In shallow environments, frequent unsteady flows such as tides, waves or wind-driven currents modulate the diffusive boundary layer thickness that controls the exchange of electron acceptors for mineralization and oxidation processes in surficial sediment. An oscillating grid mesocosm was developed in order to study organic matter degradation under controlled unsteady turbulence. The ability of this oscillating grid mesocosm is to (1) produce homogenous control of diffusive flux at sediment-water interface of 36 sediment cores with an oscillating grid ; (2) simulate diffusive boundary layer thickness dynamics at different time scales by easy control of turbulence intensity and (3) study transient oxygen dynamics along organic matter mineralization under unsteady diffusive boundary layer thickness. Two experiments of organic matter degradation were investigated under different hydro- dynamics : firstly, an experiment (protein enrichment) simulating a mortaly of consumers in a closed system (i. E. Lagoon) with a progressive sediment reoxygenation by short turbulences and secondly, an experiment (carbohydrates enrichment) simulating a terrestrial vegetal input in an open system under waves (i. E. Coastal zone), with 4 daily periods of high turbulence. During these experiments, oxygen, organic matter and biotic (microfauna and meiofauna) dynamics were measured. Moreover, these experiments were investigated to study experimentally organic matter and microbial population dynamics under different oxygen transients to complete early diagenesis model under unsteady flows. For each experiment, total sedimentary oxygen demande was oscillated with time, depth and oxygen supply. Oxygen global affinity and oxygen specific affinity were calculated and show that sedimentary oxygen demand reflects aerobic microbial community dynamics in carbohydrates enrichment experiment. To conclude, oscillating grid mesocosm is a promising tool to carry out benchmark expe- riments to investigate the coupling between microbial community dynamics and hydrodynamic fluctuations, which occurs in coastal waters with different periods and intensities (tides, waves, seiches)