When considering processes in seagrass meadows, spatial homogeneity is usually assumed, but in nature, patchy seagrass landscapes are more common than homogenous ones. This patchiness may have a major effect on physical, geological, chemical, and biological processes. For example, gaps in the canopy allow an accelerated influx of water into the canopy (Granata et al., 2001), replenishing nutrients, introducing spores and propagules, eroding sediment and increasing mixing in general. The distance currents can penetrate the edge of a seagrass bed before becoming equilibrated with respect to momentum has been estimated to be between 1 m (Fonseca and Fisher, 1986) and 50 boundary layers (assumed to be equal to the height of the canopy; Nowell and Jumars, 1984; Granata et al., 2001).
These values can be affected by the density of the bed and the vertical distribution of biomass. In dense beds, unidirectional water flow is smoothly directed over the top of the seagrass canopy as "skimming flow" (Nowell and Jumars, 1984; Fonseca and Ken-worthy, 1987), effectively trapping a layer of water within the canopy (Koch and Gust, 1999), i.e. increasing the residence time. Under such conditions, nutrient concentrations within the vegetation may be quite low (Moore et al., 1996). In contrast, a reduction in shoot density leads to increased flow intrusion and velocity within the canopy (van Keulen, 1997). Therefore, reducing seagrass density could permit an increase in turbulence and mixing within the meadow, with an associated increase in nutrient exchange and uptake, and the potential for increased sediment resuspension. This subject needs further attention.
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