TWO HUNDRED YEARS OF COASTAL CHANGE IN THE MOBILE BAY ESTUARY, NORTHERN GULF OF MEXICO

In an effort to document how the environmental and sedimentological regimes have evolved in the face of natural and anthropogenic pressures, foraminifers and sediment geochemistry of Mobile Bay, Alabama (USA) were analyzed. Twelve surface-sediment grab samples collected throughout the bay for three seasons show seasonal and spatial variability in the foraminifers; agglutinated species dominate the modern microfauna throughout most of the bay. Living foraminiferal densities are greatest in the southern bay and the more industrial western bay. Similarly, organic matter increases in a southerly or seaward direction, indicating a portion of riverine and/or deltaic organic matter bypasses the pro-delta region and deposits in the southern portion of the bay. Higher organic matter and salinities favor the greater living benthic foraminiferal densities observed in the southern portion of the bay. Living foraminiferal densities and associated assemblage composition indicate two biofacies (upper and lower bay), which are used to help reconstruct longer-term environmental change in Mobile Bay.

Study of seven box cores, with ²¹⁰Pb-derived chronologies, show that significant environmental changes occurred in the middle of the last century. Lower-density calcareous foraminiferal assemblages at depth are replaced in the near-surface sediment by higher-density agglutinated assemblages indicative of eutrophic conditions. This complete faunal turnover and introduction of more severe eutrophic conditions began impacting the microfauna between 1950 and 1960, around the same time that many of the oyster reefs in the northwestern portion of the bay became unharvestable. Increases in the sediment mass-accumulation rates and geochemical evidence from bulk stable carbon and nitrogen isotopes support increased terrestrial, freshwater, and sediment input over the same interval. These biological and physical changes all indicate significant anthropogenic forcing. Although the data set is not complete, patterns have been identified in the data that indicate a significant change in Mobile Bay salinity, structure, and environmental health over the last century.