Impact of fiddler crabs and plant roots on sediment biogeochemistry in a Georgia saltmarsh

The influence of macrofauna and macrophytes on sediment biogeochemistry was quantified in a Spartina alterniflora (Loisel) saltmarsh, with emphasis on sulfur and iron cycling. Vertical profiles of sediment geochemistry and rates of microbial metabolism at 3 sites with different abundances of fiddler crab Uca pugnax burrows, vegetation coverage and hydrology were supplemented with high-resolution radial profiles around burrow walls and S. alterniflora roots. Carbon oxidation was measured as sulfate reduction using the S-35 technique, as total anaerobic CO2 production, and as Fe(III) reduction by monitoring Fe(II) evolution. Depth-integrated (0 to 10 cm) sulfate reduction was 25% lower, while total Fe and Fe(III) concentrations were 1.5 and 6 times higher, respectively, in bioturbated than in nonbioturbated sediment. Low sulfate-reduction rates adjacent to burrow walls (3% of those in bulk sediment) were counteracted by very high Fe(III) reduction rates. Thus, Fe(III) reduction accounted for 54 to 86% of the total carbon oxidation within 4 cm distance of burrows, decreasing in importance with distance from the burrow wall. Overall, S. alterniflora roots showed a greater impact on sediment biogeochemistry than crab burrows. Sulfate reduction was almost absent in the rhizosphere, whereas Fe(III) reduction rates (6.2 mumol Fe cm(-3) d(-1)) were among the highest reported for marine sediments, accounting for >99% of carbon oxidation. Our results confirm the universal relationship between the contribution of Fe(III) respiration to total carbon oxidation and solid Fe(III) concentrations that has been suggested based on studies of subtidal marine sediments. The importance of Fe(Ill) respiration was strongly dependent on Fe(III) concentrations below levels of 30 mumol cm(-3), whereas above this level almost all anaerobic respiration was mediated by Fe(III) reduction in saltmarsh sediments.