Relationships of Light Transmission, Stratification and Fluorescence in the Hypoxic Region of the Texas-Louisiana Shelf in Spring/Summer 2009

The growth of phytoplankton in hypoxic waters requires nutrients and light. In river plumes of the coastal ocean, river borne surface nutrient concentrations decrease with distance from the river mouth. Light availability at the surface also changes with distance from the river source, as particulates and other materials sink through the water column. Using in situ CTD sensors, water samples of nutrients, and ship flow-through data from two Louisiana Shelf research cruises, April and July 2009, the relationship between transmissivity, photosynthetically available radiation (PAR), stratification, and fluorescence are analyzed. PAR is measured using a silicon photovoltaic detector and is measured in the 400 to 700 nm range. A fluorometer is used to measure the fluorescence of chlorophyll a in g/L and is a proxy indicator for phytoplankton biomass. The Louisiana Shelf is home to the largest hypoxic zone in the western hemisphere, covering on average more than 15000 sq. km. The Louisiana hypoxic zone occurs seasonally in summer and is typically dissipated by physical processes in fall. Hypoxia is defined as waters that have a dissolved oxygen concentration of 1.4 ml/L and is the level at which marine organisms are typically adversely affected. Preliminary results show expected relationships between transmissivity and fluorescence in the water column indicating a positive correlation between light availability and fluorescence. The spatial distribution of stratification, light availability, fluorescence, and nutrients are compared for the different seasons