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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Climate change and engineering activities have modified the hydrology and morphology of estuaries. However, the potential effects of these modifications on vegetation succession in estuarine marshes are still poorly understood. Therefore, we studied temporal changes in tidal habitats of the Elbe estuary over a period of 30 years. We compared vegetation maps from 1980 to 2010 and calculated the change in area of habitats with respect to three salinity and three elevational zones. To analyze the direction of the temporal change, we differentiated between progressive and regressive succession. By using regression tree models (conditional inference trees), we identified the most influential factors determining progressive or regressive succession of low marshes. The total area of the estuarine tidal marshes at the Elbe increased by 2 % from 1980 to 2010, but changes were unequal among the salinity zones. In the salt and brackish zones, the area covered by high marshes increased substantially but decreased in the tidal freshwater zone, while that covered by low marshes decreased in all the salinity zones. Additionally, we determined high persistence of tidal flats and high marshes, whereas only 19 to 28 % of the low marshes found in 1980 remained in 2010. In salt and brackish marshes, more than two-thirds of the area that had been identified as low marshes in 1980 had progressively developed into high marshes. In contrast, 44 % of the area of low marshes in tidal freshwater marshes showed regressive succession back into tidal flats. The distance to the navigation channel was the main factor determining successional direction in salt and brackish marshes. Here, greater proximity to the channel was correlated with higher risk of regressive succession. In tidal freshwater marshes, we identified both the distance to the navigation channel and the situation on the river shore (i.e. inner bank, outer bank or straight bank) as the main factors for marsh succession. Here, considerable engineering activities in the channel had simultaneously decreased the mean low water level and increased the mean high water level between 1980 and 2010, which led to an increase in tidal amplitude. It is quite likely that these changes negatively modified marsh distribution, increased regressive succession and, thus, lowered the quality of tidal freshwater marshes.

Accelerated sea-level rise (SLR) is threatening tidal marshes worldwide. An important control of tidal marsh survival under accelerated SLR is the sediment deposition. Therefore, factors affecting sediment deposition rates (SDR) have been studied extensively using various types of sediment traps. The efficiency of various sediment traps has been compared in several studies, but most of these were conducted in shallow lakes or rivers. In contrast, the efficiency of different sediment traps in tidal marshes is unknown. Therefore, the aim of this study was to compare the trapping efficiency of four frequently used sediment traps, namely flat traps constructed of either tiles or floor mat, and circular traps with and without a lid, under controlled experimental conditions simulating tidal inundations in a flume. The strong differences between circular sediment traps (high efficiency) and both flat surface sediment trap methods (low efficiency) found in this study were remarkable. Additionally, further evidence was found for decreases of SDR with increasing distance to the inflow of the flume (sediment source) and with decreasing suspended sediment concentration (SSC). These findings indicate that trap design has a large influence on sedimentation rate and that studies using different types of sediment traps are not directly comparable.

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