Sediment sorting within a relatively wave-exposed and sandy subtidal seagrass (Zostera marina) meadow

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Seagrasses impact on sedimentary intertidal and subtidal ecosystems by affecting local hydrodynamics, geomorphology and sediment properties. Their influence on hydrodynamics is to reduce flow velocities in their canopies, and this leads to increased net sedimentation rates and reduction of the grain size. Most investigations of the seagrass-hydrodynamics-sediment feedback system has been carried out over silt and fine-sand beds under tide-dominated conditions, mostly in the intertidal zone. Here, we use sedimentological data from a relatively wave-exposed and subtidal seagrass (Zostera marina) meadow in the Isles of Scilly with a fine-to-medium sand bed and show that the sand within the seagrass meadow is indeed finer than in adjacent, unvegetated regions. However, in contrast to previous studies, this is not due to increased mud/silt content within the seagrass meadow, but an almost 0.1-mm shift in the median sediment size across the sand fraction from 0.25 mm (fine to medium sand) to 0.35 mm (medium sand). The studied seagrass meadow is characterised by a distinct spatial structure comprising of vegetated ‘ridges’ and bare sand ‘valleys’. Even the bare sand valleys within the seagrass meadow are characterised by significantly coarser sand than the adjacent vegetated ridges, providing further confirmation of the efficiency of sediment sorting by wave processes that takes place within seagrass meadows. It is concluded that any numerical modelling involving sediment transport processes associated with seagrass environments must account for variability in the textural characteristics.