Exploring Changes in Sediment Carbon Cycling at the Tidal Raritan River Using a Natural Salinity Gradient

New Jersey, and other coastal Atlantic states are experiencing sea level rise at a faster rate than the global average due to post-glacial isostatic adjustment. In coastal river systems, sea level rise promotes the increased inland exchange of ions and nutrients between the sea and rivers via salt water intrusion. While ion concentrations increase due to this intrusion, the ratio of sulfate to chloride is lower than anticipated for a conservative mixture. Previous studies suggest that this decrease may be due to an increase on bacterial sulfate reduction. Enhanced mineralization of organic matter by freshwater bacteria, via the coupling of organic matter oxidation to sulfate reduction, can result in an accelerated flux of carbon between sediment and atmosphere. We assessed the chemical and biological profile of sediments in the Raritan River, New Jersey. Core samples were taken from three sites with different salinities (1.8%, 1.0% and <0.3%). Incubations were established by exposing sediment from a site to its own water, water from a higher salinity site, and seawater. We anticipated a decrease in sulfate and an increase in sulfide, CO₂, and N₂O as we exposed the sediment microbial communities to higher salinity and higher sulfate concentrations. However, on short time scale incubations, iron reduction, resulting in millimolar iron(II) production, was the predominant biogeochemical process. Over longer incubations, sulfate reduction increased and a corresponding community shift was observed.