The Limits of Vertical Accretion in Sediment-Poor Microtidal Estuaries

A meta-analysis of sediment bulk density and loss-on-ignition (LOI) data from marshes in thirteen estuaries on the U.S. Pacific, Gulf, and East Coasts consisting of over 2400 samples shows that dry bulk density can be predicted very well as a function of LOI (r2=0.8) using a conservative mixing model. The estimated bulk densities of pure organic and mineral matter were 0.08 and 1.9 g/cm3, respectively, as determined by a best fit of the mixing model. The composite bulk density and LOI of a mixture are best explained as the result of a conservative mix of these two end members. These bulk densities are useful in establishing limits on the vertical accretion in tidal wetlands. For example, if we assume a maximum belowground production of 2.5 kg/m2 and that 10% of this is preserved, then the maximum vertical accretion from this amount of belowground production is 3 mm/yr (0.1 x 2500 g m-2 y-1 × m2/1000 cm2 ÷ 0.08 g/cm3 = 0.3 cm/y). For a sediment-poor estuary with a suspended inorganic sediment concentration in flood water of 20 mg/L, flooding a marsh surface twice daily to an average depth of 12.5 cm, the vertical accretion rate would be no greater than about 1 mm/yr. This assumes that the surface captures 100% of the sediment load. The maximum vertical accretion is the sum of the two components, and in a sediment-poor, microtidal estuary (typical of many on the Gulf Coast) this should be no more than about 4 mm/y.