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

Abstract

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.

Turbulence Within Natural Mangrove Pneumatophore Canopies

“…Measured eddy viscosities ranged from O (10 −5 –10 −3 ) m 2 /s, with the maximum values reaching 1.9 × 10 −3 m 2 /s. These eddy viscosities were consistent with simple models (Henderson et al, ; Lowe et al, ; Weitzman et al, ) and were also comparable to eddy viscosities of O (10 −5 ) to O (10 −4 ) m 2 /s measured in situ within seagrass ( Zostera marina ) by Ackerman and Okubo (). In numerical models, vegetation effects are simulated through user‐prescribed background eddy viscosities that differ between vegetated and nonvegetated areas.…”

“…Nevertheless, most eddy viscosities were positive, and in the range 0–1.9 × 10 −3 m 2 /s. For comparison, adopting a standard mixing‐length model (Lowe et al, ; Weitzman et al, ) and using the scaling in Appendix A.1 of Henderson et al () suggests a theoretical eddy viscosity of ΛC f u 0 ℓ , where ℓ is a mixing length, u 0 is a typical velocity magnitude (here taken to be the r.m.s. velocity), C f = 0.1, Λ = (4 π ) −1 au 0 T w , and T w is a typical wave period.…”

A simple and practical model for combined wave–current canopy flows

“…In order to leverage this finding, a simple N-box model was developed. The model is an extension and improvement over the 2-and 3-box models presented by Lowe et al (2005) and Weitzman et al (2015). The N-box model allows accurate prediction of the volume-averaged streamwise velocity profile based on a pressure gradient and a canopy density profile.…”

“…NRI provides accurate estimates of the in-canopy velocity profile, as well as the average velocity between z = h and z = 2h. NRI is closely related to the 3-box model of Weitzman (2013) and Weitzman et al (2015), which employs two in-canopy boxes to account for polycultural vegetation with a sparse upper canopy and a dense understory. NRI demonstrates how the N-box model could be used to generalize the 3-box approach to incorporate higher-resolution vegetation density profiles.…”

Toward a universal mass‐momentum transfer relationship for predicting nutrient uptake and metabolite exchange in benthic reef communities

“…The use of canopy models to predict flow structure within natural, homogeneous canopies has proven highly successful thus far [e.g., Lowe et al ., ; Luhar et al ., ; Weitzman et al ., ]; however, a number of problems arise when trying to use them to predict rates of canopy mass transfer at the scale of an entire organism or community. First, calculation of flow within more complex and/or heterogeneous communities requires extensive measurements of the canopy architecture in order to constrain the model with the necessary statistical rigor [ Lowe et al ., ].…”