Temporal variability in the relative strength of external drivers controlling ecosystem succession in a coastal wetland near Bayou Lafourche, southeast Louisiana, USA

Abstract

The coastal wetlands of Louisiana are extremely dynamic due to their location on the edge of the continent, the heavy but spatially and temporally variable influence of the discharge of the Mississippi River, and the effects of both chronic (sea-level rise, coastal retreat) and abrupt (hurricanes, lobe switching) stressors. In these coastal wetlands the timing and mode of ecological succession can result from a variety of external factors and vary from sudden to gradual. This paper uses a palynology-based multi-proxy analysis of sediment cores retrieved from a small pond on a mangrove-dominated island in Bay Champagne near Port Fourchon, Louisiana to describe and date the successive ecosystems that have covered this site since 4000 BP and applies a PCA-based interpretation to infer the drivers responsible for each shift. During the late Holocene, the most important controlling agents have been the Mississippi River delta development, sea-level rise, climatic variability, shoreline retreat, tropical cyclones, and, more recently, anthropogenic effects. These processes have progressively transformed the site from a freshwater to a saline environment, consisting of four distinct ecosystems: a Taxodium-Nyssa freshwater forest (4.0–1.3 cal yr BP), a Typha-Bacopa freshwater deltaic-lake (1.3–0.7 cal yr BP), an intermediate and brackish marsh (0.7–0.2 cal yr BP), a Spartina-dominated saline marsh (since 0.2 cal yr BP), and a short-stature Avicennia forest since the early 1900s. The Mississippi River delta cycle (lobe switching) was the dominant driver of ecosystem changes early in the site's history, while relative sea-level rise, shoreline retreat, tropical cyclones, climate, and anthropogenic effects have been the main drivers since the 19th century.