Reconstructing the development of eutrophication in Louisiana salt marshes

We collected sediment cores from three salt marsh ponds in coastal Louisiana to test the usefulness of proxies of eutrophication. Onecentimeter increments of 210Pb- and 137Cs-dated sediment were analyzed for diatoms, pigments (phaeophytin and chlorophyll a), biogenic silica, percentage organic matter, percentage carbon, and percentage nitrogen. Both sediment chlorophyll a and a diatombased trophic index (TI) were significantly and positively correlated with riverine or local nutrient indices. Two diatom species, Amphora copulata Giffen and Navicula yarrensis Grunow, were significantly and negatively correlated with riverine and local nutrient indices. These results suggest that these variables can be used as potential indicators of trophic status. Results from a complete-linkage cluster analysis on the diatom assemblage data demonstrated that the sediment cores could be split into three time periods: early 1900s (pre-1930s/1940s), mid-1900s (1930s/40s to 1960s/1970s), and late 1900s (1960s/70s to 1990s). Examination of the sediment chlorophyll a and TI data over these time periods, coupled with an analysis of variance of nutrient inputs between the time periods, suggests that nutrient loading increased dramatically from the mid- 1960s to the mid-1970s. This study demonstrates that (1) a retrospective analysis of sediment cores can be conducted in highly variable salt marsh ponds and (2) these salt marsh environments are already affected by the higher nutrient loads from both riverine and local processes occurring over the last 50 years. Additional nutrient loading, e.g., from river diversion projects for the lower Mississippi River, may exacerbate eutrophication already evident in the marsh environment.