Sediment Deposition and Accretion Rates in Tidal Marshes Are Highly Variable Along Estuarine Salinity and Flooding Gradients

Abstract

Vertical accretion in estuarine marshes depends on rates of sediment deposition and is a complex function of different interacting variables. In times of climate change and associated sea-level rise, knowledge about the relation between these variables and sediment deposition and accretion rates is gaining high importance. Therefore, we studied spatial and temporal variation in short-term sediment deposition rates and its possible predictors in three marsh types along an estuarine salinity gradient. Between March 2010 and March 2011, bi-weekly sediment deposition was quantified along three transects, reflecting the variability in elevation (low to high marsh) and distance to the sediment source, in each of one tidal freshwater, brackish, and salt marsh at the Elbe Estuary (Germany). Simultaneously, water-level fluctuations and suspended sediment concentration (SSC) were recorded, and aboveground plant biomass was sampled once in late summer and once by the end of winter, respectively. Annual sediment deposition (17.5 ± 4.0 kg m−2) and calculated accretion rates (20.3 ± 4.7 mm year−1) were highest in the brackish low marsh and were between 51 and 71 % lower in the low tidal freshwater and the salt marsh, respectively. Highest SSC and longest inundations were found during fall and winter. Flooding duration and frequency were higher in the tidal freshwater than in the brackish and the salt marsh. Aboveground, plant biomass of the regularly flooded vegetation stratum (0–50 cm above soil surface) did not differ between marsh types, but the spatial pattern changed between late summer and early spring. In all three marsh types, decreasing sediment deposition rates with increasing distances from the sedimentation source were recorded. The applied multiple regression models were able to explain 74, 79, and 71 % of variation in sediment deposition patterns in tidal freshwater, brackish, and salt marshes, respectively. SSC was the most important model predictor variable. Our results emphasize the importance of considering spatial and temporal variations in sediment deposition rates and its predictors. According to our findings, sediment deposition rates in the investigated tidal low marshes of the Elbe Estuary seem to be sufficient to compensate moderate rates of sea-level rise. Contrastingly, high salt marshes might be vulnerable due to insufficient input of sediment and might regress into low marshes, partly

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Improved modelling of the impacts of sea level rise on coastal wetland plant communities

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