Water flow through seagrass beds transports nutrients, affects sediment stability and chemistry, and imposes hydrodynamic forces on shoots that alter canopy configuration. Past studies done under diverse conditions yielded conflicting results about the effects of shoot density on flow through seagrass bed canopies. We used eelgrass, Zostera marina, to study how the density of flexible shoots affect the hydrodynamics of seagrass beds in unidirectional water flow. By exposing randomly-arranged shoots of uniform length to current velocities controlled in a flume, the effects of shoot density and distance downstream from the bed edge could be determined without confounding factors. Comparison of velocity profiles within beds to those upstream of beds showed that flow was slower in the beds. However, shoot density, downstream distance, and current velocity did not affect the percent reduction in flow velocity in a bed. Turbulence enhances mixing of substances carried in the water. Here, turbulence intensity (index of the importance of turbulent velocity fluctuations relative to average current velocity) was lower when ambient flow was faster, but was not affected by shoot density or downstream position, Drag (hydrodynamic force on a shoot that bends it over in the flow direction) provides another measure of how the canopy affects flow experienced by a shoot. Drag was not affected by current velocity, shoot density, or downstream position in the bed. Gaps between shoots can enhance light and flow penetration into the canopy, but when shoots are bent over by flow, they can cover gaps. Faster ambient currents caused greater gap closure, which at each current speed was greater for high shoot densities. Thus, canopy gap closure did not correlate with percent flow reduction in grass beds or with drag on individual shoots, both of which were independent of shoot density and ambient current velocity. Since changing shoot density does not affect the flow in a grass bed exposed to a given ambient current, our results are inconsistent with the hypothesis that the high shoot densities observed in grass beds in habitats exposed to rapid flow are due to a direct, adaptive response of the grass to the flow environment.
Canopies formed by aquatic vegetation, such as mangroves, seagrass, and kelp, play a crucial role in altering the local hydrodynamics in rivers, estuaries, and coastal regions, and thereby influence a range of morphodynamic and biophysical processes. Prediction of the influence of canopies on these hydrodynamic processes requires a fundamental understanding of canopy drag, which varies significantly with both flow conditions and canopy properties (such as density and submergence). Although our knowledge on canopy drag has increased significantly in recent decades, a conclusive, physics-based description for canopy drag that can be applied to both emergent and submerged canopies is currently lacking. Here, we extend a new theoretical canopy drag model (that employs the velocity between canopy elements as the reference velocity) to submerged aquatic canopies. The model is validated for the first time with direct measurements of drag forces exerted by canopies across broad ranges of flow conditions and canopy density and submergence. The skill and broader applicability of the model are further assessed using a comprehensive set of existing experimental data, covering a broad range of natural conditions (including flexible vegetation). The resulting model provides a simple tool to estimate canopy drag forces, which govern hydraulic resistance, sediment transport, and biophysical processes within aquatic ecosystems.
Local effects of flow interaction with seagrass structure modify meadow scale hydrodynamics, resulting in lower current velocities and wave heights within a seagrass meadow. This attenuation promotes the deposition of suspended sediment, increasing the light available locally to benthic organisms. To elucidate the relationship between small-scale hydrodynamics that occur at the sea floor and the meadow scale effects of seagrass, high resolution velocity profiles were recorded adjacent to the sediment-water interface within a Zostera marina seagrass meadow in South Bay, Virginia. Additionally, instrumentation was deployed across the meadow to seasonally monitor corresponding changes in wave height across the seagrass meadow. Results show that wave height was reduced by 25–49% compared to an adjacent bare site, and by 13–38% compared to an analytical model of wave attenuation over an unvegetated seafloor with the same bathymetry. The greatest attenuation of wave height occurred during the spring and summer when seagrass biomass was greatest, while the lowest attenuation occurred in winter, corresponding to periods of minimal seagrass biomass. Significant wave height attenuation coefficients, αw, calculated for the meadow ranged from αw = 0.49 in spring to 0.19 during winter, but were highly dependent on wave conditions, with greater αw for larger wave heights and longer period waves. Within the seagrass meadow during summer, the highest measured bed shear stress was τbed = 0.034 ± 0.022 Pa, which occurred during peak wave conditions. This suggests that during high biomass conditions, the bed shear stress rarely exceeds the critical bed shear, τcrit = 0.04 Pa necessary to initiate sediment resuspension. This is in contrast to the bare site which showed elevated values of τbed above the critical threshold across all seasons. These findings suggest the seagrass meadow does exert significant control over both wave heights and the hydrodynamic conditions at the sediment-water interface, and this control is due to the attenuation of wave motion by drag induced from the seagrass over the expanse of the meadow.
Ice can be an important structuring factor physically removing intertidal flora and fauna. At high latitudes in particular, the removal of canopy-forming algae by ice scour may be important as their canopy may serve to modify the extreme environment for marine organisms at low tide. We simulated the effect of ice scouring by manipulating the biomass of the canopy-forming algae Ascophyllum nodosum in a sub-Arctic fjord [“Full canopy,” “Reduced canopy,” “Bare (start),” “Bare (annual)”]. Over a three-year period, we quantified key physical parameters and the recolonization of flora and fauna to test the hypothesis that A. nodosum and rock rugosity facilitate recolonization of sub-Arctic intertidal fauna and that potential facilitation could rely on an ability of A. nodosum canopy to modify air temperature and ice scour. Finally, we estimated the recovery period of A. nodosum canopy height to pre-disturbance levels based on estimated early growth rates. We found that A. nodosum canopy facilitated higher species richness and recolonization of dominating faunal species (Littorina saxatilis, Littorina obtusata, Mytilus edulis, and Semibalanus balanoides), and also significantly reduced the high temperatures in summer and raised the low temperatures in winter. The abundance of M. edulis and A. nodosum recolonization increased significantly with rock rugosity and the recovery of A. nodosum canopy height was estimated to a minimum of 15 years. We conclude that algal canopy and rock rugosity play key roles in structuring sub-Arctic intertidal communities, likely by modifying environmental stress such as extreme temperature, desiccation, and by increasing the settling surface and the habitat complexity. As the distribution of canopy-forming algae is expected to shift northward, they may act as a key habitat facilitating a northward colonization of intertidal fauna in the Arctic. We highlight the importance of considering scales relevant to biological communities when predicting impacts of climate change on distributional patterns and community structure in the Arctic intertidal.