Hydrologic variability in a salt marsh: Assessing the links between drought and acute marsh dieback

Abstract

It has been hypothesized that acute marsh dieback (AMD) observed along the Gulf Coast and South Atlantic Bight in the early 2000s was the result of drought-induced changes to porewater and sediment chemistry through hypersalinity or through mobilization of metals and acidification associated with redox changes. The impact of drought on coastal wetlands remains unclear because the hydrology of these wetlands is strongly influenced by regular tidal inundation. In order to test the links between hydrologic variability and changes to marsh groundwater conditions that may be stressful to the salt marsh grass Spartina alterniflora, we installed piezometers and passive diffusion samplers in a salt marsh island at North Inlet, South Carolina, where AMD was observed in fall 2001. Significant variations in tidal inundation, rainfall, evapotranspiration, groundwater dynamics, and porewater chemistry were observed. The island was typically inundated twice daily, but there were occasional 19–21 h periods in winter and spring when the marsh was not inundated and a singular event when the marsh was not inundated for three days (March 2008). Enhanced exposure resulted in seasonal redox chemistry changes, as indicated by changes in the ratio of ferrous iron [Fe(II)] to total iron [Fe(II) + Fe(III)], but our observations do not support redox and pH changes as the cause of AMD at this site. Porewater salinity varied from 14 to 40 in the upper 1 m of the marsh. Salinity was most variable near the surface and increased with depth, reflecting root zone transpiration and downward movement of porewater through the marsh mud into the underlying confined sand aquifer. Pearson Correlation tests among porewater constituents and hydrologic parameters indicated significant associations between porewater salinity, tidal inundation, rainfall, and ET, and additional associations between porewater iron concentration, speciation, and tidal inundation. Linear regression model estimates of porewater salinity for 2001–2002 did not indicate the development of hypersalinity during that period. However, these estimates did predict a dramatic increase in salinity that coincided with the beginning of drought conditions just prior to the observation of AMD, suggesting this as a cause for AMD at this site. Drought is predicted to increase over the next century; damage caused by potential increases in the frequency of drought-related AMD may limit the ability of intertidal salt marshes to accommodate sea level rise.