Rates and controls of anaerobic microbial respiration across spatial and temporal gradients in saltmarsh sediments

This study was undertaken to determine the rates and controls of anaerobic respiration reactions coupled to organic matter mineralization as a function of space and time along a transect from a bioturbated creekbank to the midmarsh in Georgia saltmarsh sediments. Sulfate reduction rates (SRR) were measured at 3 sites during 5 sampling periods throughout the growth season. The sites differed according to hydrologic regime and the abundance of dominant plants and macrofauna. SRR and pore water/solid phase geochemistry showed evidence of enhanced sediment oxidation at sites exposed to intense bioturbation. Iron(III) reduction rates (FeRR) were directly determined in saltmarsh sediments for the first time, and in agreement with measured SRR, higher rates were observed at the bioturbated, unvegetated creekbank (BUC) and bioturbated, vegetated levee (BVL) sites in comparison to a vegetated mid-marsh (MM) site. An unexpected result was the fact that SRR varied nearly as much between sites (2-3 x) as it did with temperature or season (3-4 x). The BVL site, vegetated by the tall form of Spartina alterniflora, always exhibited the highest SRR and carbon oxidation rates (> 4000 nmol cm-3 d-1) with high activity levels extending deep (≥ 50 cm) into the sediment, while the MM site, dominated by the short form of Spartina, always exhibited the lowest SRR which were localized to the top 15 cm of sediment. SRR and FeRR at BUC were intermediate between those measured at the BVL and MM. Acetate was the most abundant microbial fermentation product (concentrations up to > 1 mM) in marsh porewaters, and its distribution reflected respiration activity. Chemical exchange, caused by bioturbation, appeared to be the primary control explaining trends in rates of sulfate and Fe(III) reduction with macrophytes and carbon source acting as secondary controls.