TEMPORAL INFLUENCES OF SEASONAL HYPOXIA ON SEDIMENT BIOGEOCHEMISTRY IN COASTAL SEDIMENTS

This research addresses both spatial and temporal variability in sediment biogeochemistry in the southeastern region of Corpus Christi Bay, TX where seasonal (summer) hypoxia occurs.  Traditional techniques for determination of a variety of dissolved and solid components, benthic oxygen demand, and sulfate reduction rates were augmented by measurements using solid state microelectrodes to simultaneously determine concentrations of dissolved O₂, Mn²⁺, Fe²⁺, and ΣH₂S in multiple small - interval (1 mm) depth profiles.  Studies of changes in sediment biogeochemistry from norm - oxic to hypoxic conditions were complimented by sediment microcosm studies where oxygen concentrations in the overlying water were manipulated and electrode depth profile measurements were made over the ~ 500 hours of experimentation. 

Laboratory and field microelectrode results were in good agreement for both norm - oxic and anoxic periods.  Results indicated that iron (Fe²⁺) and sulfide (ΣH₂S) were the major dissolved redox - reactive ions in these sediments. During hypoxic conditions an upward migration of dissolved Fe²⁺and ΣH₂S through the sediment column and, at times, into the overlying water was observed as the oxygen levels decreased.  A corresponding decline in the vertical extent of these redox species occurred when the overlying water was re - oxidized.    During hypoxic regimes both constituents were more than double their concentrations in the sediment from norm - oxic periods.  When both dissolved iron and sulfide coexisted, FeS minerals were formed in the sediment, preventing sulfide diffusion into the overlying water. However, after extended periods of hypoxia (> 200 hours) this buffering capacity was exceeded and sulfide penetrated into the overlying waters.  Moreover, in the southeastern region of the Bay, where mixing was minimal and the water column was shallow, the sediments alone may have caused the onset of the hypoxic event in a relatively short time period (< 5.5 days).  These results demonstrated that in shallow marine environments where seasonal hypoxia occurs, such as Corpus Christi Bay, the associated major changes that take place in the sediment biogeochemistry must be included in benthic - pelagic models for overlying water hypoxia.