Overlooked Mineral in Lake Mud Traps Phosphorus and Could Slow Algal Blooms, Study Finds

Some lakes remain plagued by harmful algal blooms even after phosphorus pollution has been reduced, and part of the answer may lie in the sediment beneath the surface. A study from Concordia University, published in the journal Scientific Reports, found that mackinawite, an iron sulfide mineral that forms in oxygen-depleted lake sediments, can trap phosphorus under conditions where other phosphorus-binding minerals become unstable, preventing it from returning to the water and fueling excessive algae growth.

Phosphorus is an essential nutrient for aquatic life, but too much of it can trigger 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 each summer. Scientists have long known that iron-rich minerals can lock away phosphorus when oxygen is present in sediment, but these minerals break down once oxygen levels decline, releasing phosphorus back into the water. Lead author Milad Ezzati, a PhD candidate in Concordia's Department of Chemistry and Biochemistry, and colleagues wanted to know whether mackinawite, which also forms under oxygen-poor conditions, might continue trapping phosphorus as those other minerals disappear — and found that it can.

Until now, researchers generally recognized two main ways phosphorus becomes permanently stored in oxygen-poor sediment: through the burial of organic matter or the formation of another iron-phosphorus mineral called vivianite. The study identifies mackinawite as a third, previously overlooked pathway, though the team also found that natural organic matter competes with phosphorus for space on the mineral's surface, reducing how much phosphorus mackinawite can retain.

"Our interpretation is that interactions between phosphorus and mackinawite provide an additional pathway for removing phosphorus from lakes," Ezzati said. He said the importance of the process still needs to be quantified at the scale of a whole lake, but that it may help scientists better predict how long it takes eutrophic lakes to recover once phosphorus pollution is reduced. The team next plans to investigate how different forms of natural organic matter interact with mackinawite.