Isolated Green Lake buoys measure water quality

The continuous monitoring buoys provide high-resolution data documenting short-term fluctuations in oxygen, water temperature and seasonal trends. submitted photo

by Alison Thiel

Those recreating near Green Lake’s Dartford Bay or Sandstone Bluff may have noticed a new presence floating by.

The areas have become home to two temporary monitoring buoys taking continuous water-quality measurements as part of a major lake study.

“The buoys suspend dissolved oxygen sensors at multiple depths in the water column,” explained Cory McDonald, limnologist at Superior Hydroscience. “They provide high-resolution data documenting short-term fluctuations in oxygen and water temperature as well as seasonal trends.”

The data collected will provide valuable insight into the processes leading to Green Lake’s low dissolved-oxygen zone, or “dead zone.”

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 WDNR recommends that lakes have a dissolved oxygen concentration of at least 5 milligrams per liter. In August 2014, for example, this zone in Green Lake had a concentration of 0.3 milligrams per liter, or nearly 17 times lower than the recommended concentration.

McDonald explained that the team has several theories behind Green Lake’s low dissolved-oxygen layer, most directly or indirectly tied to phosphorus.

However, the exact mechanisms causing the dead zone are not known. Research is required since the lake phenomenon is rare and little is known about its cause.

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 second year of a three-year lake study to determine factors driving Green Lake’s impairment,” said 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 why it is happening, the type of solutions required, and the scale that we need to implement those strategies.”

Dale Robertson, research hydrologist with the U.S. Geological Survey (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,” Robertson said.

This will allow the researchers to predict the lake’s future water-quality conditions and the management strategies needed to improve Green Lake’s water quality.

“There is no doubt that good things are happening in Green Lake and the Green Lake watershed,” Prellwitz said. “We owe it to the community to balance the duty to act now to improve water quality with a responsibility to make sure our strategies are on target and based on sound science.”

McDonald agrees with this parallel-path strategy. “It is exciting to see a commitment to both current efforts in the watershed to improve water quality and also to better understand the lake to guide future actions.”

This lake study is made possible by a grant awarded by the WDNR to the Green Lake Association with substantial support contributed by the Green Lake Sanitary District and USGS.

Alison Thiel is the Project Manager for the Green Lake Association, a local not-for-profit that works to improve water quality for Green Lake. The GLA is a guest columnist on behalf of the LMP team.