TRAVERSE CITY, Mich. (AP) — Reducing phosphorus levels in Lake Erie is a worthy goal but not necessarily a cure-all for one of the lake's biggest environmental hazards: "dead zones" with oxygen levels so low that fish can't survive, scientists said Tuesday.

Researchers with the Carnegie Institution for Science said Erie's biggest dead zone on record formed in summer 2012. While phosphorus-laden fertilizer runoff from farms played a part, weather conditions including drought and low flows from tributary rivers and streams were even more influential, they said.

The findings suggest policymakers working on plans for combating the lake's dead zones — and its worsening problem of harmful algae blooms — should consider meteorological trends as well as agricultural management practices, said environmental scientist Anna Michalak, who led the study. That's especially the case as climate change brings more extreme weather, she said.

"It means we need to be careful in our assumptions about how management is going to affect" oxygen levels in the water, Michalak said. "Reducing nutrient input won't always give you as much bang for the buck as you might assume."

Although Erie is the shallowest of the five Great Lakes, with an average depth of just 62 feet, it produces more of the types of fish that people eat — such as walleye and perch — than the others combined.

But it has suffered some of the worst pollution damage. Phosphorus and other nutrients in runoff from farms, sewage plants and other sources deplete oxygen and enlarge Erie's annual dead zones.

The nutrients also feed massive blooms of cyanobacteria — commonly known as blue-green algae. An outbreak last August prompted a two-day drinking water ban in northwestern Ohio, including Toledo, and southeastern Michigan. When the algae die and decay, oxygen levels drop further, sometimes reaching levels too low to sustain life.

The largest known algae bloom formed in 2011, following a stormy, wet spring. The Carnegie researchers found that the next year, which featured drought and little algae, a dead zone as large as Yellowstone National Park spread across the lake's central basin. It covered about 3,400 square miles — the biggest since measurements began in the mid-1980s and more than double the average size of the zone, which typically peaks in late summer.

The fact that the record-setting dead zone formed in a low-algae year shows the crucial role of weather, Michalak said. Water flow from tributaries into the lake was the lowest ever measured. Additionally, strong northwesterly winds pushed nutrients from the western basin into the central basin.

"What this research is showing is that ... the meteorological factors are as important in determining the size of the dead zone in a given year as nutrient loading is," she said.

Great Lakes scientists who didn't participate in the study agreed weather affects Lake Erie's dead zones, but said cutbacks in phosphorus runoff are still urgently needed.

"Only a few things are in our control, and how much phosphorus comes into the lake is one of them," said Raj Bejankiwar of the International Joint Commission, a U.S.-Canadian agency that in 2014 called for cutting phosphorus runoff by nearly half over three to six years.

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