Elevation-dependent surface elevation gain in a tidal freshwater marsh and implications for marsh persistence

Daniel Cadol, Katharina Engelhardt, Andrew Elmore and Geoffrey Sanders

Limnol. Oceanogr., 59(3), 2014, 1065-1080 | DOI: 10.4319/lo.2014.59.3.1065

ABSTRACT: Tidal marshes in equilibrium with rising sea level accrete material and gain elevation at a rate similar to sea level rise (SLR). Increased inundation, erosion, and marsh area loss occur where alterations to ecogeomorphic processes and feedbacks cause surface elevation gain to be less than SLR. Relationships between marsh platform elevation and surface elevation change have been observed in tidal salt marshes. In this study we expand this concept to tidal freshwater marshes by monitoring elevation change using surface elevation tables (SETs) and repeated surface elevation surveys. A novel method of survey differencing revealed elevation-dependent rates of elevation change in our study site. Greater elevations were generally associated with decreased elevation gains. This relationship, however, was not monotonic but included a reversal to increasing gains from ∼ 0.2 m to 0.3 m elevation, leading to a local maximum elevation gain rate at ∼ 0.3 m elevation, about 0.3 half tide units above mean tide level. This localized accretion maximum could be caused by the elevation-controlled distribution of marsh plant species, among other possibilities. Model projections of modern elevation change rates into the future suggested that high- and low-marsh platforms would be maintained and even expanded for SLR rates between 2 and 4 mm yr−1. In our projections, rates of SLR < 2 mm yr−1 led to conversion of low marsh to high marsh, whereas rates from 4 to 6 mm yr−1 led to the reverse. SLR > 6 mm yr−1 led to conversion of marsh to open water. On average, surface elevations appear to be increasing at a rate similar to recent local SLR, although elevation change at some specific locations and within some elevation ranges may be less than SLR.

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