Whole-lake methane emissions from two temperate shallow lakes with fluctuating water levels : Relevance of spatiotemporal patterns

Globally, shallow lakes are an important source of methane (CH4) emissions to the atmosphere. Previous studies of such lakes have rarely focused on the within-lake spatiotemporal variability, which is critical for generating representative whole-lake fluxes, and better understanding and constrain large-scale emissions. To address this issue, we determined the variability of CH4 fluxes and CH4 concentrations in two small shallow (&amp;lt;= 150 cm) lakes in Central Europe biweekly over almost 2 years. We found that both lakes were a source of CH4, mainly by ebullition. At the shallower Lake Heideweiher, which temporarily dried out, the average flux was 7.2 mmol m(-2) d(-1), the average flux from Lake Windsborn that never dried out, was 3.5 mmol m(-2) d(-1). The spatial differences (between and within lakes) were most strongly related to sediment C-content and quality, which in turn was linked to depth or distance to shore. The highest fluxes occurred in the central parts of both lakes. The temporal variability of the fluxes was primarily correlated with sediment temperature and degree of drying measured as the time since drying up. The whole-lake estimates were dominated by low water periods and the warm summer months. Overall, we show that short-term and small-scale measurements cannot account for the high variability of CH4 fluxes from small lakes, and that reliable large-scale assessments need to consider such spatiotemporal variability.Funding Agencies|Deutsche Forschungsgemeinschaft (German Research Foundation, DFG)German Research Foundation (DFG) [BL563/25-1]; European Research Council (ERC) under the European UnionEuropean Research Council (ERC) [725546]</p

Organic matter and sediment properties determine in-lake variability of sediment CO2 and CH4 production and emissions of a small and shallow lake

Inland waters, particularly small and shallow lakes, are significant sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. However, the spatial in-lake heterogeneity of CO2 and CH4 production processes and their drivers in the sediment remain poorly studied. We measured potential CO2 and CH4 production in slurry incubations from 12 sites within the small and shallow crater lake Windsborn in Germany, as well as fluxes at the water–atmosphere interface of intact sediment core incubations from four sites. Production rates were highly variable and ranged from 7.2 to 38.5 µmol CO2 gC−1 d−1 and from 5.4 to 33.5 µmol CH4 gC−1 d−1. Fluxes ranged from 4.5 to 26.9 mmol CO2 m−2 d−1 and from 0 to 9.8 mmol CH4 m−2 d−1. Both CO2 and CH4 production rates and the CH4 fluxes exhibited a significant and negative correlation (p<0.05, ρ<−0.6) with a prevalence of recalcitrant organic matter (OM) compounds in the sediment as identified by Fourier-transformed infrared spectroscopy. The carbon / nitrogen ratio exhibited a significant negative correlation (p<0.01, ρ=−0.88) with CH4 fluxes but not with production rates or CO2 fluxes. The availability of inorganic (nitrate, sulfate, ferric iron) and organic (humic acids) electron acceptors failed to explain differences in CH4 production rates, assuming a competitive suppression, but observed non-methanogenic CO2 production could be explained up to 91 % by prevalent electron acceptors. We did not find clear relationships between OM quality, the thermodynamics of methanogenic pathways (acetoclastic vs. hydrogenotrophic) and electron-accepting capacity of the OM. Differences in CH4 fluxes were interestingly to a large part explained by grain size distribution (p<0.05, ρ=±0.65). Surprisingly though, sediment gas storage, potential production rates and water–atmosphere fluxes were decoupled from each other and did not show any correlations. Our results show that within a small lake, sediment CO2 and CH4 production shows significant spatial variability which is mainly driven by spatial differences in the degradability of the sediment OM. We highlight that studies on production rates and sediment quality need to be interpreted with care, though, in terms of deducing emission rates and patterns as approaches based on production rates only neglect physical sediment properties and production and oxidation processes in the water column as major controls on actual emissions

Methane ebullition, sedimentation rates, organic matter quality and environmental parameters at small and shallow temperate Lake Windsborn, Germany in 2017 and 2018

To determine spatiotemporal variability of methane (CH4) ebullition and its drivers, we measured CH4 ebullition rates, sedimentation rates and characteristics of the sedimented material, sediment chemical and physical characteristics and environmental parameters at Lake Windsborn in 2017 and 2018. Measurements of CH4 ebullition were conducted bi-weekly from May to October 2017 and April to November 2018. Sedimentation rates were measured in 2018 in four-week intervals. Characteristics of the sedimented material were measured in 2017 and 2018. Sediment characteristics were measured in November 2017, spring 2018 and August 2019. Meterological parameters (temperature and air pressure) were constantly measured from a floating platform in the lake center