Lake mixing is a natural phenomenon; in most Wisconsin lakes, it happens every spring and fall. During summer, distinct layers form, with a cool bottom layer (the hypolimnion) isolated from warmer surface waters. This pattern of mixing and stratification has been going on in Wisconsin lakes since the retreat of the glaciers. But in other parts of the world, mixing follows a different schedule. For example, NTL researchers Trina McMahon, Tim Kratz, and graduate students Stuart Jones and Ashley Shade, documented erratic mixing of Yuan Yang Lake in Taiwan caused by typhoons through their involvement in the NTL Microbial Observatory (MO) and the Global Lakes Ecological Observatory (GLEON). The unexpected mixing events reshuffled microbial communities, but recovery was rapid.
These dramatic events gave McMahon an idea: what would happen if a typhoon hit Wisconsin? Thus was hatched a plan that involved collaboration between engineering and limnology, and between NTL, MO, and GLEON research.
McMahon and Shade honed their question to ask how microbial communities would respond to the novel disturbance of summertime lake mixing. They enlisted the services of another NTL graduate student, Jordan Read, to figure out how to mix a lake--and to do so without stirring up sediments or bubbling in air that might overly disrupt the microbes. The solution: the Gradual Entrainment Lake Inverter, or GELI (like the donut). An aluminum ring was fitted with a PVC hose so that air could be added or removed, and a geo-membrane was stretched across the ring like a trampoline. Adding air causes the GELI to rise; removing it causes it to sink. Rising pulls cold water from the hypolimnion up while falling pushes warm surface waters down to mix the lake.
Two GELIs were deployed in North Sparkling Bog, along with a GLEON buoy, to monitor lake dynamics, in 2008. After 5 days of floating and sinking, the GELIs converted the stratified lake into a thermally uniform system. Mixing produced a number of changes, some expected and others unexpected. The microbial communities were drastically altered, but recovered to pre-mixing conditions within 20 days. The lake re-stratified quickly, but unlike the microbes, was not quite the same for the rest of the summer. Both during and after the mixing, the hypolimnion reached temperatures that were likely never been experienced by the lake. The normally cool, 10° water reached levels as high as 20° during and after the peak mixing period. And this led to another idea, this time posed by Jake Vander Zanden: could mixing be used to warm up a lake to get rid of an invasive fish that needs cold water?
So now, the GELIs are being moved up the road, and in 2011, will be put to work in Crystal Lake in hopes of eliminating rainbow smelt, a non-native fish that preys upon and outcompetes several native species. Mixing the lake should temporarily eliminate the cold or deepwater refuge that rainbow smelt alone occupy, and will allow Vander Zanden and colleagues to determine if ecosystem changes caused by smelt invasion are reversible. Crystal Lake is one of 11 NTL core study lakes, so 30 years of long term data provide the storyline of pre-smelt conditions and how the lake changed in response to smelt invasion in the 1980s.