Vertical Movement and Substrate Characteristics of Oligohaline Marshes Near a High-Sediment, Riverine System

Abstract

In coastal Louisiana, large sediment diversions from the Mississippi and Atchafalaya Rivers have been planned to enhance marsh accretion and productivity and reverse an overall deterioration of coastal marshes. Located between these rivers, there are extensive buoyant marshes in the freshwater and oligohaline zones (about 70% of the vegetated area), and little information exists on how buoyant substrates respond to mineral sediment introduction. The purpose of this study was to understand the vertical movement (buoyancy) and substrate properties of oligohaline marshes with increasing distance from the Atchafalaya River-Fourleague Bay system, a significant mineral sediment source. To do this thirty-seven stations were established (within a study area of 125 km²) where vertical marsh movement and marsh hydrology were monitored for a year, and soil cores were collected and analyzed for bulk properties. With increasing distance inland from the sediment source, there were significant increases in vertical movement of the marsh mat and decreasing mineral sediment content in the soil profile. In the interior marshes farthest from the sediment source, the average mat movement was greater than 15 cm, and some sites exceeded 25 cm. Non-buoyant marshes with high substrate bulk density were located closest to the sediment source. Mineral sediment deposition appeared to be a discontinuous process controlled by large events such as hurricanes and tropical storms, rather than average yearly flood events. A comparison of our data (from oligohaline marshes) with other studies (from freshwater marshes) suggests that increases of mineral matter within the upper peat matrix may decrease the vertical movement of marsh substrates.