Authors: Guilherme Calabro Souza
Source: theses.fr (ABES) (preprint, 2022-06-07)
Field: Materials Science — Biomaterials

Measurement of nutrient and contaminant flux dynamics at the water-sediment interface in aquatic environments. Freshwater quality in natural environments is a major concern for the agricultural, industrial and domestic sectors. Despite decades of efforts to reduce pollutant inputs, water quality recovery can be slow, due to the pollutant stock in the sediments of aquatic ecosystems. For example, the so-called internal load significantly contributes to the dynamics of eutrophication. These benthic fluxes (at the interface between the sediments and the water column) result from microbiological and chemical activity in the sediments. They have different drivers including hydrodynamics. Turbulence affects the benthic boundary layer, more precisely the fluxes through the thickness of the diffusive sub-layer (DBL). Different techniques are widely used to quantify benthic fluxes (benthic chambers, dialysis chambers, etc). Only the eddy covariance (EC) technique can be used to measure turbulent benthic fluxes over a large area of sediment without disturbing the flow, but for a limited number of fluxes (heat, dissolved oxygen, nitrates, particles and alkalinity). The adaptation of relaxed eddy accumulation (REA) to the aquatic environment has been shown to be theoretically feasible. REA consists of sampling conditioned by the direction of the turbulent fluctuation of the vertical velocity: different pairs of samples are accumulated for upstream and downstream currents and the benthic flux is proportional to the difference in concentration between these sample pairs. Our objective was to develop a prototype, validate it, and acquire the first hourly time series of turbulent benthic nutrient fluxes.During the development, the principles of REA were transferred to aquatic environments through various experiments and simulations. These were carried out in collaboration with the Institute for Environmental Studies of the University of Koblenz-Landau. The validation of the aquatic REA was carried out by comparing oxygen fluxes measured simultaneously by REA and EC. In 2020, campaigns were conducted at four sites with different hydrodynamic and ecological conditions, ranging from a laboratory canal to a highly eutrophic shallow lake.The benthic fluxes measured by four different techniques were compared in collaboration with the University of South Bohemia (Czech Republic). The first Fick's law was applied to: diffusive fluxes in ex-situ sediment cores by measuring concentration microprofiles with diffusive thin films; and to turbulent fluxes by measuring in situ the concentration difference between diffusive and turbulent sublayers of the benthic boundary layer and calculating a mass transfer coefficient proportional to the friction velocity (MTC, another innovative technique). The prototype equipped for EC and REA flux measurements was also deployed. Dissolved oxygen and phosphate, ammonium, sulphate, iron and manganese ion fluxes were measured in the Lake Champs-sur-Marne (June 2021). The turbulent fluxes of nutrients and metal ions measured by REA and MTC were comparable, increasing confidence in both new techniques.The development, validation and application of aquatic REA are described and discussed in this work. The measurement requirements and range of use are also illustrated. Even if REA still requires further development, nutrient and metal ion fluxes have been measured by this new technique and have exhibited rapid dynamics. All these elements allow us to present the REA as a reliable technique for monitoring the rapid dynamics of benthic fluxes, and thus their contribution to biogeochemical processes in aquatic ecosystems. Nutrient ; Sediment ; Environmental science ; Flux (metallurgy) ; Interface (matter) ; Dynamics (music) ; Hydrology (agriculture) ; Ecology ; Engineering ; Geology ; Chemistry ; Geotechnical engineering ; Geomorphology ; Biology ; Physics ; Chemical engineering Diatoms and Algae Research