Vegetation effects and sediment deposition in a mesotidal wetland

Many studies have been carried out in the last decade to assess the rates of sediment transport and deposition on tidal flats and salt marshes, however, a need to characterize the transport fluxes between the various habitats as a function of tidal range, their position relative to mean sea level, and flow-asymmetries in the vegetation effect remain. This study uses fieldwork data to characterize the sediment fluxes and deposition from the tidal flats towards the marsh platform, in a channel margin of the Ria Formosa coastal lagoon (south Portugal). Sediment fluxes were measured in a cross-shore transect, during neap and spring tide conditions. The dominant intertidal species are Spartina maritima and the seagrass Zostera noltei. Current measurements were used to assess bottom shear stress conditions. Deposition rates, instantaneous suspended sediment, and near-bed velocities were linked through theoretical formulas and used to characterize time-averaged conditions for sediment delivery and deposition to the site.

The results showed that suspended sediment concentrations and sediment deposition varied across-shore with no specific relation to elevation. Maximum current velocities were recorded in the vegetated tidal flat, in the order of 0.20 m/s, and in the low marsh due to flow-plant interactions and an increase in turbulence. Deposition rates ranged between 20 to 45 g/m²/hr, after a complete tidal cycle, and were higher in the mid-upper marsh. The hydroperiod was not the main contributor to sediment deposition in the study area. Measured sediment transport was tidally driven, with shifting current angles during the cycle and major alongshore components during peak flood velocities. Flow-spartina interference in the low marsh significantly affected local sediment resuspension. The obtained results provide insights into the dynamics and variability of flow and mass transfer along a transition from the vegetated tidal flat to the upper marsh and can be used in sediment transport models for mesotidal marsh systems.

Acknowledgments: A. Rita Carrasco was supported by the contract DL57/2016/CP1361/CT0002, and Katerina Kombiadou was supported by the institutional contract CEECINST/00146/2018, both funded by Fundação para a Ciência e Tecnologia (FCT). This study had also the support of FCT under the project LA/P/0069/2020 granted to the Associate Laboratory ARNET and CIMA BASE UID/00350/2020.