Attenuation of Currents by Seagrass Canopies A Classical Concept

The presence of seagrass canopies in the benthic boundary layer (BBL) alters the roughness of the bottom (Fonseca and Fisher, 1986; Nepf and Vivoni, 2000; Granata et al., 2001). As a result, the vertical flow profile shown in Fig. 1 is altered (Fig. 5), especially when the plants occupy a large portion of the water column, i.e. when H/h < 10 (H is the water depth and h the canopy height; Nepf and Vivoni, 2000). Reducedflows are common within the canopy

Fig. 6. Vertical velocity (U) profile (solid line) showing relative flow intensification near the bottom, which is a result of the vertical seagrass biomass (B) distribution (shaded area). Z, distance above the sediment interface. Adapted from Ackerman and Okubo (1993; Zostera marina). Also observed for Thalassia testudinum (koch, 1996), Amphibolis griffithii (van Keulen, 1997), and Amphibolis antarctica (Verduin and Backhaus, 2000).

Fig. 6. Vertical velocity (U) profile (solid line) showing relative flow intensification near the bottom, which is a result of the vertical seagrass biomass (B) distribution (shaded area). Z, distance above the sediment interface. Adapted from Ackerman and Okubo (1993; Zostera marina). Also observed for Thalassia testudinum (koch, 1996), Amphibolis griffithii (van Keulen, 1997), and Amphibolis antarctica (Verduin and Backhaus, 2000).

due to the deflection of the current over the canopy and a loss of momentum within the canopy (Fonseca et al., 1982; Fonseca and Fisher, 1986; Gambi et al., 1990; Koch, 1996; Wallace and Cox, 1997; Koch and Gust, 1999; Verduin and Backhaus, 2000; Peterson et al., 2004). As a result, depending on the seagrass species and shoot density, water speed in the canopy can be 2 to >10 times slower than outside the bed (Ackerman, 1986; Gambi et al., 1990). This process can also trap water within dense seagrass canopies during low tide, leading to a water height difference between vegetated and adjacent unvegetated areas (Powell and Schaffner, 1991). Velocities within seagrass canopies are commonly < 10 cm s-1 but can be as high as 100 cm s-1 (see review by Koch, 2001). Even relatively short seagrasses (Zostera novaze-landica, 15 cm) or beds with relatively low densities (Zostera marina, 100-200 shoots m-2) still seem to reduce velocity (Worcester, 1995; Heiss et al., 2000).

When measuring velocities at a relatively fine scale (cm), flow intensification near the bottom (i.e. relatively faster flows in the region of the sheaths or vertical stems, Fig. 6) may be observed depending on the vertical biomass distribution (Ackerman and Okubo, 1993; Koch, 1996; Koch and Gust, 1999; Nepf and Vivoni, 2000; Verduin and Backhaus, 2000; van Keulen and Borowitzka, 2002). This is due to the fact that the sheaths (e.g. Thalassia testudinum and Zostera marina) or stems (e.g. Amphibolis griffithii and A. antarctica) are less effective in reducing the flow and extracting momentum (Fig. 6) than the vegetated regions above the sheaths and stems that are filled with leaves. Similarly, velocities increase near the top of the canopy as the leaf area is reduced and eventually disappears.

A number of canopy flow models have been applied to terrestrial plant canopies using empirically-fit parameters to modify the law of the wall (review in Okubo et al., 2002). This approach has been recently

Fig. 7. Wave attenuation (open circles) as a function of water depth/tidal fluctuation (black boxes). Wave attenuation was based on the significant wave height in a Ruppia maritima bed in comparison to an adjacent unvegetated area at Bishop's Head Point, Chesapeake Bay, USA. Note that these data were collected in June when the plants were reproductive. Wave attenuation was highest at low tide when the canopy occupied the entire water column. Negative wave attenuation represents periods in which wave height was larger in the vegetated site than the unvegetated site. Source of data: E.W. Koch.

Fig. 7. Wave attenuation (open circles) as a function of water depth/tidal fluctuation (black boxes). Wave attenuation was based on the significant wave height in a Ruppia maritima bed in comparison to an adjacent unvegetated area at Bishop's Head Point, Chesapeake Bay, USA. Note that these data were collected in June when the plants were reproductive. Wave attenuation was highest at low tide when the canopy occupied the entire water column. Negative wave attenuation represents periods in which wave height was larger in the vegetated site than the unvegetated site. Source of data: E.W. Koch.

applied to Z. marina with some success, although there were a number of inconsistencies with field observations as would be expected (Abdelrham, 2003; Peterson et al., 2004). This type of approach provides some indication of the general pattern of flow within an eelgrass canopy, but its utility will likely be limited by species-specific differences in canopy vegetative profiles and the lack of detailed studies of canopy flow in these systems. Future development will need to apply the mixing layer analogy. Realistically, the ability to model canopy flow phenomena is a goal that speaks to the need for detailed canopy flow profiles in the laboratory and the field.

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