Long-term sediment accretion and nutrient deposition in a tidal marsh of the Delaware Bay

Abstract

Long-term sediment accretion and nutrient concentrations were investigated in tidal marsh sediments of the Murderkill River, Delaware. Radionuclides (210Pb and 137Cs), stable isotopes (13C and 15N), organic matter concentrations and nutrient concentrations were measured. Tidal marsh sediment accumulation rates ranged from 0.10 to 0.20 g cm-2 yr-1, and accretion rates ranged from 0.31 to 0.74 cm yr-1. The Carbon (C) to nitrogen (N) ratio (∼24) was higher in the downcore of upstream sites, and a similar C/N ratio was identified in surface sediment cores at the downstream sites. The sediment N concentration in the surface sediments was highest at the upstream sites (mostly ≥1% of N) and decreased downstream. Total nitrogen concentrations were highest from 1910 to 1920 (∼1.5% N), decreased to approximately 0.6% N by 1940, and then remained constant to present. The dominance of the C4 plant Spartina (cordgrass) over C3 plants after the 1940s may be due to an upward movement of saline water associated with hydrologic changes and a rise in relative sea level. Surface sediments in all of the cores have more total phosphorus (TP) than below ground. The downstream site closest to the estuary had an abrupt increase in surface TP (i.e., 0–10 cm). Sediment TP increased slightly from 2003 to present time (i.e., from 0.07 to 0.11% P). Sediment accumulation and accretion rates in the Murderkill River marshes are similar to the accretion rates determined by previous studies in the region.