Dissolved organic carbon production and consumption in anoxic marine sediments: A pulsed-tracer experiment

The degradation of phytoplankton and seagrass organic carbon (OC) in sulfate-reducing (SR) and methane-producing (MP) sediments was tracked by measuring concentrations of particulate OC (POC), hydrolysis products (HP), fermentation products (FP), and inorganic end products (EP). This experiment used the novel approach of amending sediment with organic substrates having delta C-13 values unique from that of the OC pool originally present in the sediment. As a result, we could monitor changes in the dynamic dissolved OC pool and gain insight into processes that control the fate of OC. Rates of hydrolysis, fermentation, and terminal metabolism were greater in the phytoplankton-amended treatments than in the seagrass-amended treatments during the period of active decomposition. At the end of the incubation, concentrations of HP and FP in amended treatments were not significantly different from those in the controls. An analysis of the delta C-13 values of HP from the amended treatments indicated that the addition of fresh organic matter stimulated the decomposition of carbon present in the sediment at the time of collection. Hydrolysis of this carbon accounted for greater than or equal to 50% of the total carbon hydrolyzed in the sediment amended with seagrass.