gas exchange with the atmosphere but neglecting vertical mixing in the water column. During two months in summer the average lake net production was close to zero suggesting at most slightly autotrophic conditions. However, the lake emitted O(2) and CO(2) during the entire time period suggesting that O(2) and CO(2) emissions from lakes can be decoupled from the metabolism in the near surface layer.