Burrows of the Semi-Terrestrial Crab Ucides cordatus Enhance CO(2) Release in a North Brazilian Mangrove Forest

Ucides cordatus is an abundant mangrove crab in Brazil constructing burrows of up to 2 m depth. Sediment around burrows may oxidize during low tides. This increase in sediment-air contact area may enhance carbon degradation processes. We hypothesized that 1) the sediment CO(2) efflux rate is greater with burrows than without and 2) the reduction potential in radial profiles in the sediment surrounding the burrows decreases gradually, until approximating non-bioturbated conditions. Sampling was conducted during the North Brazilian wet season at neap tides. CO(2) efflux rates of inhabited burrows and plain sediment were measured with a CO(2)/H(2)O gas analyzer connected to a respiration chamber. Sediment redox potential, pH and temperature were measured in the sediment surrounding the burrows at horizontal distances of 2, 5, 8 and 15 cm at four sediment depths (1, 10, 30 and 50 cm) and rH values were calculated. Sediment cores (50 cm length) were taken to measure the same parameters for plain sediment. CO(2) efflux rates of plain sediment and individual crab burrows with entrance diameters of 7 cm were 0.7–1.3 µmol m(−2 )s(−1) and 0.2–0.4 µmol burrows(−1 )s(−1), respectively. CO(2) released from a Rhizophora mangle dominated forest with an average of 1.7 U. cordatus burrows(−1 )m(−2) yielded 1.0–1.7 µmol m(−2 )s(−1), depending on the month and burrow entrance diameter. Laboratory experiments revealed that 20–60% of the CO(2) released by burrows originated from crab respiration. Temporal changes in the reduction potential in the sediment surrounding the burrows did not influence the CO(2) release from burrows. More oxidized conditions of plain sediment over time may explain the increase in CO(2) release until the end of the wet season. CO(2) released by U. cordatus and their burrows may be a significant pathway of CO(2) export from mangrove sediments and should be considered in mangrove carbon budget estimates.