Rates of sulphate reduction were determined in sediments from the eutrophic Bay of Arcachon, France. A comparison was made between sediment overgrown with the eelgrass Zostera noltii and unvegetated sediment. Rates of sulphate reduction were about twice as high in the root zone from 1 to 4 cm depth in the Z. noltii sedunents (about 600 nmol cm-3 d-l) as compared to the activity in the equivalent layer of the unvegetated sediment. The sulphate reduction in the root zone was not stimulated by

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... t, which in other studies with other submersed macrophytes has been shown to promote sulphate reduction due to heightened excretion of organic molecules. Furthermore, in a field experiment, we were not able to detect any diurnal variation in the activity within the root zone. The stimulatlon of sulphate reduction by the presence of the Z. noltii was most likely due to degradation of leaf and root fragments which were found mlxed into the sediment. lncubatlon times with radioactive sulphate had to be kept very short (preferably 15 min) to a v o~d a substantial reoxidation of sulphide and hence an underestimation of the rate of sulphate reduction.