Nutrient dynamics in mangrove crab burrow sediments subjected to anthropogenic input

Bioturbation by burrowing macroinvertebrates has a major impact on sediment properties, pollutant redistribution, and biogeochemical cycling. We assessed the impact of bioturbation on the nutrient dynamics and organic matter of sediments receiving anthropogenic inputs in the Manko wetland, located in southern Okinawa, Japan. We compared sediments that were and were not subjected to the activities of the tidal-flat crab Helice formosensis. The fatty acid composition of sediments indicated that different sources contributed to the organic matter profile. Agricultural and domestic waste discharge seemed to induce a high amount of green macroalgae and bacteria in the tidal flat. Sediments without crabs exhibited 2.1–2.4× more NH4-N release, which was associated with a low C/N ratio during the summer. In the crab burrow sediments, NO3-N concentrations were 1.4–1.9× more pronounced during winter. A significant correlation existed between NO3-N and NO2-N concentrations in sediments of the burrow chamber (r = 0.837, p < 0.02) and the mixed zone of burrow opening shaft and chamber (r = 0.885, p < 0.01). We suggest that burrow wall sediments provide ideal conditions for nitrate reduction (denitrification). H. formosensis thus contributes to balancing the effects of anthropogenic inputs by removing nitrogen loads in sediments.