REDUCED MIXING IN A MARINE MACROPHYTE CANOPY

1. We report the effects of a marine plant canopy on flow in the coast al environment. In addition to reducing the magnitude of the ambient f low, a Zostera marina canopy was observed to undulate under unidirecti onal flow. Spectral analyses of the velocity fluctuations in the canop y revealed a fundamental frequency of approximately 0.125-0.156 Hz. 2. These fluctuations were not caused by ambient flow conditions but rat her by the hydroelasticity of the plants ('monami'). 3. Flow within th e canopy can be, therefore, considered a function of plant movement ra ther than ambient conditions. Calculations incorporating these observa tions and the resistance of the canopy indicate that eddy viscosity is between 10(-5) and 10(-4) m2 s-1. 4. Mixing within Z. marina canopies is reduced and more like the deep ocean than the coastal environment. This reduced level of mixing has considerable implications for the pr oductivity and physiology of marine macrophytes.