Folks have heard a great deal of talk about the risks of algal blooms in the Gulf of Mexico in recent years, and more recently extensive research aimed at not only determining why these hypoxic zones, or "dead zones," are problematic but also at finding solutions for how the threat can be addressed..
So far, scientific research and study has determined—much to the dismay of the agricultural industry—that these anoxic conditions are largely the result of a change in water chemistry, specifically the ratio of dissolved silica to dissolved inorganic nitrogen.
A spike in nitrogen levels has been tied to agricultural runoff from fields and farms up and down rivers and streams that drain into the Gulf. Many of the rich agricultural regions of the Midwest are credited with using substantial amounts of nitrogen in farming, but nitrogen in the waterways snowball as fields downstream add to problem.
Scientists say this nitrogen-enhanced water collects behind dams, creating an imbalance between nitrogen and dissolved silica that contributes to the excessive bloom of algae. When this water is subsequently released, it travels south to the open Gulf where dead zones develop.
While the Mississippi River contributes the vast majority of this nitrogen-rich water to the Gulf, smaller rivers also add to the drastic change in water chemistry in the Gulf. These dead zones are responsible for the bloom of multiple types of algae, which result in a host of potential problems. Nutrient overloading and algal blooms lead to eutrophication, which has been shown to reduce benthic biomass and biodiversity. Hypoxic water supports fewer organisms and has been linked to massive fish kills.
Major negative impacts to the seafood industry result. The Gulf supplies 72 percent of U.S. harvested shrimp, 66 percent of harvested oysters, and 16 percent of commercial fish. In addition, coastal states claim the dead zones hamper tourism and recreational industries. Economic damages have resulted, with the prospect of increased damage in the years ahead.
LARGER DEAD ZONE
A recent published study credits the latest research as timely because algae, agriculture and weather combined to create an above-average size dead zone in the Gulf this year (2016). Starting at the mouth of the Mississippi River, the ratio of silica to inorganic nitrogen has shifted dramatically toward nitrogen in recent years, with more changes expected in the years ahead.
Two possible explanations for the change in ratio are that dams and reservoirs may remove silica, and agriculture supplies so much nitrogen that the ratios are forced downward. This study shows that reservoirs do not remove silica, whereas nitrogen increases greatly when more than 60 percent of land is dedicated to agricultural production.
The study also indicates excess dissolved nitrogen in coastal zones like the Gulf of Mexico and Lake Erie fuels the growth of algae, which in turn precipitates anoxic conditions, depending on the runoff of fertilizers each year. Wet years cause higher concentration of runoff, which subsequently feed these anoxic conditions.
"It is easy to change a phytoplankton community just by altering the availability of a few important compounds such as dissolved silica or dissolved inorganic nitrogen," said co-author John Downing, director of the University of Minnesota Sea Grant College Program and the study's principal investigator. "In the Gulf we want to see diatoms, a type of phytoplankton that needs about equal amounts of dissolved silica and dissolved inorganic nitrogen. What we've created is a nitrate-rich condition that favors other, harmful types of phytoplankton."
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Downing and colleagues from the University of Washington and Boston University confirmed that increased nitrogen loss from agricultural fields sufficiently explains the formation of these dead zones in the northern Gulf of Mexico. But they also identified methods that may lead to better techniques for managing landscapes and the water they influence.
Researchers suggest that managing landscapes at watershed levels could significantly improve water quality, especially in wet years. They also point to reservoirs and similar water impoundments as areas where heavy nutrient loads could be diminished without affecting silica concentrations before the water reaches the Gulf of Mexico.
"We need to be vigilant about our land use and water quality," said Downing. "Climate change and increased storminess will likely exacerbate the skewed ratios we found and the extent of harmful blooms in coastal areas if we don't manage agricultural runoff more effectively. Harmful algae blooms cost the U.S. seafood, tourism and health industries over $80 million a year, according to the National Oceanic and Atmospheric Administration, and we know we can do better."
LOOKING FOR SOLUTIONS
Another recent study offers hope for significant improvement in nitrogen-rich runoffs. In the new study, an Iowa State University agronomist shows that an increase in perennial bioenergy grasses throughout the Corn Belt would lead to a significant reduction in nitrogen moving down the Mississippi River and into the Gulf of Mexico.
The study used computer models to simulate how various levels of perennial grasses might affect the level of nutrient runoff from Midwestern farmland. The study's findings largely corroborate those of the Iowa Nutrient Reduction Strategy, a state plan designed to reduce nitrogen and phosphorus levels that contribute to the Gulf of Mexico's hypoxic zone, said Andy VanLoocke, an assistant professor of agronomy and co-author of the study.
"This study adds to the portfolio of evidence that perennials can be a major part of the solution to nutrient runoff," VanLoocke said. "This is a process that we'll continue to refine, but it adds to the conversation."
VanLoocke's study, published in the academic journal Global Change Biology Bioenergy, found that perennials could reduce nitrogen runoff by more than 70 percent on farmland where they're planted.
The Iowa Nutrient Reduction Strategy identified the planting of perennial crops among several strategies with the potential to significantly reduce nitrogen runoff. Researchers have pointed to plants such as miscanthus and switchgrass as promising perennial options.
VanLoocke's study used computer models for climate and soils data across the Midwest, as well as historical maps of where corn and soybeans have been planted. The study analyzed different thresholds of crop replacement of current farmland between 5 percent and 40 percent. The study also examined different levels of fertilizer use for perennial grasses, since researchers are still experimenting with how best to fertilize perennials.
VanLoocke said perennial crops tend to start growing earlier in the spring than corn and soybeans. That longer growing season means they soak up more nitrogen, which keeps the nutrients from washing downstream.
He said the study's simulations can't account for every variable in every field, but said he and his co-author will continue to refine the models and improve their usefulness.
"The new research shows that a perennial market would have positive effects on water quality," VanLoocke said. "With this model in place, we'll be able to play with more scenarios and land-use practices."