News•April 1, 2013

Warming May Mean More Toxic Algae Blooms for Lake Erie

Toxic algae blooms in Lake Erie could come more often and be more intense in coming decades thanks in part to torrential rains intensified by global warming, according to a study published in Monday’s Proceedings of the National Academy of Sciences.

Heavy runoff from farmland, say the authors, can carry nutrient-rich fertilizer into the western part of the lake, triggering a population explosion of blue-green algae that pump poisons while they live and can rob the water of oxygen when they die and decompose.

Aerial shot of a Lake Erie algae bloom.
Click Image to enlarge. Credit: NOAA.

And according to the study, if farming practices stay the same, the millions of people who depend on Lake Erie for drinking water and recreation — and by extension, those who depend on the other Great Lakes as well — could see water quality degrade significantly.

The report’s assertion isn’t just theoretical. In 2011, an enormous algae bloom struck Lake Erie, spreading to cover about 2,000 square miles — a record, and some three times more than the biggest of the algae blooms that grabbed headlines in the 1960s and 1970s.

“Lake Erie was famously declared dead at the time,” said co-author Don Scavia, an environmental engineer at the University of Michigan, in an interview. But in the following decades, strict controls on sewage treatment plants reduced pollution and allowed the lake to recover.

Then came torrential rains in May, 2011, including a storm that dumped 2 inches of water on the region in 24 hours. Fertilizer from thousands of square miles of corn, soybean and wheat fields was flushed into the lake, and algae literally ate it up.

The rains alone wouldn’t have caused such a problem, Scavia said, but starting in the 1990s, several changes in farming practices left more fertilizer on top of the ground. One was the increasing application of fertilizer in the fall, where it would sit on the frozen fields through winter, ready to go to work as soon as crops began sprouting in spring. Another was an increase on the acreage devoted to corn, a fertilizer-intensive crop — especially over the past few years as the federal government encouraged the use of corn ethanol to replace gasoline.

Blue green algae bloom on the shore of Catawaba Island, Ohio in Lake Erie. Summer 2009.
Click Image to enlarge. Credit: NOAA Great Lakes Environmental Research Laboratory/ flickr.

Yet another factor was a movement toward no-till agriculture, which eliminates annual plowing. “The irony is that this change was driven by valid environmental concerns,” said lead author Anna Michalak, a statistician with the Carnegie Institution for Science, in Stanford, Calif., in an interview. Old-fashion plowing is being phased out because it loosens the soil and promotes erosion; it also churns up nitrogen and carbon, putting extra heat-trapping greenhouse gases into the atmosphere. But if fertilizer is spread on top of the ground rather than being plowed under, it’s also easier to wash away in a heavy downpour.

Such downpours are expected to come more often as the world warns: a warmer atmosphere can hold more water vapor, but it also has more to dump during rainstorms. In fact, scientists have already seen both dry spells and heavy rains increase across the globe, and climate models generally agree that this so-called hydrological cycle will continue to become more extreme. “If you combine the changes in farming with the changes in frequency and intensity of severe rains, we think those things are conspiring to make algae blooms worse,” Scavia said.

In fact, the scientists did simulations to test this hypothesis. “We took 1970s weather combined with the agricultural practices of the 2000s, and 2000s weather with 1970s agriculture,” Scavia said. In both cases, the amount of fertilizer entering Lake Erie was significantly lower. When the team ran climate-model simulations of future precipitation in the area, said University of Michigan climatologist Allison Steiner, another co-author, in an interview, “we saw an increase both in intense and very intense rain events.”

The biggest lesson, however, has to do with the danger of looking at just one aspect of how climate change might affect a specific part of the world. Increased rainfall alone didn’t cause the 2011 algae bloom, Michalak said, and adapting to climate change will require scientists and policymakers to look beyond just what’s happening to the climate.

“It speaks to the need to look at the environment as a very complex, interrelated system,” Michalak said.