Benthic Biofilm Potential for Organic Carbon Accumulation in Salt Marsh Sediments

Coastal salt marshes are productive environments with high potential for carbon accumulation and storage. Even though organic carbon in salt marsh sediment is typically attributed to plant biomass, it can also be produced by benthic photosynthetic biofilms. These biofilms, generally composed of diatoms and their secretions, are known for their high primary productivity and contribution to the basal food web. The growth of biofilms and the preservation of carbon produced by biofilms depends on the amount of sedimentation; low sedimentation rates will favor decomposition, while high sedimentation rates could decrease biofilm productivity. In this study, we conducted laboratory experiments to test (1) if biofilms can potentially accumulate carbon in marsh soil and (2) how different sedimentation rates affect the amount of carbon accumulation. Containers filled with a settled mud bed were inoculated with natural biofilms collected from a marsh surface and allowed to grow with favorable light exposure, nutrient supply, and absence of grazing. Mud was added weekly in different amounts, resulting in an equivalent sedimentation rate from 12 to 189 mm/yr. After 11 weeks, the sediment columns were sampled and analyzed for chl a , organic matter via loss on ignition (LOI), and total organic carbon (TOC). Chl a accumulation rates ranged from 123 to 534 mg/cm 2 /yr, organic matter accumulation ranged from 86 to 456 g/m 2 /yr, and TOC accumulation rates ranged from 31 to 211 g/m 2 /yr. These values are on the same order of magnitude of marsh carbon accumulation rates measured in the field. All three metrics (chl a , organic matter, and TOC) increased with increased sedimentation rate. These results show that biofilms can potentially contribute to carbon accumulation in salt marsh soils. Furthermore, areas with high sedimentation rates have the potential for higher amounts of organic matter from biofilms in the sediment.