The use of diatoms to infer environmental change of the lower White River, southeastern Arkansas.

摘要

This dissertation describes the limnology of the lower White River oxbow lakes in southeast Arkansas, inundated by the major 2008 spring flood. The flood water and sediment deposited in oxbow lakes were analyzed for water chemistry, sediment geochemistry and diatom assemblages and used as an analogue for past floods. The 2008 spring flood inundated and homogenized the entire floodplain; also represented by the predominantly planktonic diatom assemblage suggesting riverine input. Indicator species for eutrophication were also recorded consistent with the high nutrient content in the lakes. However, the variance in the surface diatom assemblage was best explained by the lake sill height and was used to develop a transfer function. The transfer function was applied to fossil diatom assemblage from Adams Bayou Lake sediment core (36 cm length) to test it as an archive for past White River floods. Cross validation of the developed model was done with river stage and the groundwater level data (proxy for increasing agricultural activity). The diatom inferred sill height was not successful in identifying the flood events in the lower section of the sediment core due to lack of modern analogue. The fossil assemblages show that the Adams Bayou was a stable, less disturbed system abundant in benthic diatoms prior to 1996 (estimated age) later converting into an open, nutrient rich system abundant in planktonic, eutrophic indicator diatoms. This shift in lake conditions towards the nutrient rich system also coincided with the lowering of groundwater due to increased agricultural practices. Finally, the history of flooding and land use for the past 750 years ( 137Cs and AMS used for chronology) was investigated from a 400 cm long sediment core from the Adams Bayou using sediment geochemistry. The lake was a steady system from 1300-1600 AD, but experienced major land use changes and flooding from 1800 AD till present. This study provides insights about current and past environmental conditions of the lower White River floodplain. It underlines the limitations of working in floodplain systems and also serves as a foundation for future paleolimnological studies in the study site as well as other floodplain river systems.