Satellites give scientists an eye in the sky on harmful algal blooms
Tyler King, a research hydrologist with the U.S. Geological Survey, hopped into a truck at the agency’s Boise headquarters.
The destination was Lake Lowell, a reservoir some 20 miles west.
“Lake Lowell, as is common with many other water bodies in the western United States, grows an awful lot of plant matter,” he said.
It can also grow cyanobacteria, one of earth’s oldest organisms and a key source of our atmosphere’s oxygen. But they can produce toxins harmful to humans and other animals, and are one type of algae that can cause harmful algal blooms – or HABs.
Much monitoring is done by water managers and members of the public. That’s resource-intensive work with limited reach, as King and his coauthors noted in a paper published late last year.
Thus the interest in satellites.
“So, basically for the past 50 years, there have been satellites orbiting the planet, basically taking earth selfies,” King said.
Some can identify cyanobacteria and blooms in the ocean and other large water bodies. But the resolution doesn’t work well with the narrow, snaking arms of some Idaho lakes and reservoirs.
Others with finer resolution can’t make those IDs. But they can spot chlorophyll, suggesting the presence of cyanobacteria – and at certain concentrations, a potentially hazardous bloom.
Combing the scientific literature, King’s team found some 17 methods for extracting that information from satellite images.
“And none of them worked particularly well just by themselves, so we built an ensemble out of all of them,” he said. “And that worked reasonably well to identify when you have chlorophyll greater than 10 micrograms per liter.”
At that threshold, the World Health Organization says the risk rises from low to moderate, according to King.
High over the other side of Earth, a speeding satellite was on an intercept course with Lake Lowell. It was one of the European Space Agency’s Sentinel-2 polar-orbiting satellites, whose images form the core of the monitoring effort.
“That's actually why we're going out here today,” King said. “We want to be on the water body at the same time that satellite image is collected.”
At a boat launch, King’s colleagues Rob Florence and John Carricaburu slowly backed a simple, aluminum boat into the water. Before the boat even got underway, troubling signs were already clear from the deck.
“You see these clumps of clumps of algae here?” King said, pointing out a swarm of green, bunched-together particles. “It looks like a cyanobacteria.”
At the first stop, Rob and his colleagues break out the gear.
“This is a water quality sonde,” Florence explained. “I'm gonna drop this in the water, and it tells us all sorts of things: it tells us water temperature …, pH, dissolved oxygen, a bunch of water quality parameters.”
They also gather lake water to test for chlorophyll concentration, and churn it for uniform sampling.
“After the fact, we will take the field data and we'll match it up with the satellite based products…to quantify how accurate they are,” King said. “And then eventually to improve the next generation of remotely sensed products.”
One task remained at the first stop: snapping a photo with a highly specialized camera.
“And right now there's a cloud in the way,” he said. “So we'll wait for that to move out of the way so we can get similar illumination conditions to the satellite.”
Every five or so days, updated maps are uploaded to the public web application known as REACT, or the remote aquatic chlorophyll tracker.
Some Idaho water managers are intrigued by its potential to save time and resources monitoring.
The work typically involved in sampling is “very time-intensive,” said Elizabeth Spelsberg, a senior water quality standards scientist at the Idaho Department of Environmental Quality.
The satellite tool is “new enough that we haven't really started to utilize it yet, but it might really help in narrowing down locations for us to go sample,” she said.
King said that the tool is currently calibrated for Idaho water bodies.
“And as a scientist, you don't like to apply models outside of the bounds where they were developed,” he said. “But with validation data, on the ground observations, this product could be applied essentially globally.”
Even in its current state, it’s being used elsewhere in the West.
“It’s really been a game-changer these last few years,” said Kurt Carpenter, a fellow research hydrologist for the geological survey in Portland.
“Having the application calibrated for more areas could be helpful,” he added. “But there's really no reason to believe that the algorithm that was developed for Idaho here, that it wouldn't hold in other places. I mean, it seems to be working beautifully in Oregon.”
He said it’s helped him in his own research, and has allowed him and his colleagues to more efficiently choose sampling sites.
Let the sun shine
Back on the boat, the sun was starting to peek through. Florence readied the camera and King was set to take notes.
“We’re going to have a short window here, shall we?” King said.
“Let’s do it,” Florence responded.
The single-pixel device is capable of capturing dozens of wavelengths of light. It’s used to check images from the satellite and account for the impact of miles of atmosphere, according to King.
“And that’s everything,” Florence said, after snagging a few more images.
Data collected, the crew stowed the gear, and Carricaburu sped the boat toward the next site.
EDITOR'S NOTE: In a previous version, several images and quotes had John Carricaburu and Rob Florence swapped.
This story was produced by the Mountain West News Bureau, a collaboration between Wyoming Public Media, Nevada Public Radio, Boise State Public Radio in Idaho, KUNR in Nevada, KUNC in Colorado and KANW in New Mexico, with support from affiliate stations across the region. Funding for the Mountain West News Bureau is provided in part by the Corporation for Public Broadcasting.