Effects of seagrass beds (Zostera noltii and Z-marina) on near-bed hydrodynamics and sediment resuspension

The main objectives of this flume study were to (1) quantify density dependent effects of the short-leaf seagrass Zostera nolti on hydrodynamics and sediment resuspension from a sandy bed, and (2) measure the erodability of 2 contrasting sediments (sandy and muddy) and the extent to which this is modified by the presence of 2 seagrass species, Z. noltii (sandy) and Z. marina (muddy). Field measurements of near-bed tidal currents, turbulence and suspended particulate matter at 2 different Z. noltii locations (low energy [sheltered] and higher energy [exposed] environments) were interpreted in the context of the flume results. Skimming flow above the high density bed of Z. noltii was accompanied by a 40 % reduction in near-bed flow, but this was offset by a 2-fold increase in turbulent kinetic energy (TKE) and bed shear stress (tau(0)). Despite this increase in tau(0) there was an increase in sediment stabilisation with increasing seagrass density (10-fold increase in critical bed shear stress for erosion [tau(e)] from 0.1 [bare sediment] to 1.0 Pa at the highest shoot density). This was largely explained by the increased microphytobenthos abundance (reflected in the higher chlorophyll a and carbohydrate contents) and a lower density of the grazer and bio-destabiliser Hydrobia ulvae. In contrast, the muddy site was more easily eroded (10-fold higher), with Z marina having little effect on sediment erodability (bare: tau(e) = 0.05 Pa; Z. marina: tau(e) = 0.07 Pa). This higher erodability was due to differences in hydrodynamics and the physical/biological properties of the sediment.