This study aims to compare the impact of oyster cultures on diagenetic processes and the phosphorus cycle in the sediments of the Aber Benoit and the Riviere d'Auray, estuary of Brittany, France. Our results showed clear evidence of the seasonal impact of oyster cultures on sediment characteristics (grain size and organic matter parameters) and the phosphorus cycle, especially in the Aber Benoit. At this site, seasonal variations in sulfide and Fe concentrations in pore waters, as well as Fe-P concentrations in the solid phase, highlighted a shift from a system governed by iron reduction (Reference) to a system governed by sulfate reduction (beneath oyster). This could be partly explained by the increase in labile organic matter (i.e., biodeposits) beneath oysters, whose mineralization by sulfate led to high sulfide concentrations in pore waters (up to 4,475 mu mol l(-1)). In turn, sulfide caused an enhanced release of phosphate in the summer, as adsorption sites for phosphate decreased through the formation of iron-sulfide compounds (FeS and FeS2). In the Aber Benoit, dissolved Fe/PO4 ratios could be used as an indicator of phosphate release into oxic water. Low Fe/PO4 ratios in the summer indicated higher effluxes of phosphate toward the water column (up to 47 mu mol m(-2) h(-1)). At other periods, Fe/PO4 ratios higher than 2 mol/mol indicated very low phosphate fluxes. In contrast, in the Riviere d'Auray, the occurrence of macroalgae, stranding regularly all over the site, clearly masked the impact of oyster cultures on sediment properties and the phosphorus cycle and made the use of Fe/PO4 ratios more difficult in terms of indicators of phosphate release.