Monday, December 16, 2013

Too Much of a Good Thing in Lake Owasso?

Lake Owasso, late November.  Photo by Sage Passi

By Bill Bartodziej

As we all prepared for our Thanksgiving celebrations, something very disturbing was taking place on Lake Owasso. Nearby residents ventured onto the lake to take a break from the holiday hustle, and many were shocked to see dead fish scattered just under the newly formed ice. Immediately after the discovery, residents made phone calls and shot off emails that alerted state and local agencies. News crews were on the scene to cover the story.  Since the onset of the reports, managers and research scientists have been working to determine what caused the fish kill. Below is an update.

The fish kill was initially investigated the Saturday after Thanksgiving (November 30th) by DNR biologists. A large number of dead game fish, primarily panfish, were observed in Owasso. Newspapers articles showed dead largemouth bass, walleye, and muskie just under the ice. Fish were found mainly in the shallows, scattered around the lake.

Photo courtesy of TJ DeBates, MN DNR East Metro Fisheries Supervisor

A large muskie beached on the shore of
Lake Owasso.  Photo by Sage Passi

Is it possible that a chemical was responsible for the kill? Well, there were no outward signs or reports of a chemical spill around the lake. Aquatic plant control was last conducted in August, and the herbicide would be at very low or undetectable levels going into fall.

Fisheries biologists doubt that a fish disease was the culprit. However, samples of dead fish were brought to DNR’s pathology lab for analyses. Results should be available in a couple of weeks.

Until these other things are officially ruled out, the most plausible explanation for the kill has to do with too much of a good thing- oxygen.

Fish need ample but balanced oxygen in the water to survive, so it was one of the first things scientists tested for. Dissolved oxygen testing on Saturday revealed high levels in and around the shallows where dead fish were identified. Additional oxygen testing was performed by the Ramsey County Public Works Environmental Services staff on Monday, December 2nd. Again, test results pointed to normal to high dissolved oxygen levels.  

Although uncommon in lake systems, water can actually accumulate too much gas, like oxygen, under certain conditions. This phenomenon is called “super-saturation.”

The super-saturation fish kill hypothesis that is currently being talked about has to do with photosynthesis and ice formation. The thesis goes like this: There were unusually high levels of algae (phytoplankton) in Owasso going into fall. As an early-season ice sheet formed on the lake, we experienced sunny periods without any snow cover. With these conditions, the algae likely had a high rate of oxygen production (through photosynthesis), and this oxygen could not diffuse into the air because of the newly formed ice sheet. With a lot of algae, a clear ice sheet, and sunny conditions, it is possible to have super-saturated oxygen levels in lake systems.

Photo courtesy of TJ DeBates, MN DNR East Metro Fisheries Supervisor

With the excess oxygen levels in the water, fish experience symptoms like the “bends” that scuba divers suffer when they ascend too quickly. Gas bubbles form in the blood stream, causing a blockage in blood flow, which can eventually lead to death. This condition, however, is quite difficult to detect in nature, due to the gas bubbles dissipating quickly after a fish dies (usually less than 24 hours). Because of this, it is too late to run tests on the Owasso fish and gain meaningful results.

Although uncommon, fish kills have been reported on other lakes in the Midwest immediately after ice formation. For instance, a similar occurrence happened on Forest Lake last year. Dead fish were tested for disease, but nothing turned up.

TJ DeBates, DNR’s East Metro Fisheries Supervisor, will be evaluating the extent of the fish kill in the spring. He states that DNR may adjust the stocking rates of walleye and muskie to compensate for any substantial game fish loss that was experienced from the unusual fish kill.

This kill has reminded all of us that our urban lakes are complex systems. Although we can’t say with certainty that too much algae caused the fish kill, we can state that it is a condition that we just don’t want to see. Watershed, lake management, and individual homeowner activities are critical in working to bring phosphorus levels down, which will in turn, reduce algal blooms.

If recent fish tissue or water quality testing reveals any novel findings, we will let you know.

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