REDUCED MIXING IN A MARINE MACROPHYTE CANOPY

1. We report the effects of a marine plant canopy on flow in the coastal environment. In addition to reducing the magnitude of the ambient flow, a Zostera marina canopy was observed to undulate under unidirectional flow. Spectral analyses of the velocity fluctuations in the canopy revealed a fundamental frequency of approximately 0.125-0.156 Hz. 2. These fluctuations were not caused by ambient flow conditions but rather by the hydroelasticity of the plants ('monami'). 3. Flow within the canopy can be, therefore, considered a function of plant movement rather than ambient conditions. Calculations incorporating these observations 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 productivity and physiology of marine macrophytes.