Oh buoy! Watch out, Green Lake Association warns

Ben Siebers, hydrologic technician at the U.S. Geologic Survey; Mahta Naziri Saeed, graduate student at Michigan Tech; Dale Robertson, research hydrologist at the USGS; and Cory McDonald, assistant professor of civil and environmental engineering at Michigan Tech install a monitoring buoy on the west end of Big Green as part of a major lake study. submitted photo

by Alison Niescier

Take it from the unfortunate boater who accidentally struck and severed a monitoring buoy on Big Green last September:

Check your surroundings before going full steam ahead.

The orange and white device was one of two temporary monitoring buoys that were installed on opposite ends of Green Lake last year as part of a major study to better understand the processes leading to the lake’s low dissolved oxygen zone, or “dead zone.” This year, project planners from the Green Lake Association, U.S. Geological

Survey (USGS) and Michigan Technological University decided to install a single buoy on the west end of the lake near Sugarloaf.

Aside from reducing the odds of another buoy accident, installing one buoy versus two allows the team to double the monitoring equipment attached to a single device.

“We had buoys at each end of the lake last year in order to test whether oxygen levels are changing on one end of the lake faster than the other. Looking at those data, that doesn’t seem to be happening — oxygen levels are going down across the lake at the same time,” said Cory McDonald, assistant professor of civil and environmental engineering at Michigan Tech and project partner. “By doubling the monitoring sensors on our buoy, we can see with a finer scale what’s happening at these various lake depths.”

Green Lake’s invisible dead zone stretches across the entire lake and is present from about 30 feet to 60 feet below the water’s surface from late spring to early fall. The dead zone is not a health hazard and has not contributed to any fish kills.

However, the layer of low dissolved oxygen has been progressively worsening since about the 1970s. In response, in 2014 the Wisconsin Department of Natural Resources (WDNR) listed Green Lake as an impaired waterbody for this long-term degradation.

The team’s goal is to get ahead of the worsening trend and improve Green Lake’s water quality to the point of being able to “delist” Green Lake as an impaired waterbody.

“We are in the final year of a three-year lake study to determine factors driving Green Lake’s impairment,” explained Stephanie Prellwitz, executive director of the Green Lake Association. “Before we can improve this dead zone and remove Green Lake’s impairment listing, we need to know what is causing the dead zone and the type of strategies required to fix it.”

The 5-foot-long, foam-filled device may appear to be your average lake buoy bobbing along the south end of Norwegian Bay, but there is more than meets the eye.

Approximately 6 feet below the waterline, subsurface floats connect 100 feet of vertical monitoring equipment that provide high-resolution data documenting short-term fluctuations in oxygen, water temperature and seasonal trends.

Dale Robertson, research hydrologist with the USGS, will use the lake buoy data to create a computer model that simulates water quality changes in the lake and its complex processes. When combined with historic and new lake data, the computer model will describe how the temperature, nutrients and algae have changed in the lake over time. This will enable the researchers to predict the lake’s future water-quality conditions and the management strategies needed to improve Green Lake’s water quality.

After the monitoring equipment’s wild ride to the bottom of Green Lake and epic scuba diving recovery mission last September, the team is hoping for a less “active” monitoring season. In particular, the team asks that boaters slow down on the west end of the lake and keep an eye out for their orange and white buoy this summer.

The dissolved oxygen study is funded by a WDNR lake-protection grant with cash and in-kind contributions from the Green Lake Association, Green Lake Sanitary District and USGS.

Alison Niescier is the project manager for the Green Lake Association, a local not-for-profit that works to improve water quality for Green Lake.