At least two additional aspects have to be considered in relation to leaf nutrient uptake by seagrasses. First, the resistance to nutrient uptake induced by the unstirred boundary layer. The influence of diffusive boundary layers (DBLs) on several biological processes has received much attention (e.g. Denny and Wethey, 2001; Koch et al., Chapter 8; Larkum et al., Chapter 14), and it is known that, under certain conditions, it can curtail solute exchange. The relevance of such boundary layers in seagrass biology has been examined mostly related to inorganic carbon acquisition (e.g. Koch, 1994; James and Larkum, 1996; Larkum et al., Chapter 14); however, there is also some evidence of its importance for nutrients in aquatic macrophytes: for example, periodic stripping of the boundary layer by passing waves has been shown to increase nutrient uptake by a factor of 10 (Stevens and Hurd, 1997; Koch et al., Chapter 8).
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