Flume observations on flow dynamics in Zostera marina (eelgrass) beds

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Abstract
Flow dynamics in Zostera marina L. (eelgrass) were studied in a large seawater flume. Velocity and turbulence intensity profiles were measured at 3 free-stream flow velocities (5, 10 and 20 cm s-1), at 5 shoot densities (1200, 1000, 800, 600 and 400 shoots m-2) and at 5 along-stream positions relative to the leading edge of the eelgrass bed (10 cm upstream of the bed; 25, 50, 75 and 100 cm downstream of the leading edge of the bed). All the profiles (75) above the canopy or over bare sand fitted a long-profile relationship. At all densities and ambient velocities tested, mean velocity increased above the canopy, while within the bed water speed dropped distinctly below the canopy-water interface. Depending on shoot density, water speed was from 2 to 10 times lower under the canopy than upstream of the seagrass bed. Shear velocities (U*) above the canopy were 2 to 11 times greater than outside the bed at equivalent height, and increased significantly with distance into the meadow. No significant differences among densities were observed. Turbulence intensity showed a dramatic increase in all the profiles at the canopy-water interface, a significant increase with distance into the bed, but showed no significant differences between densities. Fluid flux within the bed decreased significantly with distance into the meadow, but exhibited no significant dependence on density. Downstream, vertically integrated fluid flux at 100 cm into the bed ranged between 14.7 and 40.6% of upstream values. The least flux reduction occurred at the highest velocity (20 cm s-1). Trends in shear velocity and turbulence intensity show clearly that within the bed one can distinguish 2 dynamically different environments. The ''canopy-water interface'' habitat is characterized by high shear stress and high turbulence intensity; the ''below-canopy'' habitat is characterized by low shear stress and a reduction of turbulence intensity.