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We investigated the rates of the main microbiological processes (primary production, aerobic and anaerobic carbon degradation) and transport phenomena in an intertidal sand plate with a combination of in situ microsensor measurements and incubations. The sand was coarse, organically poor (0.6-1 mg of total organic carbon per gram dry weight of sediment), and highly permeable to water flow (k ϭ 1.5-7 ϫ 10 Ϫ11 m 2 ). Aerobic respiration rates ranged from 105 to 175 mmol m Ϫ2 d Ϫ1, sulfate reduction rates from 0.08 to 13.7 mmol m Ϫ2 d Ϫ1, and net primary production Ͻ35 mmol m Ϫ2 d Ϫ1 . In situ microsensor measurements showed large changes in oxygen and sulfide concentrations in the top 10 cm, depending on tides and waves. The observed dynamics and high aerobic degradation rates imply that pressure gradients drive advective influx of oxygen and organic material from the water column into the sediments. Our results show that intertidal porous sand plates have high aerobic degradation rates, despite having an organic matter content that is one to two orders of magnitude lower than that of fine-grained deposits with similar decomposition rates.

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