The attenuation of current- and wave-driven flow within submerged multispecific vegetative canopies.

Historically, submerged vegetative canopies have either been reported as or modeled after unispecific examples-communities comprised of only a single vegetative species or element type. Field surveys of a shallow Florida Bay seagrass meadow highlighted a more diverse benthic landscape. Although dominated by Thalassia testudinum, the communities were distinctly multispecific, composed of a mixture of both plant and algal species. Strap-like seagrass elements defined the upper portion of these canopies (the upperstory) while broad-bodied algal species were found concentrated close to the bed (the understory). To predict the hydrodynamic implications of this dual-story canopy structure, we derived a new canopy flow attenuation model, formulated to account for vertical canopy heterogeneities like those seen at our field site. The model was validated through a series of laboratory experiments: multispecific canopy mimics were installed in a current-wave flume and exposed to a range of unidirectional and oscillatory flows. Mean and fluctuating velocity was measured above and within each canopy to determine vegetation-induced flow attenuation. Velocities near the bed were markedly reduced through the addition of understory elements, results that were consistent with model predictions. These findings suggest that accurate prediction of flow-regulated processes like sediment transport and propagule dissemination depends on a thorough accounting of community composition. These properties are also expected to change in response to seasonal variability and episodic environmental stresses. [ABSTRACT FROM AUTHOR]