SPATIAL VARIABILITY OF ACCRETION RATES IN A DELAWARE BAY SALT MARSH

As part of a regional investigation of potential sea-level rise effects on tidal marshes, a study was conducted to examine spatial variations in marsh accretion rates at Dividing Creek, a small subestuary on the New Jersey coast of Delaware Bay. The marsh is dominated by S. alterniflora with patches of S. patens. Five different marsh platform locations around the creek, each uniformly covered in short-form S. alterniflora, were selected for core sampling. Gamma spectroscopy of ¹³⁷Cs was used to date the cores and determine an accretion rate (averaged since 1963) at each site, and loss-on-ignition and bulk-density measurements were used to characterize the volume composition of marsh soils. Accretion rates ranged from 0.39 to 0.80 cm/yr, with an overall average of 0.55 cm/yr. These rates are typical for river-estuarine marshes of Delaware and New Jersey but are higher than rates reported for back-barrier lagoon marshes of the U.S. Atlantic coast. Mineral sediment and organic matter inventories, along with the distance between coring sites and creek waters, were determined in an effort to explain variations in accretion among the sites. Regression analysis of accretion variables did not produce statistically significant results, though positive correlations between accretion rate and sediment-source proximity, and accretion rate and soil solids volume were observed, as described in the literature for salt marshes in general. Comparison between marsh accretion rates and relative sea level rise for Delaware Bay (0.35 ± 0.07 cm/yr) shows that the Dividing Creek marsh is keeping up with, and in several locations exceeds, local rates of relative sea level rise.