It’s a very visible problem for anyone living in south-central Minnesota’s lake-rich region: lakes turning a deep coral-green color during the hot summer months.

The area’s lakes, often fairly shallow, have been routinely afflicted as the sun heats up and activates phytoplankton leading to blue-green algae blooms. The sight of the blooms has become more common and intense in recent years.

Scientists know that the noxious blooms are on the rise because of climate change and the continued runoff of fertilizers that fuel algae growth. But a new study suggests the blooms are also on the rise because these phytoplankton are self-perpetuating because they use phosphorus deposits long buried in sediment.

The findings, reported by the Ecological Society of America, mean even cold-water, deep lakes, like those often seen in northern Minnesota, could see growing threats from algae blooms. Already spots along Lake Erie have experienced toxic blooms that forced large municipal water systems to be temporarily shut down.

The study found cyanobacteria seem to access phosphorus in lake bottom sediment that is typically not reached by other algae, according to The Associated Press. The cyanobacteria’s ability to capture the phosphorus leads to algae developing a stronger and perpetual cycle of turning into toxic blooms.

The problem is not just one of aesthetics. The toxin in algae blooms can be harmful to people and have killed pets that drink from lakes.

The findings add more complications to trying to manage algae blooms. But the researchers say that beyond climate change, human activity remains a big driver of the increase in blooms. Fertilizer runoff and too much sediment getting into rivers and lakes during long periods of time increases the phosphorus sitting in the bottom of lakes.

Researchers say new ways of looking at water management is needed. While those new approaches are complex and just beginning to be studied, scientists say the flow of excess nutrients into rivers and lakes from fertilizers remains the top contributor to algae blooms.

The Mankato Free Press

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