High net CO₂ and CH₄ release at a eutrophic shallow lake on a formerly drained fen

Abstract. Drained peatlands often act as carbon dioxide (CO₂) hotspots. Raising the groundwater table is expected to reduce their CO₂ contribution to the atmosphere and revitalise their function as carbon (C) sink in the long term. Without strict water management rewetting often results in partial flooding and the formation of spatially heterogeneous, nutrient-rich shallow lakes. Uncertainties remain as to when the intended effect of rewetting is achieved, as this specific ecosystem type has hardly been investigated in terms of greenhouse gas (GHG) exchange. In most cases of rewetting, methane (CH₄) emissions increase under anoxic conditions due to a higher water table and in terms of global warming potential (GWP) outperform the shift towards CO₂ uptake, at least in the short term.

Based on eddy covariance measurements we studied the ecosystem–atmosphere exchange of CH₄ and CO₂ at a shallow lake situated on a former fen grassland in northeastern Germany. The lake evolved shortly after flooding, 9 years previous to our investigation period. The ecosystem consists of two main surface types: open water (inhabited by submerged and floating vegetation) and emergent vegetation (particularly including the eulittoral zone of the lake, dominated by Typha latifolia). To determine the individual contribution of the two main surface types to the net CO₂ and CH₄ exchange of the whole lake ecosystem, we combined footprint analysis with CH₄ modelling and net ecosystem exchange partitioning.

The CH₄ and CO₂ dynamics were strikingly different between open water and emergent vegetation. Net CH₄ emissions from the open water area were around 4-fold higher than from emergent vegetation stands, accounting for 53 and 13 g CH₄ m⁻² a⁻¹ respectively. In addition, both surface types were net CO₂ sources with 158 and 750 g CO₂ m⁻² a⁻¹ respectively. Unusual meteorological conditions in terms of a warm and dry summer and a mild winter might have facilitated high respiration rates. In sum, even after 9 years of rewetting the lake ecosystem exhibited a considerable C loss and global warming impact, the latter mainly driven by high CH₄ emissions. We assume the eutrophic conditions in combination with permanent high inundation as major reasons for the unfavourable GHG balance.