Hydrodynamic effects of macrophyte microtopography: Spatial consequences of interspecific benthic transitions

Rhizophytic green algae of the genus Caulerpa are potential competitors with seagrass-dominated habitats. At Cadiz Bay Natural Park, Caulerpa prolifera and the seagrass Cymodocea nodosa co-occur in overlapping patches, which may be considered as transition zones. In these zones, changes in microtopography are stepped because patches of C. prolifera normally occur at 5 to 10 cm above the adjacent C. nodosa patches. We hypothesized that hydrodynamics influenced by this microtopography may foster sedimentation on the C. nodosa side, which in turn would facilitate the spreading of C. prolifera. To test this hypothesis, an experiment simulating the flow between both species was conducted in a race-track flume tank, allowing us to study the influence of unidirectional free stream velocity (low velocity, LV = 0.065 m s^-1, vs. high velocity, HV = 0.14 m s^-1) and microtopography (flat bottom, FB, vs. stepped bottom, SB) on shear stress (τ), turbulent kinetic energy above the canopy (TKE_above) and volumetric flow rate through the canopy (Q_c). Under our experimental conditions, comparison of τ-values with theoretical thresholds revealed sedimentation scenarios. However, at HV, the probability of sedimentation was higher in C. nodosa than in C. prolifera patches. Moreover, Q_c was higher within C. nodosa canopies than within C. prolifera, ensuring a higher source of sediment with potential to be trapped within the canopy. Although we found no species-specific differences in TKE_above values, they increased with HV increasing the vertical mixing. Overall, conditions for sedimentation seemed more facilitated in C. nodosa than in C. prolifera in SB only under HV conditions.