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The world's most extensive and active deltas, Louisiana's wetlands, are deteriorating rapidly due to multiple stressors. Their ecological and anthropogenic histories on a multimillennial timescale have not been thoroughly documented. This study investigates hydrological and anthropogenic impacts on southern Louisiana wetlands to guide future efforts toward environmental restoration. A 3.6 m sediment core (JOYWMA) extracted from a freshwater wetland adjacent to Lake Pontchartrain yielded a 5000‐year record. Multiproxy data indicate four distinct stages, with the earliest period representing an oyster reef (~4.5 cal yr bp) marked by elevated Ca and Sr concentrations. Evolution to a freshwater marsh occurred 4.5–4.0 cal yr bp during the St. Bernard delta progradation, marked by increased Zr and Br concentrations. The site transitioned to a lake due to the isolation from the St. Bernard delta and local subsidence (4.0–2.0 cal yr bp), marked by increased Ti, Fe, Mn and K concentrations. The site altered to a cypress swamp after 2.0 cal yr bp due to increased sediment supply during the St. Bernard subdelta progradation. Both natural (delta progradation and subsidence) and anthropogenic (fire, deforestation) stressors have impacted the site over the last 4500 years. Delta‐switching of the Mississippi River caused the significant geomorphological and ecological changes. [ABSTRACT FROM AUTHOR]

This study evaluates plant diversity and species richness levels and reconstructs the environmental history of a site near Little Lake in southeast Louisiana, USA over the past 3200 years. Six successive ecosystems occurred as a result of changing environmental conditions, ranging from the local (anthropogenic activities), regional (delta lobe switching), and to global (sea level rise): interdistributary bay (3.2–2.3 cal kyr BP), hydrophytic freshwater marsh (2.3–1.42 cal kyr BP), mesophyte coastal prairie (1.42–0.92 cal kyr BP), Typha marsh (0.92–0.33 cal kyr BP), intermediate marsh (0.33 cal kyr Bp – ad, 1972 CE), brackish marsh (197 CE2-present). Biodiversity was evaluated for each ecosystem, with maximum diversity occurring during the period of maximum delta-growth as the river dynamics increased landscape heterogeneity by creating diverse habitats, including marsh, swamps, oxbow lakes, and natural levees, while the transgressive phase presents the lowest biodiversity due to increasing salinity. Multivariate analysis is used to identify the primary depositional processes for each stage and infer the proximate forcing agents(s). Early ecosystem shifts were controlled by the stages of the delta cycle, while more recent shifts are associated with anthropogenic activities and the effects of global climate change such as sea-level rise, and more intense tropical cyclones. • Identifying the external forcing agents driving each deltaic environmental transition. • Multi-proxy data identify six ecological shifts since 3200 BP • Ecosystem diversity is strongly correlated with the Mississippi deltaic cycles. • Maximum diversity occurring during the period of maximum delta-growth. • Various forcing agents and the relationships between them change over time. [ABSTRACT FROM AUTHOR]

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. • A multi-proxy record details the environmental history of the Mississippi River delta. • The present black mangrove (Avicennia germinans) forest is a recent feature. • Five distinct ecosystems have covered the site since 4000 BP • Environmental agents include the delta cycle, climate warming, and sea level rise. • Palynological analyses captures the first occurrence of Avicennia germinans. [ABSTRACT FROM AUTHOR]