The relative roles of salinity stratification and nutrient loading in seasonal hypoxia in Corpus Christi Bay, Texas

Hypoxia (low dissolved oxygen concentration) is known to occur in the southeast corner of Corpus Christi Bay, Texas, USA each summer since its discovery in 1988. In 2006, ongoing field research discovered that the hypoxia area has a greater extent spatially and temporally than previously thought. Although it was known that hypoxia was associated with salinity stratification, it was not until 2007 that it was discovered that salty water from both Oso Bay and Laguna Madre were contributing to this stratification. This is interesting because there are three wastewater treatment plants that empty into Oso Creek and to Oso Bay, so there is a source of nutrients as well as salt. The purpose of the current study was to determine the spatial and temporal extent of hypoxia, to explore the role of nutrients and bay currents in the formation and frequency of hypoxia, to determine what, if any role small rain events play in the formation of hypoxia, and to test the feasibility of disseminating the data collected in this study and others to the public via the use of a standardized database schema and web services. Hypoxia was found to begin as early as the first week of June, and occur as late as the last week of August, i.e. stops when wind stops. Hypoxic conditions can extend from Ward Island to Shamrock Island, and area estimated to cover 80 km2. Nutrient concentrations are not at high levels however, ammonium levels are higher in the hypoxic zone, likely due to anaerobic remineralization of organic matter. Even small-scale rain events appear to flush nutrients from Oso Bay however, this does not appear to affect hypoxia in Corpus Christi Bay. Differences in acoustic opacity, current velocity, and current direction, and salinity between the bottom waters and those above may all be contributing to stratification, known to cause hypoxia. Hypoxia also appears to be influenced by the fortnightly lunar cycle, bathymetry, and bottom composition. Attempts to transform data from this project into a standard database schema were successful. However, not all of the complexities of biological nomenclature, multivariate data structures, and laboratory information requirements could be met with the system under study.