Functional assessment of differential sediment slurry applications in a deteriorating brackish marsh

We applied sediment slurries of varying thicknesses to deteriorating vegetated brackish marsh areas with organic soils. Our objective was to determine if overall elevation change and its component processes of soil compression and sediment consolidation are differentially affected by different amounts of sediment and to assess sediment effects on key ecosystem functions and physico-chemical drivers. We found that sediment nourishment (2.3–20.3 cm) increased soil surface elevation initially, but by the end of the ∼2.5 year study period, sediment-nourished areas, averaged over all thicknesses, subsided to pre-sediment surface elevations, and were no different from reference area surface elevations. Rates of elevation change, soil compression, and sediment consolidation were all strongly related to applied sediment thickness; plots that received more sediment lost elevation at faster rates. Elevation change following sediment nourishment was driven by soil compression that occurred within the underlying native soil. In contrast, applied sediment consolidation over time had little negative influence on elevation change, though applied sediment bulk density increased linearly with increasing sediment thickness. Sediment nourishment <10 cm thick resulted in plots having lower elevations (up to ∼2 cm lower) compared to pre-sediment conditions without enhanced function. However, plots that received >15 cm of sediment nourishment had both small elevation gains (<3 cm) compared to pre-sediment surface elevations and greater plant production compared to unamended reference plots. Factors influencing enhanced ecosystem function included a combination of greater soil bulk density, mineral matter content, available nutrient and trace metal concentrations, higher soil redox potential, and reduced salinity and sulfide stress that occurred with increasing sediment nourishment.