Diel cycles of sulphate reduction rates in sediments of a Zostera marina bed (Denmark)

Sulphate reduction was investigated over 24 h cycles in Zostera marina-bearing sediments. The experiments were carried out at the beginning (April) and end (August) of the growing season of the eelgrass. The sulphate reduction rates were 3 times higher in August (similar to 90 mmol SO42- m(-2) d(-1)) than in April. In both periods, the sulphate reduction rates were significantly higher in the light than in the dark. The stimulation in the light may be due to increased root exudation of organic matter related to photosynthetic activity. The exuded organic carbon accounted for approximately 2 to 4% of the total amount of carbon fixed by the plants. The sulphate reduction rates responded rapidly to shifts from dark to light and vice versa, indicating the presence of a limited pool of labile organic matter in the sediment. A positive correlation between sulphate reduction rates and root+rhizome biomass was observed in April and August during the daytime and in August but not April in the dark.