Biophysical Controls on Sediment Suspension in a Shallow Coastal Bay

The suspension and transport of sediments in coastal environments influences water quality through light attenuation, contaminant and nutrient transport. This regulation of water column clarity also affects the growth of pelagic and benthic photosynthetic organisms. In addition, the presence of benthic vegetation, such as seagrass, can attenuate wave and tidal energy; thereby altering suspended sediment concentrations (SSC) and microphytobenthos (MPB) biomass that may secrete biogenic compounds that increase sediment cohesion. This work investigates the dual role of seagrass and MPB in altering the seasonal critical bed shear stress necessary to suspend sediment within a restored Zostera marina seagrass meadow. Research was conducted in South Bay, Virginia, a shallow (1-2 m mean depth) subtidal coastal bay on the Delmarva Peninsula. Hydrodynamic measurements including current velocity, wave characteristics, shear stress, as well as biological samples, light and SSC data were recorded for 1-3 weeks over 2 consecutive years during the spring, summer, fall and winter seasons. An in situ flume was used to quantify the critical bed shear stress during the summer by manipulating flow velocities and examining the shear stress and SSC response. The shear stress and SSC timeseries from the seasonal deployments was used to quantify the critical stress for other seasons. The seasonal deployments correspond to distinct seagrass growth cycles, where seagrass density during the summer can be 4 times greater compared to winter. Seagrass is well known to attenuate fluid energy, creating favorable conditions for sediment deposition and reducing SSC within the meadow during peak density. However, due to seagrass shading during the summer, carbohydrate, a proxy for MPB activity, was higher during the winter (99±19 µg g-1) compared to summer (84±3 µg g-1). This potentially led to bed stabilization in the winter, as measured by an increase in the critical bed shear stress from 0.02 Pa during the summer to 0.05 Pa in the winter. These findings suggest MPB play an important role in regulating seasonal water quality, possibly providing a positive feedback for seagrass growth by maintaining bed stability during low-density periods, which primarily occur during the winter.