Correlations Between the Concentrations of Chlorophyll a in Surface Waters and Dissolved Oxygen in Bottom Waters of the Northern Gulf of Mexico

Hypoxia is an environmental condition of low concentrations of dissolved oxygen that is detrimental to marine animals. It is a result of excessive input of fluvial nutrients to the coastal environment that leads to rapid growth of phytoplankton. When phytoplankton die and sink, they consume dissolved oxygen during decomposition. Water column stratification, especially a strong stratification in summer, is essential for the development of hypoxia. Seasonal occurrence of hypoxia has been observed in many coastal waters. The most prominent hypoxia in the United States was observed in the Northern Gulf of Mexico. For more than two decades, scientists have been monitoring the hypoxia in the Northern Gulf of Mexico by conducting annual ~ 2-week-long cruise in summer and developing strategies for reducing the flux of nutrients and the size of hypoxia. There are many shortcomings in the current monitoring method. The major shortcoming is poor temporal and spatial resolutions of these field studies.

We conducted the following research project to overcome some of these shortcomings and to lay a foundation for high resolution monitoring of hypoxia with satellite remote sensing. The project was based on the hypothesis that there is a linear correlation between the concentrations of chlorophyll a in surface waters and dissolved oxygen in bottom waters. To test this hypothesis we : (1)* obtained SeaWiFS data and processed it for sea surface chlorophyll a; (2) obtained dissolved oxygen data from NOAA’s National Coastal Data Development Center (NCDDC); (3) selected chlorophyll a data that coincided with dissolved oxygen data; (4) correlated the concentrations of dissolved oxygen in bottom waters to that of chlorophyll a in surface waters; (5) correlated the water column apparent oxygen utilization (AOU) to concentrations of chlorophyll a in surface waters. If our hypothesis is proven true, the correlations to be derived will enable monitoring of the future evolution of hypoxia in the Northern Gulf of Mexico with satellite remote sensing. This will significantly increase both temporal and spatial resolutions of hypoxia mapping. The correlations could also be used to evaluate the past evolution of the hypoxia in the Northern Gulf of Mexico (i.e. from 1997 when SeaWiFS was launched).