Potential Effects of Sea-Level Rise on Salt Marsh Elevation Dynamics in a New Hampshire Estuary

Salt marsh survival in the face of sea-level rise (SLR) depends largely on a marsh’s ability to compensate for increased flooding by building in elevation, but the rate of elevation gain depends on processes that are not well-understood (i.e., belowground productivity, sediment accretion, and subsidence). An array of planted and unplanted pots was installed in the field to examine the effects of tidal flooding on productivity and elevation change. We found that belowground plant volume increased linearly as elevation increased for both Spartina patens and Spartina alterniflora. Even though the volume of roots increased by 400% with elevation for S. alterniflora and \u3e 200% for S. patens, there was no relationship between belowground volume and elevation change of the original soil surface, perhaps due to the infilling of porosity by roots. However, the soil in planted treatments subsided significantly less than in unplanted controls. Measurements from Surface Elevation Tables (SETs) indicated that local high marshes have been losing elevation relative to sea level at an average rate of 2.1 mm/year. The rate of vertical gain decreased at SET locations of greater marsh elevation, and high marshes did not keep pace with SLR even when sediment supply appeared to be high (TSS = 57 ± 7 mg/L). The high marsh accretion deficit, combined with a continuing trend of reduced belowground growth with increased flooding due to SLR, suggests that S. alterniflora will replace less flood-tolerant species over time, potentially causing loss of high marsh habitat