Water flow through a plastic tunnel (1OL x 0.80W x 0.30H m) was used to determine nutrient retention and release rates over two substrates (with and without oysters) in a natural oyster bank at the northeastern end of the Cienaga Grande de Santa Marta. In general terms, nutrient fluxes are inversally related to the water flow. Values obtained over the substrate without oysters were higher than those observed over the substrate with oysters. Such differences can be explained by the low oyster density in the area and the great microbial activity and typical sediment diffusion processes in this estuarine ecosystem.