Water overflowing from the Mississippi River rushed through the wooden slats of the Bonnet Carré Spillway on April 3, flooding Lake Pontchartrain with freshwater. A week after its opening, a bright green web lined the surface of the water along the north and south shores of the lake.
With an increase in nutrient pollution and unusually high amounts of rainfall resulting from climate change, harmful algal blooms are becoming more prominent in Lake Pontchartrain and the surrounding basin ecosystem. The headwaters of the Mississippi River flow from Lake Itasca, a small glacial lake located in northern Minnesota. As the water travels south through the Midwest, it collects agricultural runoff and pollutants. The river makes its last stop in Louisiana, ultimately flowing out into the Gulf of Mexico.
When the spillway is opened, nutrient-rich freshwater and sediment collected by the river is diverted into Lake Pontchartrain, altering the estuarine environment. Lake Pontchartrain, the center of the Lake Pontchartrain Basin, is part of a complex ecological system that includes lakes, rivers, marshes, forests and bayous. The lake contains primarily brackish water and is home to a large number of species of fish, crab and shrimp.
Nearly 1.5 million people reside on the banks of Lake Pontchartrain, making it one of the most densely populated areas of Louisiana. Historically, its close proximity to urban centers made the lake susceptible to heavy pollution and water quality issues. The Environmental Protection Agency removed Lake Pontchartrain from the Impaired Waterbodies list in 2006, but for years prior the lake was an ecological disaster.
The lake, ridden with pollutants, was unfit for swimming due to high levels of sewage and chemical runoff from the surrounding urban areas. Since the Save Our Lake campaign, the water in Lake Pontchartrain is cleaner, Pontchartrain Conservancy Water Quality Coordinator John O’Donnell said. But the organization is beginning to monitor a new issue: algal blooms.
“It’s a bit different from the old pollution, which is more bacteria based, or toxic such as a spill,” O’Donnell said. “Nutrients alone aren’t a bad thing, but it’s when they become too much is when they cause these blooms.”
In April, the U.S. Army Corp of Engineers opened the Bonnet Carré spillway for the third time in the past four years. In 2019, it was opened twice for a total of 123 days.
The spillway, completed in 1931, was designed to protect the surrounding low-lying areas of New Orleans from the flooding of the Mississippi.
Although it helps maintain the water level of the river, its opening does not come without consequence. When the spillway is open, an influx of nutrient-rich water flows into Lake Pontchartrain.
According to the U.S. Army Corp of Engineers, the long-range effect of the spillway opening can be favorable. It simulates the natural flooding cycle of the Mississippi River and replenishes nutrients to the ecosystem and surrounding wetlands. Spillway openings can be associated with increased fisheries production, particularly of crab and oyster, in Lake Pontchartrain for several years after flood events, but not all of the aftereffects benefit the health of the lake.
The water flowing out of the spillway is high in nitrogen and phosphorus, creating the perfect conditions for algal blooms. Algae thrives when temperatures are warm and there is lack of lake mixing or turnover.
The algae uses sunlight and nutrients to grow, and in the correct amount, it is a common food source for aquatic species. If there is a high concentration of nutrients in the lake, the algae grows and quickly becomes too large for its environment. It then dies off, sinking to the bottom of the lake.
“Lake Pontchartrain is a small-scale example of what is happening in the Gulf of Mexico right now,” O’Donnell said. “When you open the spillway, you get all of these nutrients pumping into the lake instead of going down the river and into the Gulf.”
As algae decomposes, it uses oxygen from the water column, creating hypoxic zones in the lake. Fish and other animals can become trapped in these areas and die.
In certain cases toxic blue-green algae, also known as cyanobacteria, can also appear in the water. When the algae begins to die, the bacteria can become stressed and will start producing toxins.
These toxins are directly harmful to people, pets, fish, shellfish and birds. Harmful algal bloom expert Sibel Bargu said humans can be exposed to these toxins if they are utilizing the water for recreational purposes.
“If people are using a contaminated area and come into physical contact with the water, they may contract the toxin through their skin,” Bargu said. Filter feeders such as oysters will ingest the phytoplankton toxins from the water and can also become contaminated. The Louisiana Department of Health advises the public against the harvest and consumption of seafood from affected areas.
According to the National Oceanic and Atmospheric Administration, the human illnesses caused by HABs, though rare, can be debilitating or even fatal, causing potential damage to the liver and nervous system.
NOAA is both leading and expanding HAB research in an effort to gain a better understanding of why these blooms occur. Their data will help improve both the prediction and detection of these events.
In Louisiana, the LA Sea Grant is receiving funds to develop predictive models to stimulate when and where blooms will occur, Bargu said. Currently, no regular HAB monitoring has been initiated in Lake Pontchartrain. “We need to establish a monitoring program to make sure that a warning to the public is both timely and correct,” Bargu said.
LSU’s College of the Coast & Environment collaborated with the Pontchartrain Conservancy to examine the algae last year, but the organization focuses mainly on water quality and beach monitoring.
The conservancy is expecting new tools to help them start studying the issue.
Algal blooms are difficult to monitor, O’Donnell said. But with new technology, the organization will have the necessary tools to collect data regularly.
“Blooms can pop up quickly and then disappear within the same week,” O’Donnell said. “But better tools are becoming available along with satellite technology. Once we receive meters of our own, we will be able to detect chlorophyll that would be indicative of an algal bloom.”
When weather permits, satellite imagery provides a complete overhead view of the lake, which spans 630 square miles. Completing individualized grab samples for Lake Pontchartrain’s large water mass is difficult logistically and expensive.
Alisha Renfro, a coastal scientist for the National Wildlife Federation, said this technology will help researchers monitor the frequency of algal blooms.
“Satellite imagery allows researchers to see these blooms when they happen and measure them more often than we were in the past,” Renfro said. “But I think we are going to see an increasing trend in the near future.”
Bella Biondini 