Subaquatic Relaxed Eddy Accumulation: A new technique to resolve benthic solute fluxes
Résumé
The transport across the sediment-water interface of solutes, dissolved oxygen, nutrients and contaminants is of major importance for biogeochemical cycles and water quality in aquatic ecosystems. This exchange depends not only on biogeochemical processes and on temperature, but also strongly on near-bed currents. Eddy covariance measurements in a lake showed that oxygen benthic fluxes can vary over minutes to hours. However, the corresponding dynamics of nutrient and contaminant fluxes is still not fully understood. Common techniques fail to capture the turbulence-driven fluxes of interfacial transport for most chemical species, particularly in situ and at high temporal resolution. At the land-atmosphere interface, this issue is addressed with the relaxed eddy accumulation (REA), a conditional sampling technique. Samples are accumulated at constant flow rate in separated containers depending on the direction of the vertical flow. The vertical flux is finally assumed to be proportional to the concentration difference of sample pairs, which might be analysed in situ or at lab.
The adaptation of the technique for measuring fluxes through the sediment-water interface is presented here. A portable prototype was developed, embedding both relaxed eddy accumulation and eddy covariance as a reference technique for validation. Vertical Dissolved oxygen fluxes were measured during ca. 10 min accumulation periods, at ca. 15 cm above the sediment, in spring and summer and at different sites: a laboratory flume with an artificial oxygen flux, a river branch, a creek, a shallow lake and an experimental lake. Oxygen fluxes measured with both techniques showed a good agreement, which validates both the technique and the prototype. The new device proves to be a promising tool to access benthic fluxes at high frequency over a flat sediment-water interface. First time series of nutrient fluxes will be presented.