This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Why do some lakes remain plagued by harmful algal blooms even after phosphorus pollution has been reduced? Part of the answer lies beneath the surface.
A new Concordia study has uncovered a previously overlooked chemical process that can help trap phosphorus in lake sediments, preventing it from returning to the water, where it can fuel excessive plant and algal growth. Published in Scientific Reports, the study shows that mackinawite—an iron sulfide mineral that forms in oxygen-depleted sediments—can bind phosphorus under conditions where other phosphorus-trapping minerals become unstable. The discovery adds an important new piece to scientists' understanding of how nutrients move through freshwater ecosystems and could help improve predictions of how lakes recover from pollution.
"Sediments play a central role in controlling the quality of the water above them," says Milad Ezzati, a Ph.D. candidate in Concordia's Department of Chemistry and Biochemistry and lead author of the study. "They store large amounts of minerals, nutrients and organic matter, some of which can move back into the overlying water. Even small changes in environmental conditions can release nutrients that directly affect water quality." Phosphorus is an essential nutrient for aquatic life, supporting everything from microscopic algae to fish.
But too much of it can upset the balance of freshwater ecosystems, triggering eutrophication, a process that leads to excessive algal growth and, in some cases, harmful cyanobacterial blooms such as those seen in many lakes across southern Quebec during the summer. Lake sediments can either help control this process by trapping phosphorus or worsen it by releasing stored phosphorus back into the water. Scientists have long known that iron-rich minerals can lock away phosphorus when oxygen is present.
But when oxygen levels decline, in what are called "anoxic conditions," these minerals break down, allowing phosphorus to escape into the surrounding water. Ezzati and his colleagues wanted to know whether another mineral that forms under oxygen-poor conditions—mackinawite—might continue trapping phosphorus as those traditional minerals disappear. Their experiments showed that it can.
"In biogeochemistry, a sink is something that removes a compound from the active environment," Ezzati explains. "Our findings suggest that mackinawite provides an additional pathway by which phosphorus can be retained under anoxic conditions, when traditional phosphorus-binding minerals are no longer stable." Until now, researchers generally recognized two main ways phosphorus could become permanently stored in oxygen-poor sediments: through the burial of organic matter or the formation of another iron-phosphorus mineral known as vivianite. Mackinawite is commonly observed in organic-rich lake sediments, but its potential role in phosphorus cycling had not been investigated.
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