Carbon cycling in mesohaline Chesapeake Bay sediments .2. Kinetics of particulate and dissolved organic carbon turnover

Temporal and depth variations in benthic carbon metabolism rates were examined in relation to particulate organic carbon (POC) deposition rates and particulate and dissolved organic carbon degradation kinetics in two sediments from the mesohaline region of Chesapeake Bay. The depth distribution of a single pool of metabolizable POC (MPOC) in mid-Bay sediments was estimated by curve-fitting of dry weight POC profiles (''1-G'' approach). Estimated MPOC pools accounted for 3-4% of total POC content in the upper 10 cm of sediment. First-order MPOC decay constants of approximate to 10 yr(-1) during the warm season were estimated from the ratio of MPOC pool size to weighted-average MPOC deposition rate derived from mid-water column sediment trap deployments. These results indicated that the MPOC pool defined by the 1-G approach corresponded to the most readily degradable component of coastal marine phytoplankton detritus. Transient-state kinetic models of MPOC turnover, based on observed MPOC deposition rates and temperature-dependent mineralization, predicted MPOC accumulation in sediments during the spring followed by depletion during the summer. The models also predicted an early summer maximum in MPOC mineralization rate associated with the degradation of MPOC accumulated during the spring, in agreement with the seasonal pattern of sulfate reduction rates in mid-Bay sediments. Model results suggested that MPOC deposition during the summer is important in maintaining high rates of benthic carbon metabolism throughout the warm season. Steady-state and transient-state models of depth-dependent POC degradation suggested that particle mixing influences the depth distribution of MPOC concentration and turnover rate within the upper 4-6 cm of mid-Bay sediments. However, because of the rapid rate of MPOC decay, random particle mixing is unlikely to transport significant quantities of MPOC below 4-6 cm. A steady-state diagenetic model was used to test the hypothesis that downward diffusion of acetate produced by anaerobic decomposition of MPOC in the upper 4-6 cm fuels sulfate reduction deeper in the sediment. The results suggest that because of the very rapid turnover of acetate pools (greater than or equal to 2 hr(-1)), acetate diffusion does not influence the depth distribution of carbon metabolism in the sediment. Therefore, sulfate reduction occurring at depths below 4-6 cm must be fueled by decomposition of some portion of the large pool of relatively refractory sediment POC. Degradation of this material is likely responsible for approximate to 1/3 of total warm season benthic carbon metabolism.