Sediment input into a floating freshwater marsh: Effects on soil properties, buoyancy, and plant biomass

Coastal restoration strategies in the Mississippi River Delta Plain include use of direct sediment additions as well as river diversions to reintroduce fresh water and sediments into coastal marshes. Often, these diversions are located in the upper parts of the coastal basins, where fresh water floating marshes predominate. Floating marshes are wetlands of emergent vascular vegetation that have a significant mat of live and dead roots, dead organic material, and mineral sediments. Mats move vertically as ambient water levels rise and fall. As such, extensive water exchange is maintained below the mat, while overland sheet flow is reduced or eliminated, limiting inorganic sediment input. Thus, the effect of sediment introduction into floating marsh wetlands is unknown. In this study, we documented marsh mat response to Mississippi River sediment addition and measured change in soil properties, species composition change, and growth of vegetation. The study included two sites located in seasonally floating thin-mat marshes dominated by spikerush (Eleocharis baldwinii). At each site, 16 plots were established. Each 1 m2 plot was randomly assigned one of four treatments: none, low, medium, and high sediment additions, with the highest sediment addition representing approximately 15 years of deposition from an existing river diversion at Caernarvon, Louisiana. With increasing sediment addition, significant increases in bulk density, and significant decreases in percent organic matter in the top 25 cm of the marsh mat were observed. Most of the sediment added remained in the top 10 cm of the marsh mat. No significant differences in water level over the mat were found among treatments, indicating that buoyancy was not affected by sediment addition. Aboveground biomass tended to increase with sediment addition, although the differences were not statistically significant. Belowground biomass was not significantly affected by the addition of sediment. These results indicate that sediment additions at the level supplied by existing diversion structures should not sink floating marsh mats and may have slight positive effects on vegetation biomass.