EUTROPHICATION IN THE NORTHERN GULF OF MEXICO: AN EVALUATION OF LONG-TERM ENVIRONMENTAL CHANGE AND HUMAN IMPACTS ON COASTAL ESTUARIES

Eutrophication and harmful algal blooms pose substantial water quality issues in coastal estuaries, often creating hypoxic conditions, ecosystem shifts, and diminished fishery production. Modern eutrophication events have been attributed to agricultural land use change and pollution. The extent of human influence in estuaries has not yet been considered in a long-term, pre-settlement context, however. Our study analyzes multiple environmental proxies within sediment cores from Mobile Bay and Weeks Bay, AL, spanning the last ~7 kyrs to evaluate the natural, pre-settlement record of flood and storm events and compare with the human influenced period.

Piston core WB-18-02 was collected in the central basin of Weeks Bay and recovered 6 m of undisturbed Holocene bay mud. Elemental composition, measured continuously throughout the core with a handheld SciAps XRF device, shows trends in Ca, Sr, Ti, and Zr that are interpreted as an increasing marine influence at the base of the core (which captures formation of the bay ~7 ka), and as gradual transition to greater terrestrial influence in the Late Holocene. Carbon and nitrogen isotope analysis reveals trends of increasing TOC and decreasing δ¹³C, interpreted as increased terrestrial-sourced productivity in the Late Holocene. Negative peaks in gamma, measured from a RS-125 scintillometer, are interpreted as flood events with terrestrial sand influx and/or increased oxidation, and are associated with negative trends in K, Fe, and V. Hg concentration increases in the core top, potentially related to anthropogenic activity.

Ongoing proxy analysis includes grain size distribution to identify floods and diatom assemblages to identify blooms triggered by floods and eutrophication. Age models are constructed using Pb-210/Cs-137 gamma spectroscopy and radiocarbon AMS dating, which are established by published studies of nearby cores. Results will help determine the natural environmental baseline before European settlement and can be used to assess the direct (eg. land use change, industrialization) and indirect (eg. climate) anthropogenic influences on these environments. Ultimately this comparison may be used to influence water policy and bay restoration efforts to improve coastal resiliency to climate change, sea level rise, and extreme weather events.