Modelling CO2 and CH4 Dynamics in a Small, Eutrophic, Monomictic Lake

English Abstract:

Lakes play an important role in processing carbon derived from the terrestrial ecosystem. This carbon can accumulate as greenhouse gases, namely CO₂ and CH₄, as a result of biological processes. The accumulated gases are potentially emitted during seasonal mixing. Nonetheless, carbon fluxes and balances are still unclear, which limits our ability to forecast future impacts such as due to climate or land use change. To better understand greenhouse gas fluxes, we investigated the dynamics of CO₂ and CH₄ in a relatively small, eutrophic lake (Lake Okaro, New Zealand). Water column concentrations of these gases were observed monthly over one-year period. In addition, peeper incubations were made in the sediment over a period of one month to estimate sediment-water fluxes of CO₂ and CH₄. Releases of CO₂ and CH₄ from the sediment into water column were 27.03 and 28.2 mmol m^-2 d^-1, respectively, and corresponded to rapid enrichment of both gases in the hypolimnion in a period of stable stratification. Calculations indicated that approximately 6.6 mmol m^-2 d^-1 of CO₂ was taken up by the lake from the atmosphere during the stable stratification. On the other hand, in the same period CH₄ was released to the atmosphere at a rate of around 0.1 mmol m^-2 d^-1. We estimated pulsed emissions were released to the atmosphere with peaks at the onset of winter mixing when 23.5 and 6.4 mmol m^-2 d^-1 of CO₂ and CH₄, respectively. To complicate this analysis, however, large releases of CH₄ to the atmosphere are suspected to occur via ebullition, when large bubbles suddenly occur and cannot easily be sampled. We used a one dimensional lake ecosystem model (GLM-FABM) to better understand the temporal variability of processes affecting releases and uptake of CO₂ and CH₄ between the lake and the atmosphere.