Organic and Inorganic Contributions to Vertical Accretion in Salt Marsh Sediments

Abstract

The contribution of organic and inorganic constituents to recent vertical accretion rates (since 1963/4) was estimated for 141 salt marshes ranging from New England to the Gulf of Mexico. The range of vertical accretion and inorganic accumulation rates were 0.09 to 1.78 cm y⁻¹, and 0.01 to 0.53 g cm y⁻¹, respectively. The volume of the accumulated organic and inorganic in all salt marshes averaged 3.8 and 4.9%, respectively, of the total, which is relatively low among soil types. The remaining soil volume is water and air. There was a direct relationship between vertical accretion and organic accumulation that explained 59% of the variation for all samples combined. In contrast, the bulk density is strongly and directly related to inorganic content, but not the vertical accretion rate. A multiple regression equation describing the vertical accumulation as a function of mineral and organic accumulation suggests that organic accumulation is five times more important than inorganic accumulation for East coast salt marshes (n=19; weight basis), but that inorganic content is a statistically-insignificant factor for Gulf of Mexico salt marshes (n=122), or for all salt marshes examined (n=141). A simple linear regression showed that a 1 cm rise in salt marsh elevation was composed of 10.9 g of organic matter. A threshold level of 0.02 g organic matter y⁻¹ accumulation can continue without inorganic accumulation. It appears that it is the accumulation of organic matter that controls inorganic accumulation in established marshes, not the reverse. These results document the dominant role of below ground plant material in maintaining salt marshes once they are established. When wetland hydrology is altered, it is the organic soil constituents that are affected (through oxidation or plant growth below ground), thus explaining salt marsh conversion to open water through indirect changes in hydrology. Salt marsh management and restoration efforts would do well to keep in mind the plant’s health, especially belowground, if the long-term and effective strategies are to be successfully implemented. The biological components, not the geological components, appear to control the fate of established salt marshes.

“There is no other case in nature, save in the coral reefs, where the adjustment of organic relations to physical conditions is seen in such a beautiful way as the balance between the growing marshes and the tidal streams by which they are at once nourished and worn away.” (Shaler, 1886). “My belief is that the present low sea marsh does not owe its origin to any silt depositing river, but is simply an accumulation of vegetable matter.” (C. W. Thomas, of the Louisiana Land and Exploration Company, in a letter about the Louisiana coast; cited in Nesbitt, 1885).