Lecture by Dr. Quay Dortch, Ecology and Oceanography of Harmful Algal Blooms (ECOHAB) Program Director

"Troubles in the Gulf of Mexico: Dead Zones, Toxic Algae, and Diversions"

Abridged Summary by Azalea Mitch, MEM 2005

Dr. Quay Dorth's lecture outlined some of the environmental and water quality problems associated with rich nutrient inputs from the Mississippi River. The Mississippi River Basin covers approximately 40% of the continuous United States and supplies the Gulf of Mexico with an overwhelming amount of nutrient and sediment rich fresh water. Unlike the nutrient laden water, sediments in the Mississippi River flow do not travel far and are deposited on the inner Louisiana shelf overwhelming the waters off Terrebonne, Barataria and Atchafalaya Bays. The development of summer hypoxia on the Louisiana shelf has been associated with an increase in nitrate loading since the 1960's. While nutrients play an important role, stratification and weather are as equally important in setting the necessary conditions for hypoxia.

Periodic monitoring for hypoxia in the Gulf began in the mid 1980's. Dissolved oxygen measurements are taken monthly along Transect "C" which cuts through the core of the hypoxic zone. Measurements for salinity, chlorophyll, dissolved oxygen and depth are taken yearly in July. Hypoxia in the Gulf is determined by the 2 mg/L dissolved oxygen cutoff. Concentrations below 2 mg/L provide unsuitable conditions for the survival of most multi-cellular organisms. Monitoring data shows that dissolved oxygen concentrations begin to decline in April continuing through July. When cold fronts move in, water layers are re-aerated and temperatures fall increasing the density in the water and the dissolved oxygen concentrations. While conditions are highly variable, winter dissolved concentrations average around 7 mg/L. Dissolved oxygen concentrations off Terrobonne Bay, however, fall below 2 mg/L 75% of the time.

The impacts of hypoxia are numerous. While the northern Gulf of Mexico is often called the "dead zone," the waters are not completely dead as bacteria and squid populations thrive. The impacts of benthic communities are deadly, however. Massive fish kills called "jubilees" occur when benthic fish die due to the lack of oxygen and are pushed out onto shore. Hypoxia also reduces suitable habitat, increasing susceptibility to predation and causes changes in food sources. The impacts on shrimp and fish landings are highly variable. Overall, there has been a decline in brown shrimp catches, but the numbers are affected by high variability. The impacts on finfish catches are harder to see due to over fishing practices. While declines cannot be directly correlated to hypoxia, changes in species composition have been noted and are reflected by an increase in pelagic fish species and a decline in benthic species. Another impact associated with increased nutrient supply is the presence of toxic algae. While no outbreaks have been documented in the Gulf, there is evidence of increased toxic algae presence. Populations are typically high in spring, not in summer and there is evidence that growth is stimulated by nutrients.

One of the consequences associated with diversions built to alleviate high Mississippi River flow was documented in 1997 when a severe algal bloom incident occurred in Lake Pontchartain. Lake Pontchartain Basin is located east of the birdfoot delta and is 24 miles wide. During high Mississippi River flow events, a spillway connecting the river to the lake provides flood control. In 1997, after the spillway was opened, a cyanobacteria bloom covered the entire lake, emitting foul odors. Microcystin concentrations, produced by cyanobacteria, were sustained at levels exceeding 10 ng/mL. The bloom was caused by an overwhelming supply of nutrient rich water that dropped the salinity concentrations in the Lake significantly and that supplied the lake with high dissolved inorganic nitrogen concentrations. The bloom dissipated once the nitrogen concentrations dropped.

Dr. Dortch lecture discussed the problems associated with eutrophication in the Gulf from an expert's perspective and offered several examples of detrimental effects. Her research in these areas should be helpful to environmental managers who are working to restore the Gulf back to health.