Importance of Biogeomorphic and Spatial Properties in Assessing a Tidal Salt Marsh Vulnerability to Sea-level Rise

Abstract

We evaluated the biogeomorphic processes of a large (309 ha) tidal salt marsh and examined factors that influence its ability to keep pace with relative sea-level rise (SLR). Detailed elevation data from 1995 and 2008 were compared with digital elevation models (DEMs) to assess marsh surface elevation change during this time. Overall, 37 % (113 ha) of the marsh increased in elevation at a rate that exceeded SLR, whereas 63 % (196 ha) of the area did not keep pace with SLR. Of the total area, 55 % (169 ha) subsided during the study period, but subsidence varied spatially across the marsh surface. To determine which biogeomorphic and spatial factors contributed to measured elevation change, we collected soil cores and determined percent and origin of organic matter (OM), particle size, bulk density (BD), and distance to nearest bay edge, levee, and channel. We then used Akaike Information Criterion (AICc) model selection to assess those variables most important to determine measured elevation change. Soil stable isotope compositions were evaluated to assess the source of the OM. The samples had limited percent OM by weight (<5.5 %), with mean bulk densities of 0.58 g cm -3 , indicating that the soils had high mineral content with a relatively low proportion of pore space. The most parsimonious model with the highest AICc weight (0.53) included distance from bay's edge (i.e., lower intertidal) and distance from levee (i.e., upper intertidal). Close proximity to sediment source was the greatest factor in determining whether an area increased in elevation, whereas areas near landward levees experienced subsidence. Our study indicated that the ability of a marsh to keep pace with SLR varied across the surface, and assessing changes in elevation over time provides an alternative method to long-term accretion monitoring. SLR models that do not consider spatial variability of biogeomorphic and accretion processes may not correctly forecast marsh drowning rates, which may be especially true in modified and urbanized estuaries. In light of SLR, improving our understanding of elevation change in these dynamic marsh systems will play a crucial role in forecasting potential impacts to their sustainability and the survival of these ecosystems.

Controls on Sediment Accretion and Blue Carbon Burial in Tidal Saline Wetlands: Insights From the Oregon Coast, USA

“…When plotted along a standardized elevation gradient, z *, ρ DB decreases with increased elevation (Figure a). Others have observed similar trends (e.g., Callaway et al, ; Roner et al, ; Thorne et al, ), whereby low tidal elevations are inundated longer resulting in greater deposition of lithogenic material (Mudd et al, ). When compared between high marshes, mean ρ DB were significantly different between estuaries as well, the highest and lowest of which were in Coquille and Youngs, respectively (Kruskal‐Wallis test, p = 0.007, n = 7).…”

Vegetation Community Response to Tidal Marsh Restoration of a Large River Estuary

“…Many plant species are found across a narrow elevation range on the Pacific coast, such as the 53-cm elevation range (this study) and at San Quintin Bay, Mexico (Zedler et al 1999), and that small differences in elevation (~4 cm) may shift the competitive dominance of species distributions (Scholten and Rozema 1990). Furthermore, vegetation within such narrow elevation zones suggests high vulnerability to projected sea level rise for the Washington coast by 2100 (63 cm mid scenario, 142 cm high scenario; NRC 2012) and supports the need for high resolution elevation studies on local management parcels for climate change models and adaptation scenarios (Takekawa et al 2013, Thorne et al 2014. Climate change effects including sea-level rise (SLR), extreme storm events, and increased salinity are projected to increase and greatly reduce the extent of existing salt marsh areas.…”

Sediment transport‐based metrics of wetland stability

“…Coastal wetlands are vulnerable to sea level rise and anthropogenic disturbance [ Gedan et al ., ; Kirwan and Megonigal , ; Deegan et al ., ; Weston , ]. Their vulnerability is related to mineral sediment availability because marshes build vertically in part due to deposition on the marsh surface [e.g., Allen , ; Friedrichs and Perry , ; Day et al ., ; Thorne et al ., ], but net sediment input is also critical to maintain the geomorphic planform of tidal channels, intertidal flats, and the marsh plain under conditions of sea level rise and lateral erosion [ Fagherazzi et al ., , ; Mariotti and Fagherazzi , ]. However, attempts to quantify these dependencies are challenging, and relationships between suspended sediment concentration (SSC) and wetland vulnerability are generally weak [ French , ; Ensign et al ., ].…”