MARSH SEDIMENT ACCUMULATION AND ACCRETION IN A RAPIDLY FRAGMENTING WETLAND COMPLEX

Bombay Hook National Wildlife Refuge in coastal Delaware protects one of the most expansive salt marsh systems on the U.S. Mid-Atlantic seaboard. In recent decades, the Refuge experienced a substantial decrease in salt marsh area along the Delaware Bay boundary, as well as within wetland complex by conversion of marsh to open water. To investigate relationships between marsh fragmentation and sedimentation, we developed ¹³⁷Cs and ²¹⁰Pb chronologies for 19 marsh sites throughout the Refuge, and identified patterns and rates of recent historical marsh loss (1961-2012) through aerial photograph analysis.

Results indicate that Bombay Hook NWR has lost ~20% of its marsh platform since 1961 (~8.6 million m²). Most of this loss was caused by the formation of inland pools (~50% of lost area) and shoreface erosion along the Delaware Bay boundary (~35%). Shoreface erosion was most prevalent in the southern half of the Refuge with locations experiencing up to ~12 m/yr of shoreline retreat since 1961. Formation and expansion of inland pools were concentrated in the northern half of the Refuge, adjacent to three freshwater impoundments constructed by the U.S. Fish and Wildlife Service in the late 1930s.

Salt marsh accretion and mass accumulation rates measured for this study fall within the middle to upper range of similarly determined rates for undisturbed marshes of the Delaware Estuary. Accretion rates (¹³⁷Cs) for low marsh sites averaged 0.65 cm/yr and were higher than rates at high marsh sites (0.42 cm/yr). Combined mineral sediment and organic matter mass accumulation rates (¹³⁷Cs) exhibited a similar difference between low and high marsh sites, averaging 0.31 g/cm²/yr and 0.13 g/cm²/yr, respectively. Comparison of mineral sediment lost and gained in the complex during the period of interest suggests that sediment produced internally by marsh edge erosion could sustain the measured rates of sediment accumulation if it were redeposited on the marsh platform. However, we observed no spatial relationship between rates of marsh sediment accumulation and erosion. This study demonstrates that in rapidly fragmenting wetland complexes, sediment accumulation and accretion rates may fall within the normally observed values for a geographic region.