Study shows nitrate plays counterintuitive role in lake eutrophication
By Cathryn Delude
Civil & Environmental Engineering Correspondent
Natural ecosystems need nutrients, such as phosphorus and nitrogen, to sustain plant life. But too many nutrients running into water bodies from fertilized farms and lawns or from wastewater can over-fertilize algae and aquatic plants and cause a major environmental problem called eutrophication. Waters may become fouled with scum, and the eventual bacterial decay of the plants and algae can deplete oxygen, which in turn kills fish and leads to dead zones, like those in the Gulf of Mexico and the Baltic Sea, toxic red tides in coastal waters, and cyanobacterial blooms in lakes and rivers.
Efforts to prevent or reverse eutrophication in freshwater typically aim to decrease the amount of phosphate entering the lake or river in runoff from the watershed. But a new CEE study suggests that phosphate control measures that simultaneously decrease nitrate inflow could, paradoxically, result in an increased release of phosphate from lake sediments that have become enriched after years of heavy phosphate inflow. Incorporating this information into engineering models of lake eutrophication could make them more accurate and useful.
“We observed that nitrate plays a role somewhat analogous to oxygen at our study site. So, counterintuitively, a lake with less nitrate inflow might have more phosphate released back into the water from the sediment,” said Harry Hemond, the William E. Leonhard Professor of Civil and Environmental Engineering. Katherine Lin ’05, who worked with Hemond on this research, was at the time an undergraduate student participating in the Undergraduate Research Opportunities Program at MIT.
Their study, which appeared in the June 2010 issue of Water Research, looked at the Upper Mystic Lake, a freshwater lake about seven miles from the MIT campus that is fed by the Aberjona River and feeds into the Mystic River and Boston Harbor. Over centuries, arsenic and other toxins from surrounding industry have accumulated in the lake sediment, and the lake has also become eutrophied from nutrient runoff.
The researchers focused on the water chemistry at the lake bottom because the intent was to study the potential release of nutrients from the lake sediment. Specifically, they looked at how the iron redox cycle (sometimes dubbed the “ferrous wheel”) controls phosphate cycling between the sediment and the water.
In this cycle, insoluble oxyhydroxide iron (III) particles in the water absorb phosphate and drag it to the sediment, preventing it from promoting algal growth. However, iron in the sediment can be chemically reduced into ferrous iron (II), which dissolves and releases its absorbed phosphate back into the water, thus promoting eutrophication. Oxygen in the water, however, can react with this soluble ferrous iron to recreate oxyhydroxide iron (III) particles, which reabsorb phosphate and settle back to the sediment.
It was thought that oxygen depletion in the deep water stops the downward portion of this ferrous wheel and prevents the re-absorption of phosphate.
But when Lin analyzed water samples collected at Upper Mystic Lake over the course of two summers and falls and compared the patterns of oxygen, nitrate, phosphate and iron concentrations, she found that even when oxygen became depleted, there was little net phosphate release from the sediments – as long as nitrate was still present in water.
“Only at depths and times where nitrate disappeared from the water column did phosphate begin to be released to the water,” said Lin, who is now an attorney in Chicago.
Previous work by Hemond and David Senn Ph.D. ’01 had shown a similar dynamic with arsenic and the ferrous wheel. The fact that arsenic and phosphate are chemically similar led to the current study testing the hypothesis that it was not the seasonal depletion of oxygen, but rather of nitrate that initiates phosphate release from the sediment.
This finding is important because engineering models used to calculate how much incoming phosphate must be decreased to mitigate eutrophication in a lake do not account for nitrate’s role in controlling the internal source of phosphate arising from lake sediment.
“Of course, adding more nitrate to a lake is not a solution because nitrate contributes to eutrophication downstream in coastal waters,” said Hemond. “But if models included the effects of nitrate on lake chemistry, they could result in more cost-effective strategies for controlling nutrients in lakes and rivers and mitigating eutrophication in these as well as in downstream coastal waters.”