Marsh sediment accumulation and accretion on a rapidly retreating estuarine coast

Sommerfield, ChristopherBombay 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 has experienced a substantial decrease in salt marsh area along the Delaware Bay boundary by shoreface erosion and in the marsh interior by inland pool expansion. Although the origin of the pools is unknown, it has been suggested that the supply of allochthonous mineral sediment from tidal waterways to the marsh platform may be a contributing factor. To investigate whether vertical accretion of Refuge marshland is limited by sediment accumulation, a study was conducted to measure rates of mineral sediment and organic matter accumulation (mass/area/time) and accretion (length/time) using 137Cs and 210Pb chronologies developed for 19 marsh sites throughout the Refuge. To establish patterns and rates of recent historical marsh loss, an analysis of historical aerial photographs was undertaken. ☐ Results indicate that Bombay Hook NWR has lost a total of ~8.6 million m2 of marsh area since 1961. This loss was mostly caused by the formation of inland pools (~50% of area lost) and shoreface erosion along the Delaware Bay boundary (~35%), with a smaller contribution by waterway channel widening (~15%). Shoreface erosion was most prevalent in the southern half of the Refuge with some locations experiencing up to ~12 m/yr of retreat since 1961, while the northern section experienced far less retreat at 0.6 m/yr. The formation and expansion of inland pools were mostly concentrated in the northern half of the Refuge, adjacent to three freshwater impoundments constructed by the United States 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, and rates based on 137Cs and 210Pb methods were largely in agreement. Accretion rates (137Cs) for low marsh sites averaged 0.65 cm/yr and were significantly higher than rates at high marsh sites, which averaged 0.42 cm/yr. Combined mineral and organic 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. Mineral and organic mass accumulation rates correlated strongly with rates of accretion (R2= .85 and .79 respectively), revealing that both mineral sediment and organic matter drive marsh accretion at the Refuge, and that belowground biomass accumulation and aboveground mineral sediment deposition set the minimum and maximum rates of accretion, respectively. ☐ Marsh accretion rates measured in this study met or exceeded the rate of recent relative sea-level rise for the middle Delaware Estuary, based on the NOAA tide gauge record for Reedy Point (0.35 ± 0.05 mm/yr, 1956-2015). This result, in combination with the high rates of mineral accumulation measured throughout the Refuge, makes clear that there is not a marsh accretionary deficit related to insufficient mineral sediment. Moreover, the marsh soil record provides no evidence that the formation and expansion of marsh pools since the 1960s is related to low rates of marsh accretion or sediment supply. Additional research on historical changes in tidal inundation, marsh accretion, and elevation change is needed to better understand the nature of pool expansion and marsh loss at the Refuge.University of Delaware, School of Marine Science and PolicyM.S