From sky and lake, researchers study blue-green algae
It’s an unseasonably warm, early-April Friday afternoon in Madison, Wisconsin, and the calm day polishes the Lake Mendota surface to a sheen that’s just shy of glass.
A vintage flat-bottomed Boston whaler chugs slowly away from a dock adjacent to the famed University of Wisconsin-Madison Memorial Union Terrace. Soaking up the sun and expansive lake views, hundreds of Terrace-goers stare bemusedly at the whaler: Bobbing at the end of its tow rope is a 6-foot-tall, sunshine-yellow oceanographic buoy.
Its destination is the geographic center of Lake Mendota — essentially, the lake’s deepest point.
Once anchored to metal weights nearly 80 feet below the surface, this important tool (affectionately dubbed “David Buoy”) will help UW-Madison faculty, staff and student researchers track toxic blue-green algae in what is one of Wisconsin’s deepest — and, thanks to the university — most studied lakes. (Lake Mendota is a key site in the Global Lake Ecological Observatory Network, or GLEON.)
The buoy regularly and automatically collects meteorological and water-quality data and transmits it wirelessly back to land, where researchers in engineering, limnology and space science couple that information with UW-Madison satellite images of Lake Mendota. “We’re trying to forecast blue-green algae blooms based on what’s happening outside the lake,” says Trina McMahon, an associate professor of civil and environmental engineering and bacteriology at UW-Madison. “Being able to track how well we’re doing with that forecasting means measuring what’s actually going on.”
Above the water, the buoy detects wind speed, wind direction, air temperature and relative humidity. Below the surface, there are individual sensors that register the amount of oxygen in the water; detect algae; and identify phycocyanin, a pigment that signals the presence of blue-green algae. In addition, a string of sensors spaced every three feet measures the water temperature from lake surface to bottom. Every few days, McMahon’s students also hand-sample water from Lake Mendota and use powerful genetic detection methods to identify the blue-green algae. All of these measurements are key to learning more about how blue-green algae grow and move throughout the lake. Ultimately, the researchers hope to accurately predict where and when a bloom will occur.
However, one limitation of using the buoy to forecast a blue-green algae bloom on a 10,000-acre lake is that the buoy is anchored in one spot. That’s where satellite imagery comes in, says Robert Holz, an assistant scientist in the UW-Madison Space Science and Engineering Center (where a cluster of eight satellite dishes atop the building ingests data from NASA, National Oceanographic and Atmospheric Administration and other satellites).
Focused mainly on weather and climate, some Space Science and Engineering Center researchers write computer algorithms that process satellite data and generate real-time information. Organizations worldwide leverage the information and images for everything from generating weather forecasts and monitoring fires to predicting hurricanes and studying the Arctic and Antarctic. Satellite images also provide clues about what’s living in the lake. “When you look at Lake Mendota, you’ll see that it changes color,” says Holz.
Sunlight reflects off a lake differently for each of its biological constituents. For example, a green lake means lots of phytoplankton; a preponderance of blue signals a toxic blue-green algae bloom. “Your eyes are remote sensors; your brain is a remote sensor,” says Holz. “Satellites do the same thing — but they can divide up that light and different colors into very discrete wavelength bins.”
Space science and engineering postdoctoral researcher Colleen Mouw is developing algorithms to help make sense of the satellite data. Her software will derive from the data the concentration of biological tracers related to the type and concentration of algae in the lake.
Combined, the manual water samples, data from the buoy, and satellite images can help paint a more accurate picture of the ecology and water quality in Lake Mendota. Ultimately, the data also will help “calibrate” satellites for this kind of finely tuned biological remote sensing in other lakes, without the need for the buoy. “The idea is to scale all the way from genes to satellites,” says McMahon.
In its two previous seasons on Lake Mendota, the buoy has gained somewhat of a cult following. Real-time weather and water conditions are publicly available at metobs.ssec.wisc.edu/buoy and via the iPhone application “Lake Mendota Buoy Data.” McMahon and Holz say they’ve heard positive feedback from many public data users, including fishermen who find game fish based on the water temperature, crew teams and sailors interested in wind speed, and the National Weather Service, which exploits the buoy’s wide-open location to collect more accurate peak wind-speed information during severe thunderstorms.
The buoy remains in Lake Mendota until mid-November, when thunderstorms generally turn to snowstorms and frigid water and impending ice dictate its removal.