An integrated methodology for assessing ecosystem response to environmental stressors under changing climatic conditions in coastal wetlands.

Three cores were taken along the salinity gradient (n-s) in the coastal wetlands of Louisiana; an intermediate marsh, a brackish marsh, and a mangrove swamp. The cores display remarkable stratigraphic and chronologic correlations, representing six successive ecosystems and environments, namely: interdistributary bay, freshwater marsh/swamp, deltaic lake, freshwater marsh/swamp, intermediate marsh, and brackish/saline. Sedimentary, geochemical, and palynological data were used to reconstruct the paleoenvironments, including ambient environment and ecosystem types. Concentrations of Ba and Br, along with six elemental ratios (Ca/Rb, Zr/Rb, Ti/Rb, K/Ti, Mn/Rb, S/Rb), were employed to infer proxies for a range of environmental conditions (waterlogging, redox levels), depositional processes (fluvial vs marine or in situ), and sediment characteristics (grain size). Correlating the identification of environment types, inferred depositional processes, and the known history of the Mississippi delta cycle with the ecosystem reconstruction provides insight into ecosystem response to a variety of stresses, which information can be used to better understand and predict present and future responses to the ongoing stresses. Additionally, a simple elemental ratio (Zr/Rb) was used to produce a continuous (2 cm resolution) estimate of grain size along the length of the cores. The close correlation between the estimated grain size and measured samples shows that this ratio is a valid method for quickly assessing rough grain size, and is especially useful for identifying sedimentary inflection points.

The use of multivariate PCA dataset in identifying the underlying drivers of critical stressors, looking at global problems through a local lens.

Palynology-based multivariate datasets including geological, ecological, and geochemical data identified the relative importance of the underlying drivers of critical stressors to coastal wetlands by identifying and distinguishing between fluvial flooding, saline water intrusion, delta switching, and the landward migration of coastal plants. A sediment core was retrieved using a vibracorer from an intermediate marsh in Lake Salvador, Louisiana, USA. X-ray Fluorescence (XRF) quantified fluvial and marine elemental concentrations (Cl, Sr, Ca, Mn, K, Ti, Fe, Zn, Zr, Br). Palynology-based agglomerate hierarchical analysis of thirty-two pollen taxa was employed to define ecological clusters. The implementation of multivariate principal component analysis (PCA) to geochemical and ecological variables inferred the source of sedimentary material by correlating four taxonomic groups (floodplain trees, upland trees, tidal freshwater herbs, and inland herbs) to specific geochemical signatures and facilitated the testing of potential correlations between geo- and hydrological-conditions and the six ecosystems (interdistributary, delta-plain, deltaic lake, bottomland and swamp forests, freshwater marsh, and intermediate marsh) depicted in each PCA biplot. The PCA scores quantified the relative importance of multiple variables. The squared cosine function, which demonstrates the relative importance of a variable for a given observation, was used to estimate the representation of each variable on the principal component biplots. Multivariate statistical datasets can be valuable to any scientist working across the spectrum of environmental and planetary science fields as a means of identifying the relative importance of diverse background parameters in controlling ecological and environmental conditions. This methodology is applicable across both natural and social sciences as a means of distinguishing natural and anthropogenic impacts.

Identifying forcing agents of environmental change and ecological response on the Mississippi River Delta, Southeastern Louisiana.

Freshwater wetlands on the Mississippi River delta plain, containing extensive swamps and marshes, have experienced land loss of 5197 km2 since the 1930s as the ocean has transgressed landward, resulting in landward retreat of bottomland forest, and transition of fresh to intermediate marsh. The timing and rapidity of these ecotonal transitions, and the impacts of natural and anthropogenic forces on this deltaic environment are uncertain. This study details a 448 cm sediment core retrieved from the intermediate marsh on the northern edge of Lake Salvador in southeastern Louisiana. Multi-proxy data identify six ecological shifts since 3500 BP. The ecosystem has shifted from interdistributary environment with high concentrations of such terrestrial and marine elements as Ca, Zr (3.5-3.0 cal kyr BP), to a freshwater deltaic-plain with an increase in freshwater herbs and trees (3.0-2.6 cal kyr BP), to a lacustrine environment marked by high Mn, Fe concentrations (2.6-2.2 cal kyr BP), to a swamp ecosystem with high concentrations of Zn, Br (2.2-1.4 cal kyr BP), to freshwater marsh with an increase in marsh plants (1.4-0.3 cal kyr BP), and to an intermediate marsh marked by Typha and Baccharis with elevated marine elements (since 0.3 cal kyr BP). The study identified the external forcing agents driving each deltaic environmental transition using multivariable analyses. Ecosystem dynamics are highly associated with the St. Bernard deltaic cycles, with dominant fluvial processes introducing freshwater ecosystems while forming geomorphological features such as levees, oxbow lakes, and back swamp and marsh during delta progradation. Thereafter, reduced sediment supply and decreased freshwater flow during delta transgression caused land subsidence and uneven topography. As a result, the swamp converted to marsh. Eighteenth century logging and canal development by Activities from French and Spanish settlements reduced the cypress forests and enlarged the coastal lakes.