From the September 2018 Desktop News | Nutrients. Just the word sounds good. Wholesome. We want food packed with nutrients. Everything a growing child needs. Junk food doesn’t have nutrients, we believe. Nutrient-rich food keeps us healthy.
More than just our bodies thrive on nutrients. At its foundation, the world’s ecosystem is based on the consumption of nutrients that begins with tiny plant and animal life in water. Without nutrients, life does not thrive.
However, excess nutrients, the inverse, can be just as damaging. With increased population along the world’s coasts and industrialization of more economies, excess nutrients are a global problem.
Mobile Bay in Alabama is no different.
“The productivity of Mobile Bay depends on nutrients coming into the system, but too much nutrients flip it to the point where it’s no longer productive,” said Dr. Behzad Mortazavi, a marine scientist at The University of Alabama who works out of Dauphin Island Sea Lab near Mobile.
Specifically, nitrogen and phosphorus are necessary nutrients in a healthy aquatic system, but too much causes an overgrowth of algae, or small marine plants. Some algae produce toxins and are known as harmful algal, and when they bloom, it can be detrimental to fish and other marine life. These blooms can have severe economic impact on the seafood industry and tourism.
Dr. Natasha Dimova, an environmental geochemist at UA, says that rivers are often considered culprits for nutrient spikes—due to their contamination by agricultural and industrial run-off as well as human and animal waste; however, in Mobile, groundwater contamination is also a big problem.
“Although everybody assumes that the river is the major player in terms of nutrient fluxes, we found very recently that groundwater plays an important role during certain times of the year, and thus cannot be neglected,” Dimova said.
Common sense reveals this, too. Because groundwater takes longer to reach the ocean than a river, groundwater has the time to accumulate more chemicals, nutrients and heavy metals.
Dimova recently discovered that the potency of the groundwater is boosted just before it leaks into the bay, causing her team to consider, for the first time, that the coastal sediments may be a source of contamination for the groundwater.
“What we sample immediately at the point of discharge in the bay has a very different composition than what is on land,” she said. “All the data falls in this corner. That tells us the quality of the groundwater coming out is largely determined by what happens in the coastal sediments just before the discharge.”
When sorting out the influence of groundwater on the health of the bay and influence on harmful algal blooms, Dimova wants to know what is in the sediment. Below the surface, the sediment can tell the history of the bay, shedding light on when there have been harmful algal blooms.
Along with the nutrients affecting the bay today and potentially affecting it in the future, the sediment tells an intricate history of sea level and environmental changes in the bay. That is where Dr. Rebecca Totten Minzoni, a UA assistant professor of geological sciences, comes in.
Sediment cores provide a longer timeline to help understand how and when increased contributions of nutrients occurred. Totten Minzoni can reconstruct long-term climatic and environmental changes since Mobile Bay formed roughly 8,200 years ago.
“With the cores we collect in Mobile Bay, we can step back in time with each layer to tell us about how environments changed and, ultimately, produce a long-term frequency of the harmful algal blooms over time,” Totten Minzoni said.
While nutrients themselves do not preserve well in the sediment record, silica walls of microscopic, single-celled organisms called diatoms, a large part of harmful algal blooms, are left behind, preserved for millions of years.
These microscopic fossils in the sediment mark occurrences of past algal blooms in Mobile Bay. Their presence in a core of course sediment sourced from rivers can help determine whether the ancient blooms were triggered by nutrients from rivers or from other sources such as groundwater.
That can show whether the blooms and jubilees seen since European settlement, and the subsequent industrialization, are normal compared to the pre-recorded history.
“Once we understand the significance of human impact – specifically whether there is significant human impact – then we can help steer restoration efforts and any kinds of policies and best practices we should apply to water or agriculture in our state, for instance,” Totten Minzoni said. “Ultimately, we need a baseline of what was normal before land use changed, so we can really assess human environmental impacts and how to mitigate them.”
Sediments might also tell us about the bay once their contents begin leaching into it. That’s the hope of a collaboration between Dr. Sagy Cohen, in geography, and Dimova. The goal is to use satellite imaging to gauge where nutrients and heavy metals are in the bay.
“We can see sediments, but we can’t see the heavy metals or nutrients themselves,” Cohen said. “If we know the average concentration of heavy metals or nutrients attached to the sediment, then we will have a good sense of the concentration and where it is moving.”
Cohen’s and Dimova’s labs are calibrating water samples and turbidity measurements, or the cloudiness of the water with particles, with imaging from satellites. Once the data taken by hand and instruments in the bay matches with the imaging, Cohen hopes to use only the imaging to understand the flow of nutrients and heavy metals around the bay.
“It will give us not only a near real-time estimate of sediment and nutrients and heavy metal concentrations in the bay, but can also give us quite a long record in the past because there’s a good archive of satellite imagery going back to the 1970s,” he said.
When heavy metals and nutrients come downstream from the Mobile River Basin, they lodge in the sediment. The Gulf of Mexico is rising at a rate not seen since the period when the bay formed, and, as sea level rises, it changes the interaction with the sediment, Cohen said. Saltier water reacts different chemically from freshwater.
“We need to get a better sense of both how fast things are moving, some insight into how the mechanism works,” Cohen said. “A healthy ecological system can only tolerate so much specific nutrients and pollutants.
“We need that for a healthy aquatic environment both in the rivers and in the coasts,” he continued. “It’s very easy to pollute the water source, but it’s not that easy to clean it and reestablish a healthy ecosystem.”
Aspects of this research are funded by the National Science Foundation through the Chemical, Bioengineering, Environmental and Transport Systems (CBET) Division and the EPSCoR program investing in innovations at the nexus of food, energy and water systems; the Gulf of Mexico Research Initiative; and The University of Alabama.