High net CO<sub>2</sub> and CH<sub>4</sub> release at a eutrophic shallow lake on a formerly drained fen

Abstract

<p><strong>Abstract.</strong> Drained peatlands often act as carbon dioxide (CO<sub>2</sub>) hotspots. Raising the groundwater table is expected to reduce their CO<sub>2</sub> contribution to the atmosphere and revitalize 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 exchange (GHG) exchange. In most cases, methane (CH<sub>4</sub>) emissions increase under anoxic conditions due to a higher water table and in terms of global warming potential (GWP) outperform the shift towards CO<sub>2</sub> uptake, at least in the short-term. Based on eddy covariance measurements we studied the ecosystem–atmosphere exchange of CH<sub>4</sub> and CO<sub>2</sub> (NEE) at a shallow lake situated on a former fen grassland in Northeast (NE) 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 <i>Typha latifolia</i>). To determine the individual contribution of the two main surface types to the net CO<sub>2</sub> and CH<sub>4</sub> exchange of the whole lake ecosystem, we combined footprint analysis with CH<sub>4</sub> modelling and NEE partitioning. The CH<sub>4</sub> and CO<sub>2</sub> dynamics were strikingly different between open water and emergent vegetation. Net CH<sub>4</sub> emissions from the open water area were around 4-fold higher than from emergent vegetation stands, accounting for 53 and 13 g CH<sub>4</sub> m<sup>−2</sup> a<sup>−1</sup>, respectively. In addition, both surface types were net CO<sub>2</sub> sources with 158 and 750 g CO<sub>2</sub> m<sup>−2</sup> a<sup>−1</sup>, 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 CH<sub>4</sub> emissions. We assume the eutrophic conditions in combination with permanent high inundation as major reasons for the unfavourable GHG balance.</p>

Spatiotemporal Variability in Lake‐Atmosphere Net CO2 Exchange in the Littoral Zone of an Oligotrophic Lake

“…The CO 2 efflux from the littoral zone of Lochaber is intermediate along the spectrum of exchange rates observed within oligotrophic to hypereutrophic lakes in Finland, which range from 0.004 to 0.814 μmol CO 2 · m −2 · s −1 , respectively (Huttunen et al, ; Repo et al, ). The lake‐atmosphere net CO 2 exchange we observed in the littoral zone of Lochaber Lake was low relative to other more eutrophic systems, such as Lake Soppensee in Switzerland, which releases 0.278 μmol CO 2 · m −2 · s −1 as observed by Eugster et al (), and Polder Zarnekow in Germany, which was found to release 0.566 μmol CO 2 · m −2 · s −1 in a study by Franz et al (). Compared to a global average obtained from 20,632 partial pressure CO 2 measurements performed over 7,939 lakes and reservoirs, the lake‐atmosphere net CO 2 exchange we quantified in the littoral zone of Lochaber Lake was relatively low, as the review of Raymond et al () reported an average exchange rate of 2.812 μmol CO 2 · m −2 · s −1 .…”

“…The shallow water column, metabolism of macrophyte communities, predisposition to surface flows, and often intensified influence of break‐wave action in the littoral zone all encourage enhanced rates of CO 2 exchange relative to the pelagic zone. In terms of CO 2 efflux, the littoral zone can be thought of as an intermediate ecotone between that of a wetland and a pelagic lake ecosystem, with greater carbon processing rates than would be predicted by pelagic nutrient status (Franz et al, ). Relative to the pelagic zone, littoral zone carbon dynamics are thought to be more closely regulated by environmental fluctuations and substrate heterogeneity (Tonetta et al, ).…”

Prompt rewetting of drained peatlands reduces climate warming despite methane emissions

“…In light of the strong and not yet completely understood impact of CH 4 on global warming 4,5 it may seem imprudent to knowingly create or restore an additional source. Furthermore, there is considerable uncertainty on emissions from rewetted peatlands and some studies have reported elevated emissions of CH 4 compared with pristine peatlands [6][7][8][9] .…”

Where old meets new: An ecosystem study of methanogenesis in a reflooded agricultural peatland

“…Our ability to interpret CH 4 fluxes in restored wetlands and predict longer term restoration trajectories (Hemes et al, ) is limited by a lack of detailed process studies of restored wetland C cycling under field conditions. Restoration can move wetland ecosystems into new biogeochemical states (Moreno‐Mateos, Power, Comín, & Yockteng, ) and some of these changes are likely to influence CH 4 fluxes, such as eutrophication (Franz, Koebsch, Larmanou, Augustin, & Sachs, ). Though archaeal methanogenesis (Nisbet & Nisbet, ) has been well‐studied in intact wetlands (Bridgham et al, ; Segers, ), the few studies in rewetted peatlands and restored wetlands have focused exclusively on microbial community composition (e.g., Jerman, Metje, Mandić‐Mulec, & Frenzel, ; Putkinen, Tuittila, Siljanen, Bodrossy, & Fritze, ; Urbanová, Bárta, & Picek, ; Wen et al, ) rather than biogeochemical process rates.…”