Vertical Accretion in Microtidal Regularly and Irregularly Flooded Estuarine Marshes

Abstract

Vertical accretion rates were measured in microtidal (tidal amplitude less than 0·3 m) regularly (flooded twice daily by the astronomical tides), and irregularly flooded (inundated only during spring and storm tides) estuarine marshes in North Carolina to determine whether these marshes are keeping pace with rising sea-level and to quantify the relative contribution of organic matter and mineral sediment to vertical growth. Accretion rates in streamside and backmarsh locations of each marsh were determined by measuring the Cesium-137 (137Cs) activity in 2 cm soil depth increments. Soil bulk density, organic carbon (C), total nitrogen (N) and particle density also were measured to estimate rates of accumulation of organic matter (OM), mineral sediment and nutrients.With the exception of the backmarsh location of the regularly flooded marsh, vertical accretion rates in the marshes studied matched or exceeded the recent (1940-80) rate of sea-level rise (1·9 mm year-1) along the North Carolina coast. Accretion rates in the irregularly flooded marsh averaged 3·6 ± 0·5 mm year-1 along the streamside and 2·4 ± 0·2 mm year-1 in the backmarsh. The regularly flooded marsh had lower accretion rates, averaging 2·7 ± 0·3 mm year-1 along the streamside and 0·9 ± 0·2 mm year-1 in the backmarsh.Vertical accretion in the irregularly flooded marsh occurred via in situ production and accumulation of organic matter. Rates of soil OM (196-280 g m-2 year-1), organic C (106-146 g m-2 year-1) and total N (6·9-10·3 g m-2 year-1) accumulation were much higher in the irregularly flooded marsh as compared to the regularly flooded marsh (OM = 51-137 g m-2 year-1, C = 21-59 g m-2 year-1, N = 1·3-4·1 g m-2 year-1). In contrast, vertical accretion in the regularly flooded marsh was sustained by allochthonous inputs of mineral sediment. Inorganic sediment deposition contributed 677-1139 g m-2 year-1 mineral matter to the regularly flooded marsh as compared to only 147-311 g m-2year-1 to the irregularly flooded marsh.The irregularly flooded marsh exhibited an accretionary balance (vertical accretion rate minus local apparent sea-level rise) of 0·5-1·7 mm year-1 suggesting that these microtidal marshes would be capable of maintaining their elevation if the rate of sea-level rise increased. In the regularly flooded marsh, the streamside zone had an accretionary balance of 0·8 mm year-1 while the backmarsh had a deficit of 1·0 mm year-1. Our results suggest that microtidal regularly flooded marshes would be susceptible to submergence by an increase in the rate of sea-level rise unless accompanied by an increase in mineral sediment deposition.