Comparison of sediment accumulation and accretion in impounded and unimpounded marshes of the Delaware Estuary

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University of Delaware
Coastal marshes represent a small land area relative to the ecosystem services they provide. Humans have altered the coastal environment in a number of ways, one being the tidal restriction of marshes. One type of tidal restriction involves marsh impoundments, which are managed by humans to provide ecological benefits including mosquito control and waterfowl habitat. In the state of Delaware, impounded marshes are faced with rising sea level just like their unrestricted counterpart. Currently, coastal managers are concerned about the fate of these systems and are considering what actions (if any) can be taken to preserve them. Accretion, or vertical growth of the marsh platform by accumulation of mineral and organic material, is an important part of understanding marsh elevation change through time. This study compares impounded and unimpounded tidal marshes of coastal Delaware to examine 1) the relative accretionary status of selected managed and natural marshlands, 2) the relative influences of mineral and organic solids accumulation on rates of accretion. Accretion rates were measured using 137Cs and 210Pb radiometric methods, which are commonly used in tidal marsh studies but not always together. This study explores the value of using both radiometric methods and compares the results. Gravimetric, loss-on-ignition, and radionuclide analyses were conducted on over 500 subsamples of 44 marsh cores collected at eleven sites along the western coast of the Delaware Estuary. Radionuclide analysis was performed via gamma spectroscopy, which allows simultaneous measurement of 137Cs and 210Pb activity. Accretion and accumulation rates were calculated using activity-depth profiles of 137 Cs and excess 210Pb, using the 1964 reference horizon and the Constant Initial Concentration model, respectively. The two radionuclide methods were found to provide similar results, suggesting that 137Cs and 210Pb are effective sediment chronometers in the study area. For unimpounded marshes, mineral and organic mass accumulation averaged 0.22 g cm-2 y-1 and 0.06 g cm-2 y-1 , respectively. For impounded marshes, mineral and organic mass accumulation averaged 0.08 g cm-2 y-1 and 0.03 g cm-2 y-1 , respectively. Accretion rates for unimpounded marshes averaged 0.57 cm y-1 and, for unimpounded marshes, 0.28 cm y-1. Overall, rates of accumulation and accretion determined for this study were comparable to rates reported in the literature for U.S. East Coast marshes. Impounded marshes investigated exhibited lower accretion rates and lower mineral sediment inventories than the unimpounded marshes. Impounded and unimpounded marshes were found to show a similar direct relationship between accretion rates and accumulated mass; accretion increased with increasing organic and mineral mass accumulation. However, accretion was more sensitive to organic accumulation than mineral sediment accumulation. In the case of impounded marshes, accretion rates appear to be limited by mineral sediment accumulation. The implication is that these marshes are deficient in suspended mineral sediment supplied by tidal flooding and deposition, perhaps due to the impoundment works. Accretion rates determined for the impounded marshes (0.11--0.72 cm y-1 range, 0.25+-0.16 cm y-1 mean) fell at or below the rate of relative sea-level rise for middle Delaware Estuary (0.36+-0.06 cm y-1 at Reedy Point). Where the rate of marsh accretion is deficient, coastal flooding and inundation related to future sea-level rise will be most pronounced. Coastal managers should consider sediment management among the various adaptation strategies implemented by the state of Delaware to mitigate effects of rising sea level on the impounded marshlands.