Measuring turbulence to keep Lake Michigan healthy
Like many other ecosystems, Lake Michigan has been infested by invasive species. To evaluate their impact, Qian Liao builds computer models that simulate the lake’s complex interplay of physical and biogeochemical processes, including its turbulence: the small-scale pattern of moving water caused by random changes in pressure, temperature and wind that is especially challenging to describe.
In 2008, Liao, Associate Professor of Civil and Environmental Engineering, deployed in Lake Michigan the first prototype of a compact device he designed to measure turbulence in natural environments: the underwater miniature particle image velocimeter (UWMPIV), essentially a waterproof digital camera and powerful laser mounted on a floating platform. In contrast to PIVs for laboratory settings, it is lightweight, battery-powered and can be left unattended for up to eight days to periodically record the movement of tiny particles at the water surface or near the bottom.
“The UWMPIV illuminates particles with a 2D laser sheet and photographs them in rapid succession so that image processing software can resolve the instantaneous velocity distribution in the entire 2D plane, and soon in 3D space as well,” Liao explains. “This is a great improvement over traditional field instruments that measure turbulent velocity at single points within a conceptual water column.”
Since more accurate estimates of turbulence result in more realistic simulations, Liao and Harvey Bootsma, a professor at the UWM School of Freshwater Sciences, are now using these models to predict the long-term impact of quagga mussels on phosphorus cycles, with funding from the National Science Foundation.
The quagga mussel, an especially notorious, cold-tolerant invasive species that colonizes the lake bottom, feeds on the floating collection of miniscule plants (phytoplankton) that forms the bottom of the aquatic food chain, along with tiny animals (zooplankton). The mussel’s enormous appetite negatively affects organisms higher up in the food chain.
By releasing dissolved phosphorus, the mussel also causes eutrophication at the bottom of Lake Michigan. This stimulates the growth of macro-algae in the near-shore area and may contribute to the formation of large-scale harmful algae blooms. To reduce the mussel’s ecological impact, regulatory agencies employ policy tools, such as nutrient management and fish stocking—and these tools rely heavily on Liao and Bootsma’s computer models.
“Our existing model uses circulation, winds, temperature and other measured parameters to generate a long-term 3D simulation of Lake Michigan,” Liao says. “The UWMPIV adds realistic boundary conditions to help answer our key questions: How much phytoplankton will the mussels consume and how much phosphorus will they release during the next 10 to 20 years?”
Liao has no shortage of future research plans. Measuring the carbon dioxide exchange rate between the atmosphere and oceanic surfaces could improve climate change models; and new techniques for water flow measurements in rivers could improve flood prediction models.
–by Silke Schmidt