In early August, Toledo, Ohio's water turned pea-soup green and undrinkable, but it appears that phosphorus, the most obvious culprit behind the huge bloom of toxic algae, wasn't acting alone. In fact, we don't know too much about what, exactly, is causing Lake Erie's water to turn toxic.
Toxin-producing cyanobacteria occurs naturally in Lake Erie, but it typically doesn't explode into huge blooms that are visible from space without a few key ingredients, the foremost among them is phosphorus. In the mid-1960s many lakes and rivers were turning green as cyanobacteria flourished on phosphates that entered the waterways in laundry detergents.
When the blue-green algae blooms and dies, it soaks up oxygen and chokes the life out of water. It not only looks gross and smells worse, but the EPA warns, "cyanotoxins in recreational water and drinking water may cause a wide range of symptoms in humans including fever, headaches, muscle and joint pain, blisters, stomach cramps, diarrhea, vomiting, mouth ulcers, and allergic reactions."
less phosphorus is required to generate a large bloom than a decade ago
So phosphate detergent was phased out and the Clean Water Act was passed in 1972, and by 1988, Lake Erie stopped experiencing giant toxic algal blooms. But by the mid 1990s, the cyanobacteria came storming back.
Changes in agricultural practices that use more phosphorus on the soil surface have increased runoff into Lake Erie, leading to the record blooms in 2011 and another devastating bloom this summer, which rendered tap water for half a million people in Ohio and Michigan undrinkable for several days over the summer.
But phosphorus apparently isn't the only party responsible. Researchers from NOAA and the University of Michigan found that the cyanobacteria seem to be getting more sensitive to phosphates in a study published in Water Resources Research.
"Phosphorus loading is still very important," Dan Obenour, the study's lead author told me. "But, there appears to be something else going on too, because less phosphorus is required to generate a large bloom than a decade ago."
Computer models that use the amount of phosphorus going into the lake to estimate late-summer blooms don't seem to adequately explain blooms from 2002-2013.
The researchers looked at Lake Erie's surface temperatures to see if they had risen and made the lake more accommodating to algae, or if levels of the most potent type of phosphorus—dissolved reactive phosphorus—could explain what was happening.
"Neither of these did a good job explaining the trend toward larger blooms," Obenour said.
Obenour told me the researchers hypothesize that invasive species of zebra and quagga mussels, which feed on cyanobacteria's competition, may responsible.
"It may also be that climate change is playing a role through changes in wind patterns that affect mixing in the lake," he said. "However, more research is really needed to explore these ideas, so that we can improve our ability to forecast and manage blooms in the future."
But that doesn't mean that phosphorus is off the hook, or that efforts to reduce blooms through reducing phosphorus are in vain. It actually means that the level of phosphorus would have to be reduced even further.
"The problem is that because of the lake's increasing susceptibility to blooms, the currently proposed phosphorus loading targets are insufficient to reduce blooms all of the way down to desired levels," Obenour said. "We either need more severe phosphorus loading reductions or we need to accept that harmful blooms will likely persist in the future."