Sediment-water gas exchange in two Swedish lakes measured by Eddy Correlation

Abstract:

Lake sediments are hotspots for carbon (C) cycling, acting both as sinks and sources through C burial and production of carbon dioxide (CO₂) and methane. The fate of this CO₂ in the water column is controlled by bottom water turbulence, a factor not accounted for in current estimates of sediment CO₂ fluxes.

This study is aimed to quantify the turbulent CO₂ flux across the sediment-water interface (SWI) by measuring the oxygen (O₂) flux with the non-invasive Eddy Correlation (EC) method that combines measurements of 3D velocity (ADV) and O₂ fluctuations with a microsensor. Using the metabolic relation (respiratory quotient, RQ) of O₂ and CO₂ derived from a sediment incubation experiment we present the first estimates of turbulent lake sediment CO₂ flux from two boreal lakes in Sweden (Erssjön and Erken, 0.07 km² and 23.7 km² respectively).

Only ~10 % of the total dataset was extracted for flux calculations due to poor signal-to-noise ratio in the velocity and O₂ signals. The sediment in Lake Erssjön was both consuming and producing O₂, related to bacterial respiration and photosynthesis. Mean O₂ flux was -0.19 and 0.17 μmol O₂ m^-2 sec^-1, comparing to 0.04 μmol O₂ m^-2 sec^-1 derived from the sediment incubation experiment. Fluxes for Lake Erken are still to be determined. Experimentally derived RQ of the both lake sediments were close to unity implying that in-situ CO₂ fluxes are of similar magnitude as O₂ fluxes, varying between -0.15 and 0.18 μmol C m^-2 sec^-1.

The first measurement of turbulent sediment O₂ flux and estimate of turbulent CO₂ flux from a small boreal lake show higher and more variable fluxes than previously found in experimental studies. The low amount of data extracted for flux calculations (~10%) point towards the difficulties in EC measurement in low-turbulence environments. On-going work focuses on the turbulence structure in lakes and its influence on the gas fluxes at the SWI.