Water Flow Around Seagrass Shoots Ecological Implications

Seagrass shoots are obstructions to flowing water. When considering the vertical scale, seagrass shoots are exposed to a gradient of velocities in the canopy and benthic boundary layers. Due to the no-slip condition, the slowest flows are found near the sediment surface and the strongest flows near the top of the canopy. As a result of this vertical difference in velocities and the horizontal differences in upstream versus downstream velocities around a shoot, a vertical pressure gradient develops on the downstream side of the seagrass shoot: high pressure near the bottom where the currents are relatively slow and low pressure farther up in the water column where currents are stronger. This leads to the development of significant ascending flows (i.e. as high as 15% of ambient) immediately downstream of seagrass shoots (Fig. 4; Nepf and Koch, 1999). Pressure gradients around shoots (Huettel and Gust, 1992) can also lead to the intrusion of water into permeable sediments upstream of the shoot (high pressure zone; Koch and Huettel, 2000) and porewater upwelling downstream of the shoot (low pressure zone) (Nepf and Koch, 1999). For example, around a single Thalassia testudinum shoot exposed to a current speed of 10 cm s-1, water was found to penetrate 2.5 cm into the permeable sediments (Fig. 4); a depth an order of magnitude larger than that affected by diffusion (Jorgensen and Boudreau, 2001). This flow-induced intrusion of water into seagrass-colonized permeable sediments may bring organic particles (Huet-tel et al., 1996) closer to the root zone and remove toxic compounds from the sediments such as sulfide (Koch, 1999a). Most of the impact of water intrusion into the sediment occurs over the first 6 h which coincides with semi-diurnal tides (Koch and Huettel, 2000). Therefore, the exchange between the sediments and the water column seems to be maximized in seagrass habitats in which the current direction changes every 6 h.

Fig. 3. Waving of seagrass leaves in wave-dominated habitats: (A) Phyllospadix torreyi at approximately 2.5 m depth at Punta Morro, Pacific coast of Mexico and (B) Halodule wrightii at approximately 1 m depth, north of Placencia, Belize. The lines were traced over individual leaves in order to emphasize the bending pattern. Note that the leaves are continuously moving back and forth every few seconds with the passage of waves. Photos: E.W. Koch.

Fig. 3. Waving of seagrass leaves in wave-dominated habitats: (A) Phyllospadix torreyi at approximately 2.5 m depth at Punta Morro, Pacific coast of Mexico and (B) Halodule wrightii at approximately 1 m depth, north of Placencia, Belize. The lines were traced over individual leaves in order to emphasize the bending pattern. Note that the leaves are continuously moving back and forth every few seconds with the passage of waves. Photos: E.W. Koch.

The combination of the pressure-induced up-welling of porewater and the vertical ascending flows immediately downstream of the seagrass shoots colonizing permeable sediments appear to generate a slow "stream" connecting the sediment porewater and the water column at mid-height in the seagrass canopy (Nepf and Koch, 1999; Fig. 4). Under these circumstances, exchanges between the sediment

Fig. 4. Vertical ascending flows and porewater flows generated by pressure gradients around a seagrass shoot. As seagrass shoots live in the benthic boundary layer (see velocity (U) profile on the left), the top of the shoot experiences faster velocities and lower pressure than the bottom of the shoot. As a result, a vertical ascending flow is generated downstream of the shoot. This water then disperses horizontally at the point where the leaves bend over with the flow. Due to the pressure gradients generated on the sediment surface when the flowing water impacts the seagrass shoot, water also penetrates into permeable sediments leading to a zone in which the porewater is washed out by the overlying water. Z , distance above the sediment interface. Modified from Nepf and Koch (1999) and Koch and Huettel (2000). It is possible that the upwelling porewater may be transported high into the water column via the above processes.

Fig. 4. Vertical ascending flows and porewater flows generated by pressure gradients around a seagrass shoot. As seagrass shoots live in the benthic boundary layer (see velocity (U) profile on the left), the top of the shoot experiences faster velocities and lower pressure than the bottom of the shoot. As a result, a vertical ascending flow is generated downstream of the shoot. This water then disperses horizontally at the point where the leaves bend over with the flow. Due to the pressure gradients generated on the sediment surface when the flowing water impacts the seagrass shoot, water also penetrates into permeable sediments leading to a zone in which the porewater is washed out by the overlying water. Z , distance above the sediment interface. Modified from Nepf and Koch (1999) and Koch and Huettel (2000). It is possible that the upwelling porewater may be transported high into the water column via the above processes.

porewater and the water column are not driven by diffusion but by advection (Huettel and Webster, 2001). This process could benefit the seagrasses by bringing recently remineralized nutrients and carbon to seagrass leaves (Nepf and Koch, 1999).

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