Tidal Marsh Restoration at Poplar Island: II. Elevation Trends, Vegetation Development, and Carbon Dynamics

Abstract

In natural tidal marshes, nutrient fertilization stimulates complex responses, bringing into question the suitability of nutrient-rich dredged material as a substrate for tidal marsh restoration. We examined vegetation and elevation trends in the constructed tidal marshes at Poplar Island, Maryland where fine-grained dredged material (high nutrient) and locally dredged sand (low nutrient) were used as substrates. Macrophyte production was higher and root:shoot ratios lower in dredged material marshes. Measured vertical accretion was similar in dredged material (7.4 ± 0.6 mm y−1) and sand (6.4 ± 0.9 mm y−1) marshes, and is keeping pace with local sea-level rise. A detailed carbon budget was developed for one dredged material marsh to examine carbon dynamics under high nutrient availability. High annual macrophyte production combined with small exports via tidal exchange and methane emissions resulted in an annual carbon burial of 206 g C m−2 y−1, and calculated vertical accretion of 6.5 mm y−1 from organic matter. This is attributed to the contribution of organic matter from aboveground biomass due to protection from perimeter dikes constructed to contain dredged material during placement, reducing marsh exposure and organic matter export. With appropriate design and management, fine-grained, high-nutrient dredged material makes a suitable substrate for tidal marsh restoration.