Though the Detroit River delivers 25% of the phosphorus that flows into Lake Erie contributing to the Western Lake Erie Basin’s (WLEB) algae bloom, there’s never been a detailed accounting of the nutrient’s urban and agricultural sources across the entire binational watershed.
That is — until now.
A new University of Michigan (U-M) report provides the most detailed characterization to date of the phosphorus sources — and its relative contributions — into the complex Detroit River watershed, which is heavily urbanized on the U.S. side of the border and mainly agricultural on the Canadian side.
U-M researchers compiled and analyzed data from U.S. and Canadian water-quality monitoring programs between 1998 and 2016 and gathered data from both countries on discrete phosphorus sources and for combined sewer overflows. In addition, they estimated phosphorus loads from tributaries to the St. Clair River, Lake St. Clair and the Detroit River.
They found that 63% of the U.S. phosphorus originating within the watershed came from single, identifiable sources called point sources, which include wastewater treatment plants and industrial facilities, such as food processing and metal finishing plants.
The 46-page report, “Watershed assessment of the Detroit River phosphorus loads to Lake Erie,” is a summary of a three-year study. The project was guided by a 30-member advisory group, including agricultural representation from Michigan Farm Bureau (MFB), Michigan Department of Agriculture and Rural Development, and Michigan Agri-Business Association.
According to MFB Ag Ecology Department Manager Laura Campbell, while the report concluded nonpoint sources primarily from agriculture represents approximately 20% of the Detroit River phosphorus load, further reductions can be achieved through a multi-pronged approach.
“The study concluded that the most effective reductions could be achieved by combining three of the following practices: planting cover crops, adding filter/buffer strips, creating or restoring wetlands, and incorporating fertilizer and manure into the soil,” Campbell said. “Additionally, focusing those practices on specific areas within fields and at positions in the watershed where phosphorus loading is highest are most effective at reducing overall phosphorus loading from agriculture.”
That finding, according to Campbell, is significant and means farmers can target limited resources in the current farm economy. Researchers also tested the theory of simply reducing nutrient rate application and found that in most cases, it was not very effective at reducing loading.
“For Michigan agriculture, it means using technology and data to make good decisions about managing both water and nutrients on our farms, and being strategic about where to implement practices to get the best bang for the buck,” Campbell said.
The researchers also found that 83% of the Canadian phosphorus originating within the watershed is from diffuse sources called nonpoint sources, which include runoff from agricultural lands (both cropland and pasture), urban land surfaces, forests and wetlands. The Canadian side of the watershed includes some of the country’s most productive cropland.
Focusing the recommended land-management practices on the 55% of farmland with the highest per-acre phosphorus losses could lead to phosphorus-load reductions on the order of the Great Lakes Water Quality Agreement target in agricultural portions of the watershed, according to researchers.
In addition to pinpointing phosphorus sources, the report also evaluated options for reducing phosphorus levels throughout the watershed.
Among the report’s key findings:
- Overall, total phosphorus levels in the Detroit River have declined 37% since 1998. The decline is chiefly due to technical improvements at the regional wastewater treatment plant in Detroit and to phosphorus sequestered by zebra and quagga mussels on the bottom of Lake Huron.
- The researchers calculated that 54% of the Detroit River’s total phosphorus load actually comes from Lake Huron — a proportion three to four times higher than previous estimates.
- After Lake Huron, the largest contributors of phosphorus to the Detroit River are the regional wastewater treatment plant in Detroit and the heavily agricultural Thames River watershed in Ontario.
- Efforts to reduce the amount of phosphorus coming off croplands should focus on land-management practices such as adding cover crops and buffer strips, creating or restoring wetlands, and applying fertilizer below the soil surface. Focused use of those practices could help reduce phosphorus loads from agricultural watersheds by 40% or more.
The study area includes the watersheds of the Detroit River, Lake St. Clair and the St. Clair River, and is formally known as the St. Clair-Detroit River System watershed. The St. Clair River is about 40 miles long and drains waters from Lake Huron into Lake St. Clair.
The Detroit River is 32 miles long and flows south from Lake St. Clair to Lake Erie. The Detroit River delivers about 80 percent of the water and 25 percent of the phosphorus that flows into Lake Erie.
By far, the largest point source, according to the study, is the regional wastewater treatment plant in Detroit, which treats sewage from 77 communities and contributes 13 percent of the Detroit River’s phosphorus load to Lake Erie.
The plant, formally known as the Great Lakes Water Authority’s Water Resource Recovery Facility, or WRRF, has cut phosphorus discharges by 44.5% since 2009, and achieving further reductions “could be very expensive,” according to the report.
Southeast Michigan, with a population of more than 3.1 million, is the primary urban phosphorus source in the Detroit River watershed, and wastewater treatment plants are the main urban point sources.
The higher-than-expected contribution from Lake Huron to the Detroit River’s phosphorus load is due, in part, to climate-driven decline in winter ice in Lake Huron and an increased frequency of intense storms, which appear to be boosting shoreline erosion and re-suspending lake sediment from the lake’s southeastern shore, washing phosphorus-rich sediments downriver and eventually into Lake Erie, according to the report.
When the researchers analyzed satellite images of southernmost Lake Huron, they spotted sediment plumes that occur frequently along the lake’s southeastern shore and that are often missed by monitoring programs.
“Our new understanding of the contribution from Lake Huron suggests that reaching Lake Erie loading targets may require greater attention to Lake Huron sources and larger reductions from the Detroit River watershed than previously thought,” said aquatic ecologist Don Scavia, the study’s lead scientist and a professor emeritus at U-M’s School for Environment and Sustainability.
U-M researchers predict their findings and the involvement of the 30-member project advisory group will inform policy and management discussions related to Lake Erie phosphorus control — something that is long overdue, according to Campbell.
“This report identifies many of the challenges with accurately assessing nutrient sources and what to do about them,” Campbell concluded. “To meet phosphorus reduction goals it is clear we will all need to work together so that everyone can be part of the solution.”