Through field experiments and chemical analysis of the sediment and pore water, we investigated the effect of the burrowing crab Chasmagnathus granulatus on the distribution and availability of electron acceptors in mudflat sediments. The results show that the presence and activity of C. granulatus and its burrows determine the chemical characteristics of pore water and the redox state of mudflat sediments. Crabs enhance the transport of particulate material in the sediment column, completely

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... xing the upper 7 cm of sediment in a few days. Comparative analyses of pore water from areas (1) with crabs and burrows, (2) with unoccupied burrows, and (3) without burrows or crabs reveal a large increase in sediment oxygenation, modification of pore water salinity and the distribution of dissolved inorganic nitrogen and sulphate when crabs are present. Denitrification and organic matter (OM) degradation rates were estimated by a diagenetic model that searches for simultaneous agreement between measured and model-calculated depth profiles . OM degradation rates (kG) were found to be greater in bioturbated (kG = 10.6 ± 6.1 [SD] µM s -1 ) than in non-bioturbated sediments (kG = 4.9 ± 17.8 µM s -1 if unoccupied burrows were present; kG = 0.02 ± 0.013 µM s -1 without burrows). Model-estimated denitrification rates revealed that OM degradation pathways are also affected by C. granulatus activities and their burrows. These changes in the sediment chemistry that are directly and indirectly produced by C. granulatus could comprise the force that drives the pathways of microbial processes and nutrient flows to neighbouring systems.