ABSTRACT

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 plots of Zostera marina (with different biomass, shoot densities and sediment properties) to gradually increased current flow velocities ranging from low (5 cm s^–1) to high (26 cm s^–1) in a hydraulic flume.

We found that higher flow velocities substantially increased (by more than three-fold) 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.

Schematic illustration of the hydraulic flume. Currents are generated with motor and flow velocity measured with an acoustic Doppler velocimeter (ADV). Sediment with seagrass is placed in the test section in the middle of the flume. Organic carbon is measured by taking water samples from the water column at different flow velocities.