Jillian Bertram

"Effects of tidal restriction on sediment and nutrient accumulation in a tidal salt marsh."
BSES Senior Research 2006

I measured concentrations, accretion, and accumulation of soil organic carbon (C), nitrogen (N), and phosphorous (P) at locations upstream and downstream of a levee and bridge across the Dean Creek (GA) salt marsh to investigate the effects of historical tidal restriction on marsh accretion, sediment deposition, and nutrient retention. Four 30 cm soil cores were taken from either side of the bridge and then divided into 2 cm depth increments which were analyzed for bulk density, organic C, total N, and total P. The depth increments were also analyzed for Cs-137 and Pb-210 to obtain short and long-term accretion rates. Soils upstream of the restriction had greater bulk densities (0.55g/cm³), more P (680 ug/g), less organic C (3.0%), and less N (0.19%) than the downstream soils (0.42 g/cm³, 480 ug/g P, 5.5% organic C, and 0.34% N). Upstream soils also contained more sand (70%), less clay (6%), and less organic matter (8%) than the soils downstream of the restriction (64% sand, 11% clay, and 11% organic matter). Accumulation of organic carbon and nitrogen in the upstream soils (26 g/m²/yr C and 1.7 g/m²/yr N) were half that of the downstream soils (59 g/m²/yr C and 3.4 g/m²/yr). My results suggest that historical tidal restriction has reduced the accretion rate and nutrient accumulation in the upstream marsh over the past 40 years.

Results from this study will be used to gauge the benefits of restoring tidal flow on marsh accretion and soil processes after the old culvert bridge is replaced with a spanning bridge.