ABSTRACT: Macroalgae can form extensive blooms in coastal areas receiving high nitrogen loading, and a large fraction of this biomass is likely to be degraded in permeable sediments. We undertook a series of experiments investigating the influence of buried macroalgae Ulva lactuca on denitrification in permeable sediments using flow-through reactor and flume experiments. Our results showed that in flow-through chamber experiments, the presence of buried macroalgae generally did not enhance rates of denitrification, although an enhancement of coupled nitrification–denitrification was observed at intermediate porewater flow rates. 2D planar optode images revealed that leaching dissolved organic carbon (DOC) from buried macroalgal sheets resulted in a plume of hypoxic-anoxic sediment that had only a very small stimulating influence on denitrification rates (observed rates range from 0.1 to 3 µmol m^-2 h^-1). Flume experiments showed that buried macroalgae sheets had a pronounced effect on porewater flow fields within the sediment, and that the effect on denitrification was highly dependent upon the position of the buried macroalgae relative to ripple geometry. Overall, our results showed that rates of denitrification were orders of magnitude lower in flume experiments with realistic flow fields compared to column experiments with artificial flow fields, in agreement with previous work. We conclude that the presence of buried macroalgae does not significantly stimulate denitrification rates within permeable silicate sands.