Last week I wrote about the serious threats to Lake Erie: the blooms of toxic algae, the colonization of its bottom by invasive mussels, and the growth of its oxygen-starved dead zone. The dumping of phosphorus into the lake is feeding the harmful algae -- and putting the lake’s wildlife and the region’s economy at risk.

Where’s that phosphorus coming from? The last time we almost lost the lake, in the 1960s and ’70s, the main offenders were the wastewater treatment plants serving cities like Detroit and Cleveland. This time, scientists consider agriculture the leading culprit. As farmers change their practices -- applying fertilizer in the winter and spreading it on the surface of the soil -- phosphorus washes more easily into the lake during storms that have grown more intense thanks to climate change.

That’s why there’s so much buzz around a new report by the U.S. Geological Survey that says wastewater plants still play a significant role. The study says 42 percent of Erie’s phosphorus comes from "point sources," where the pollution flows through a pipe, compared to 44 percent from agriculture. The Detroit River, according to USGS data, contributes more phosphorus than any tributary besides Ohio’s Maumee River. Ninety-two percent of the Detroit River’s phosphorus comes from point sources like the Detroit Wastewater Treatment Plant, which serves three million people.

For Western Lake Erie Waterkeeper Sandy Bihn, the implications are clear: With agriculture virtually exempt from the Clean Water Act, "the low-hanging fruit is the Detroit Wastewater Plant." The plant’s major federal discharge permit comes up for renewal in 2012, and Bihn plans to lobby for tougher standards this time around. "It’s one of the largest plants in North America and can give us great return the most efficiently," she says.

Several scientists I interviewed insist the USGS study is flawed. Satellite imagery shows the algal blooms are not forming near the Detroit River -- they’re forming near the Maumee, where the main phosphorus source is agriculture, says Peter Richards, a research scientist at Heidelberg University. "The phosphorus coming down the Detroit River is very diluted by the huge volumes of water coming from Lake Huron," he says. "Low concentrations make it impossible to support large densities of algae." What’s more, USGS only looked at the U.S. side of the lake. Richards says Canada’s contribution comes largely from farms. His own research attributes 72 percent of Erie’s phosphorus to agricultural and other surface runoff.

Still, some environmentalists say there’s reason to take the USGS findings to heart. State regulators use agriculture "as an excuse for not requiring more from wastewater treatment plants," says NRDC attorney Thom Cmar. "Ultimately, given the Clean Water Act’s exemption for agricultural runoff, any solution to this problem is going to have to address both wastewater treatment plants and agriculture both. For better or worse, the wastewater treatment plants need to be brought in because that’s where the regulatory tools are -- and only after they are brought in can creative approaches such as trading programs be established that include agriculture."

UPDATE: Immediately after I posted this, Thomas Bridgeman, an aquatic ecologist at the University of Toledo’s Lake Erie Center, sent me a message reasserting that wastewater in the Detroit River is not causing the blooms. "An algal cell in the water does not care one iota about how much phosphorus is dumped into the water. The cell only cares about the concentration of phosphorus," he wrote. "And the closer to the Detroit River you go, the lower the phosphorus concentration." Maybe Detroit needs to clean up its act for other reasons, Bridgeman added, but removing nutrients from the city’s wastewater discharges "would only have the effect of making the concentration in the Detroit River go from 'way too low to cause an algal bloom' to 'way, way too low low to cause an algal bloom.'"

Image of Lake Erie algae on September 3 via NOAA