Organic matter mineralisation in the hypolimnion of an eutrophic Maar lake

Abstract

Constraints on hypolimnetic methane production in productive lakes are of interest owing to the importance of methane emissions from lakes for the atmospheric methane burden. We studied carbon fluxes and terminal electron accepting processes in the hypolimnion of eutrophic Meerfelder Maar, Germany. Carbon fluxes from epilimnion and sediments were estimated using sediment traps, porewater analysis and incubation techniques. The contribution of various redox processes to overall dissolved inorganic carbon (DIC) production was quantified from a seasonal hypolimnetic mass balance. Carbon sedimentation rates were 22 mmol m−2 d−1 from the epilimnion and 20–34 mmol m−2 d−1 from the hypolimnion. Anaerobic respiration accounted for a DIC production of 21 mmol m−2 d−1, and was thus capable of using a substantial part of primary production. The diffusive C flux from the sediment was small in comparison, although potential respiration rates in incubation experiments reached 63 – 75 mmol m−2 d−1. Ebullition of methane occurred, but could not be quantified. Following depletion of dissolved oxygen (DO) and nitrate, iron and sulphate reduction and methanogenesis proceeded concurrently in the hypolimnion. Estimated hypolimnetic DIC production decreased in the order methanogenesis (2267 mmol m−2) > oxic respiration (880 mmol m−2) > sulphate reduction (575mmolm−2) > denitrification (159 mmol m−2) > acetogenesis (157 mmol m−2) > iron reduction (19 – 144 mmol m−2). Methane production thus dominated respiration and was not inhibited by sulphate and iron reduction. It also strongly accelerated with increased carbon sedimentation rates in September, which apparently eased limiting factors on the process. Hypolimnetic methane production is thus likely an important process in similar lakes, even in the presence of other electron acceptors, and will contribute to methane emissions during fall turnover.