Net Ecosystem Production

This study presents a novel concept for estimating net ecosystem production (NEP), the export of organic carbon (OC) from the productive surface layer to the deep-water (hypolimnion) of eleven seasonally stratified lakes, varying in depth and trophic state. As oxygen remineralizes settling OC at a constant ratio, NEP is equivalent to the areal hypolimnetic mineralization rate (AHM) plus burial in the sediment (net sedimentation, NS). Two major interferences have to be considered, however. First, OC from terrestrial sources, not originating from primary production, consumes a fraction of oxidants. Second, sediment diagenetic processes of lakes in trophic transition (e.g. undergoing eutrophication or reoligotrophication) that are not in quasi-steady-state with actual fluxes of OC in the productive surface layer, bias the estimation of NEP. In these cases, we suggest subtracting the flux of reduced substances diffusing from the sediment. This results in some overestimation for lakes with high allochthonous loads, and slight underestimation in lakes that are not in quasi-steady-state, because the fraction of the actual sediment burial of autochthonous OC is small but not negligible. The presented approach requires data from routinely available chemical monitoring and thus can be applied to historic data. The seasonal time integration makes the estimation of NEP quite robust. Exemplary, NEP of Lake Geneva was estimated from the export of P and N from the productive zone during the summer season to the hypolimnion assembling seasonal budgets. Based on a historic data record of 47 years, NEP estimations from AHM rates agreed well with P and N budgets and helped to verify and constrain the uncertainty of the estimates.