Comparison of Marsh Sediment Accumulation and Accretion Rates in Two Contrasting Estuaries

A comparative study of salt marshes in two different U.S. Mid-Atlantic estuaries was conducted for insight on hydrodynamic and geomorphic controls on sediment accumulation and accretion. Cores were collected in Barnegat Bay (back-barrier lagoon estuary) and Delaware Bay (coastal plain river-estuary) and analyzed for soil physical properties and radionuclide chronometry with ¹³⁷Cs and ²¹⁰Pb. Results indicate that the volume of soil organic solids was comparable between the two estuaries, yet the volume of mineral solids was significantly larger in Delaware Bay marshes. Rates of mineral and organic mass accumulation were significantly higher in Delaware Bay marshes (2.6 ± 2.0 kg m^-2 y^-1, 0.65 ± 0.25 kg m^-2 y^-1) than in Barnegat Bay (0.31 ± 0.24 kg m^-2 y^-1, 0.29 ± 0.08 kg m^-2 y^-1). The relatively higher rate of mineral sediment accumulation in Delaware Bay marshes is most likely due to the larger tidal range and higher suspended-sediment concentrations in estuarine source waters. Organic mass accumulation in both estuaries was positively related to rates of mineral sediment accumulation. The time frame of hydraulic modifications to Barnegat Inlet corresponds to changes in accumulated mineral sediment in Barnegat Bay marshes. Rates of marsh accretion were significantly higher at the Delaware Bay sites (0.79 ± 0.37 cm yr^-1) than in Barnegat Bay (0.28 ± 0.06 cm yr^-1) associated with higher mineral and organic mass accumulation. Based on these data, Delaware Bay marsh accretion over the last 60 -100 years was greater than local relative sea-level rise, whereas Barnegat Bay marsh accretion rates were at the low end, or fall below, sea-level rise over the same time period. These findings support prior research suggesting that the upper and lower limits of marsh accretion are set by rates of mineral sediment and organic matter accumulation, respectively.