Eroding Streambanks May Not Be Big Source Of Lake Champlain Phosphorus

Boats moored on Lake Champlain. (Credit: Travisleehardin via Creative Commons 3.0)

Like many of the United States’ freshwater lakes, Lake Champlain is dealing with the unwanted effects of phosphorus pollution. These include algal blooms that have those living nearby in a clamor to clean it up. State lawmakers, as well, are working to do their part to enact laws that target the pollution at its source.

But as recent research completed by the University of Vermont has uncovered, some of those sources being targeted, at least according to science’s current understanding of them, might not be the right ones.

Scientists at the university zeroed in on the banks of streams located in the mostly suburban Chittenden County that flow into Lake Champlain on its Vermont side. Their findings upend a common conception of phosphorus pollution that streambank erosion is one of its big contributors and could have implications for bodies of freshwater elsewhere.

At the outset, U. of Vermont scientists gathered LIDAR (light detection and ranging) images of streams they planned to study: Indian Brook, Alder Brook, Allen Brook and the LaPlatte River. These were used to stake out just how much the streams had shifted over time, essentially showing researchers exactly where they had undergone the most erosion.

“We digitally determined where the streams’ center lines had moved to calculate the amount of streambank loss,” said Don Ross, research professor of plant and soil science at the university.

U. of Vermont and U.S. Department of Agriculture scientists gather bulk density samples along a forested stretch of Allen Brook in Chittenden County, Vermont. (Credit: Don Ross / University of Vermont)

With the areas of erosion pinpointed, the investigators then shifted their efforts to ground-based fieldwork. They sampled in areas where soil erosion had occurred along the banks, like where large portions of streambank were missing. If an area was too harshly eroded, they got as close as they could.

Soil coring equipment, along with shovels, was used to gather samples of the eroded streambanks for analysis over a three-year period. Samples from four depths of soil were taken during each collection. These were taken back to the lab and analyzed for measurements of total phosphorus, bulk density, pH and degree of phosphorus saturation.

The pH measurement is important for phosphorus retention, says Ross. Another important metric was the phosphorus saturation rate.

Taking those into account with the other parameters, Ross and others involved with the study found a range in the erosion rates over the three-year period.

“We were able to appreciate the amount of phosphorus in the soil that eroded and used a rough calculation of the data to find that 6 to 30 percent of the total phosphorus coming out in the streams could be from streambank erosion,” said Ross. “That’s similar to what others have found.”

There is an important caveat, he notes: They established the phosphorus going into the streams, not all of which actually makes it into Lake Champlain downstream. In all, the researchers measured that somewhere less than 30 percent of the phosphorus making it into the streams came from eroding streambanks.

“Even though it’s a relatively high amount of phosphorus, the saturation index was quite low, suggesting it (the streambank) wouldn’t be releasing a lot of phosphorus,” said Ross. And so streambank erosion may not be as big of a source for the phosphorus that feeds algal blooms as has been previously thought. “But a lot of work remains to be done to verify that.”

That area is where Ross and others will be focusing some of their research efforts next. Ross says that streambank erosion is looking to be a significant source of total phosphorus but, at least in Chittenden County, perhaps not bioavailable phosphorus. He adds that the balance could be different in other areas where more active land use, like agriculture, goes right up to the bank.

Funding for the work was provided by the U.S. Geological Survey through the Vermont Water Resources and Lake Studies Center. Full results are published in the Journal of Environmental Quality.

Top image: Boats moored on Lake Champlain. (Credit: Travisleehardin via Creative Commons 3.0)