From sedimentary production to water-column accumulation: temperature and spatial controls on littoral methane

Abstract. Littoral sediments are important sources of methane (CH₄) to lake water, but it remains unclear how strongly sedimentary CH₄ production controls sediment-water fluxes and dissolved CH₄ concentrations across seasons and spatially heterogeneous littoral zones. We measured sediment organic carbon, potential CH₄ production, sediment-water CH₄ fluxes, dissolved CH₄ concentrations, and stable carbon isotopes of CH₄ and CO₂ at seven littoral sites in Upper Lake Constance (ULC) and Lower Lake Constance (LLC) over four seasons. Potential CH₄ production, sediment-water CH₄ fluxes, and dissolved CH₄ concentrations were significantly related and showed similar temperature dependencies. Their apparent activation energies ranged from 0.81 to 1.11 eV and did not differ significantly. These results are consistent with a process chain from sedimentary production via sediment-water fluxes to dissolved CH₄, although the shared temperature dependence of the three variables means that their relationships do not, by themselves, demonstrate causal propagation along this chain. Spatial patterns, however, became weaker along this process chain. Within ULC, differences in potential CH₄ production were reflected in sediment-water fluxes but were no longer significant in dissolved CH₄ concentrations. Between the two lakes, dissolved CH₄ concentrations were significantly higher in LLC than in ULC, whereas potential CH₄ production and sediment-water CH₄ fluxes did not differ significantly. Potential CH₄ production increased with sediment organic carbon, suggesting that substrate availability may have contributed to spatial differences in CH₄ production. Carbon isotope patterns were consistent with a sedimentary, mainly acetoclastic CH₄ source and indicated modification of part of the dissolved CH₄ pool after release from the sediment. However, oxidation alone could not explain the isotopic contrast between the lakes, suggesting an additional influence of lateral mixing and hydrodynamic transport. Our results suggest that sedimentary CH₄ production contributes to controlling littoral CH₄ supply, whereas oxidation and physical transport can weaken or redistribute this signal in the water column.