Effects of benthic fauna on organic matter mineralization in fish-farm sediments: importance of size and abundance

The effects of the polychaetes Nereis diversicolor and Capitella sp. I on organic matter mineralization in marine sediments collected under a fish farm were studied in vitro. The two species differ significantly in size, feeding, and burrowing activity. The presence of benthic fauna stimulated the total benthic metabolism (measured as O2uptake, CO2 release, and SO42- reduction) substantially compared to the microbial metabolism in an azoic sediment. Nereis and Capitella stimulated mineralization over a two-month period by 135% and 87%, respectively. The stimulation was primarily microbial (54%) in Capitella sediments. Sulphate reduction remained similar to azoic sediments, indicating that in particular the aerobic activity was enhanced. Microbial stimulation was less (23%) for Nereis sediments, while sulphate reduction was reduced (42%), indicating enhanced oxidation compared to azoic sediments. The fauna-mediated oxidation of the sediment probably causes increased removal of nitrogen through increased nitrification and denitrification and enhanced binding of phosphorus, thereby reducing nutrient fluxes to the water column.