The second aspect to take into consideration is the potential interference of epiphytes in nutrient uptake by leaves. The presence of small organisms on the exchange surface surely adds complexity to the diffusive process. As epiphytes increase leaf roughness, they could reduce the thickness of the boundary layer under wave or current action, as well as increasing the boundary layer under unstirred conditions. However, since most of these epiphytes are algae, they also take up nutrients from water, thus creating a competitive interference. In any case, and as far as we are aware, these aspects have not been investigated in detail (see also Section IVA, Koch et al., Chapter 8 and Borowitzka et al., Chapter 19).
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