In 2014, a Lake Erie algal bloom sent a cloud of toxins into Toledo, Ohio’s water supply, forcing the city to shut down water service to 400,000 residents. Like many lakes in agricultural areas, Lake Erie produces thick, smelly algae mats when the water gets warm. Temperatures above 60°F can trigger algal blooms, and Lake Erie—shallowest and warmest of the Great Lakes—hit nearly 80°F in 2020. In addition, the lake is the final destination for fertilizers washing off of area farms—that’s a recipe for excess photosynthesis.
“You’re cooking a perfect soup for having a very productive lake,” said Angélica Vázquez-Ortega, an assistant professor of geochemistry at Bowling Green State University.
Whereas fertilizers are a source of Lake Erie’s annual algae issue, research from Vázquez-Ortega’s lab suggests agriculture could be a partial solution, too. Instead of applying more fertilizers upstream, farmers could remove nutrients from the lake by mixing Lake Erie sediment into their soils. The research is especially timely as a new law leaves millions of tons of sediment piling up at Ohio’s ports.
The research is rooted in northwestern Ohio, a region that formerly boasted a 4,000-square-kilometer marsh dubbed the Great Black Swamp. The swamp “was the kidneys of the area, filtering the nutrients and making sure the water Lake Erie is receiving was clean,” said Vázquez-Ortega.
Colonizers sick of the knee-deep mud and clouds of mosquitoes gradually drained the area in the mid-1800s, easing navigation but increasing the export of sediments from the land. The watershed is now over 72% agricultural.
Nutrients like nitrogen and phosphorus naturally enter lakes through sediment export, but farm practices—like draining and fertilizing fields—accelerate the process. Once nutrients enter Lake Erie, they tend to stay there, eventually accumulating in sediments on the lake floor.
Those sediments require annual dredging to keep ports viable. Ohio dredges 1.5 million tons of sediment from its eight ports each year, and the Port of Toledo accounts for more than half that figure. Until recently, Toledo would dump dredged sediment into open water, a common practice that introduces phosphorus and nitrogen back into the water column and buries benthic communities on the seafloor. Ohio banned open-water dumping of dredged sediment, effective in 2020, forcing ports to find a process for storing their sediment. For now, Toledo is building artificial islands on the lakefront.
“This is a completely new challenge for Ohio,” said Vázquez-Ortega.
More Sediment, More Soybeans
Agriculture could be a possible destination for dredged sediment, according to results from Vázquez-Ortega’s lab published in Journal of Environmental Quality. In a greenhouse experiment, sediment from the Port of Toledo increased crop growth with no significant loss of nutrients in percolated water.
The study created four soil combinations, blending material from a local farm with dredged material from the Port of Toledo at sediment ratios of 0%, 10%, 20%, and 100%. Dredged sediment introduced more organic content, giving the test soils a lower bulk density and allowing roots and water to penetrate into the less compact soil. Samples with more Lake Erie sediment grew heftier root systems and generated higher soybean yields. The study demonstrated that Lake Erie sediments can improve crop yield without the use of additional fertilizers.
Farming Out the Research
Despite promising results, there’s plenty left to research. What crops grow best and at what sediment percentages? What if industrial contaminants are in the soil? Importantly, will this work outside the greenhouse on an actual farm?
“All that information is really necessary for convincing a farmer this is an option,” said Vázquez-Ortega.
Economics and logistics are other key concerns. With 1.5 million tons of material, Ohio can give nutrient-rich sediment away for free. But would anyone want it?
In the study, the greatest soybean yield came from the 100% dredged sediment sample. That’s not a feasible ratio for farms, though. Sediment is heavy, and transporting it is expensive. Even at 10% application, a farmer would need 100 tons of dried sediment per acre, estimated Keith Reid, a soil scientist with Canada’s Department of Agriculture and Agri-Food. In addition, he said, spreading tons of sediment would require heavy machinery, which would compact the soils and remove any benefits of lower bulk density.
“It’s a good start at looking at the potential for uses of soil amendment,” Reid said of the study. “It’s fair to safely say there was no negative impact. It’s hard to say if there was a real large positive impact.”
Any new method for farming must demonstrate effectiveness and affordability, and Vázquez-Ortega recognizes the work left to do. “It’s a very preliminary step,” she said of the study. She’s now collaborating with the Ohio EPA and the Ohio Lake Erie Commission, among other parties, on a 2-year farm test.
The study is a step toward finding a beneficial use for sediment that preserves the ports and protects the lake. But until the process makes economic and agronomic sense, sediments will remain all dredged up with nowhere to grow.
—J. Besl (@J_Besl), Science Writer