The role of the seagrass Zostera noltii on sand transport across an intertidal sand flat in Ryde, Isle of Wight

Abstract

Seagrass meadows are known to buffer wave energy, reduce current velocities and hence can provide stability to the coasts. The presence of seagrass on the seabed modifies the dynamics of the benthic boundary layer and thus the sediment transport. The overall aim of this project is to address the limited knowledge on the interactions between seagrass, seabed and the counteracting sedimentary processes by investigating interactions between the intertidal seagrass Zostera noltii and the surrounding sediments. Specifically, I am investigating how this species affects the mean flow, the turbulence in the canopy, and the resulting sediment mobilisation. Zostera noltii shows a strong seasonality, therefore the intertidal area of Ryde, Isle of Wight was monitored monthly over two annual cycles to assess the effect of seasonal changes on sediment characteristics and bed morphology. Grain size trend analysis was carried out in order to investigate the sand transport pathways on the intertidal flat in the region of seagrass. Sediment transport patterns were defined using Grain Trend Analysis. The Ryde intertidal flat is composed mainly of fine, well sorted and positively skewed sand. The transport vectors suggest a westward transport on the west side of Ryde Pier and an on-offshore transport on the east side of Ryde Pier. Gradients in sediment texture and composition were related to the season and influenced by meadow distribution and density. This was attributed to the mechanical trapping of particles and enhanced deposition due to dampening of current and wave action in the meadows. Locations inhabited by seagrass showed less change in bed morphology compared to bare sand areas. The tidal flat was found to be stable or exhibited minor bed accretion (cms) along vegetated profiles. Laboratory experiments carried out in a recirculating and annular flume on Zostera noltii showed that turbulence and mean flow were reduced within the canopy indicating low sediment transport in the region of the seagrass. Seagrass increases deposition in summer by modifying the boundary layer through its above-ground biomass and resists erosion in winter by increasing cohesiveness of the bed through its below-ground biomass (roots and rhizomes). Despite the reduction in seagrass canopy influence on the hydrodynamic forcing, the persistent presence of below-ground biomass all year round reduces sediment transport hence providing stability to the bed.

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ORCID for Charlotte Thompson: orcid.org/0000-0003-1105-6838

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