Metabolic threshold and sulfide-buffering in diffusion controlled marine sediments impacted by continuous organic enrichment

Abstract

The effects of organic enrichment on sediment biogeochemistry was studied in diffusion controlled sediment mesocosms, where labile organic matter (OM) (fish feed) pulses were added once a week to the sediment surface. Two types of sediments, differing mainly in content of reactive Fe, were used. The aim of this experiment was two-fold, (1) to evaluate the importance of Fe-driven sulfide buffering for sulfide accumulation in surface enriched sediments, and (2) to estimate the diagenetic capacity for degradation of labile OM near the sediment surface. The simulated OM loading rate of 375 mmol C m−2 day−1 led to a 5–6 times increase in CO2-production and a 4–5 times increase in O2-uptake. Sulfate reduction estimated by radiotracer experiments and CO2-release was 105–131 mmol m−2 day−1, but accumulation of porewater sulfide was low in both sediment types. Instead 99% of sulfide was oxidized with O2 at the sediment water interface in the low Fe treatment, whereas 46% of produced sulfide precipitated as Fe-S compound in the high Fe treatment resulting in significantly lower O2-uptake. Furthermore, the accumulation of up to 30% of added OM by the end of the experiment indicated a saturation of the heterotrophic microbial communities in the upper enriched surface layer. These results suggest a maximum diagenetic capacity for OM degradation in the range of ~25 μmol C cm−3 day−1 or 260 mmol m−2 day−1 for the present sediment types.