The influence of current speed and vegetation density on flow structure in two macrotidal eelgrass canopies

Lay Abstract The influence of eelgrass ( Zostera marina ) on near‐bed currents, turbulence, and drag was investigated at three sites in two eelgrass canopies of differing density and at one unvegetated site in the San Juan archipelago of Puget Sound, Washington, USA. The data were compared to the canopy shear‐layer model, which has been shown to describe flow through submerged vegetation in laboratory studies. Although most of the data were consistent with the model, velocity profiles from the site with lowest eelgrass density were not, particularly when currents were strong. The eelgrass attenuated tidal currents by at least 40%. Attenuation decreased with increasing current speed, and was greater than 70% at the site with greatest eelgrass density. The attenuation produces a region of low velocities within the canopy that provides a sheltered environment for zooplankton, benthic invertebrates, juvenile fishes, and microalgae. In contrast, the top of the canopy is a region of strong turbulent mixing, exhibiting large‐scale velocity fluctuations. Friction velocity (an indicator of turbulence intensity) at the canopy top was 1.5–2 times greater than at the sea floor of the unvegetated site. This strong turbulence creates a very different habitat type than within the canopy and promotes exchange of oxygen, nutrients, and organic matter between the canopy and the water above it. The eelgrass canopies generated significantly more roughness and drag than the unvegetated sea floor. The drag coefficient CD for flow over the canopies was 3–8 times greater than at the unvegetated site (0.01–0.023 vs. 2.9 × 10 − 3 ).