A DIURNAL STUDY OF THE SEDIMENT BIOGEOCHEMISTRY OF WEEKS BAY DURING BOTTOM WATER HYPOXIC AND NORM-OXIC EVENTS

During the summers of 2008-2009 field research was conducted in Weeks Bay (WB), Alabama, an area where seasonal hypoxia is prevalent. Sediment cores and water column measurements were taken from ten locations. Microelectrode profiles (-5mm-45mm) of oxygen (O₂), sulfide (H₂S), and redox potential were conducted on sediment porewaters. After profiling, the cores were sub-divided into 2cm sections down to 10cm, the porewater was removed in an N₂ environment, and dissolved anions (NO₃^2-, NO₂^-, PO₄^3-, Cl^-, SO₄^2-) were measured using ion chromatography, along with Fe²⁺ using spectrophotmetry. Additionally, overlying water quality parameters were monitored (dissolved O₂, pH, temperature, salinity, and specific conductivity) and recorded over a vertical profile on a day/night cycle. A hypoxia likelihood index for each location was determined using this data along with historically collected buoy data (1999-2009). To supplement field data, sediment cores from three locations were incubated to mimic hypoxic conditions in order to observe the sediment geochemical response during a long-term hypoxia event. Microelectrode and anion analyses were also conducted on these incubated cores.

Preliminary results indicated O₂ levels were usually greater at the surface (~11mg/L) than near the bottom of the water column (~6mg/L) and were lower at night than during the day (~3mg/L). Laboratory analysis indicated that sulfide migrated upward during low O₂ events, was found at greater depths (~4.5mm vs. <1mm), and in much lower concentrations (~5uM vs. ~120uM) than during norm-oxic periods. Anion analysis from the first sampling season showed the following levels: NO₃^2- (<1 ppm), NO₂^- (BDL), PO₄^3- (BDL to 9.2 ppm), SO₄^2- (0.8 to 1327 ppm), and Cl^- (13 to 11797 ppm). From the ten sites sampled, the site near the Magnolia River mouth showed the strongest tendency toward hypoxia (depth of 1.5m) whereas, the shallow site (depth 0.5m) isolated from the major water currents in the bay, showed the least tendency toward hypoxia. Many factors affect hypoxia formation and its associated changes, and WB is no exception to this phenomenon. The findings from this research should assist decision makers and managers in their understanding of the hypoxic events, geochemical response, sediment-pelagic coupling, and the spatial extent of hypoxia in WB.