Interactive Effects of Seagrass and the Microphytobenthos on Sediment Suspension Within Shallow Coastal Bays

Abstract

The suspension and transport of sediments in coastal environments influences water column clarity, and also affects the growth of photosynthetic organisms. 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 secrete biogenic compounds that can increase sediment cohesion. The dual role of seagrass and MPB in altering the seasonal critical bed shear stress, τc, necessary to suspend sediment was studied within a Zostera marina seagrass meadow and an adjacent unvegetated region within a shallow coastal bay in Virginia, USA. Hydrodynamics and MPB biomass were recorded seasonally to determine the critical bed shear stress and subsequent SSC response. Results show that seagrasses reduced mean currents and waves, thus lowering SSC within the meadow. In addition, seagrass created favorable conditions for MPB growth, with annual mean sediment carbohydrate concentrations, a proxy for MPB activity, to be double within the seagrass compared to the unvegetated site. Sediment carbohydrate concentrations within the seagrass bed were higher during winter than summer due to enhanced light penetration, which coincided with an increase in τc to 0.056 Pa compared to 0.024 Pa. τc was found to be 0.021 Pa at the unvegetated site, with bed shear exceeding this threshold > 85% of the time. These findings suggest both MPB and seagrass play an important and interactive role in regulating seasonal sediment resuspension, and constant reworking of the bed sediments in high shear regions prevented the establishment of MPB.