Increased current flow enhances the risk of organic carbon loss from Zostera marina sediments: Insights from a flume experiment

Hydrodynamic processes are important for carbon storage dynamics in seagrass meadows, where periods of increased hydrodynamic activity could result in erosion and the loss of buried carbon. To estimate hydrodynamic impacts on the resuspension of organic carbon (C-org) in seagrass-vegetated sediments, we exposed patches (0.35 x 0.35 cm) of Zostera marina (with different biomass, shoot densities, and sediment properties) to gradually increased unidirectional (current) flow velocities ranging from low (5 cm s(-1)) to high (26 cm s(-1)) in a hydraulic flume with a standardized water column height of 0.12 m. We found that higher flow velocities substantially increased (by more than threefold) the proportion of C-org in the suspended sediment resulting in a loss of up to 5.5% +/- 1.7% (mean +/- SE) C-org from the surface sediment. This was presumably due to increased surface erosion of larger, carbon-rich detritus particles. Resuspension of C-org in the seagrass plots correlated with sediment properties (i.e., bulk density, porosity, and sedimentary C-org) and seagrass plant structure (i.e., belowground biomass). However, shoot density had no influence on C-org resuspension (comparing unvegetated sediments with sparse, moderate, and dense seagrass bed types), which could be due to the relatively low shoot density in the experimental setup (with a maximum of 253 shoots m(-2)) reflecting natural conditions of the Swedish west coast. The projected increase in the frequency and intensity of hydrodynamic forces due to climate change could thus negatively affect the function of seagrass meadows as natural carbon sinks.