Cleaning up Lake Okabena
WORTHINGTON -- The state of Lake Okabena's health took center stage during the Okabena-Ocheda Watershed District's meeting earlier this week. For years, the Worthington lake has produced pungent summer algal blooms and given off a murky appearanc...
WORTHINGTON - The state of Lake Okabena’s health took center stage during the Okabena-Ocheda Watershed District’s meeting earlier this week.
For years, the Worthington lake has produced pungent summer algal blooms and given off a murky appearance - far from the clear, clean waters one typically expects to see in Minnesota’s Land of 10,000 Lakes.
On Tuesday, limnologists with Wenck Associates presented the results of their year-long study of Lake Okabena and delivered options for the OOWD board to consider. The lake data, which was part of a story in the Feb. 20 edition of the Daily Globe, will become part of a Total Maximum Daily Load (TMDL) study being written for the entire Missouri River Basin in Minnesota. That TMDL report should be completed in the summer of 2016, and will ultimately set daily limits on the amount of nutrient and sediment loading in the lake.
It’s proven science that reducing the amount of phosphorus in a lake will result in a reduction in algal growth, but shallow lakes are more complex, said Wenck’s Joe Bischoff.
“It isn’t just removing phosphorus that makes the lake more clear,” he said.
Lake Okabena is a shallow lake, and in its turbid state, it produces a lot of algae, has low aquatic plant productivity and few zooplankton. If the lake were to transition to a clear water state, people could expect to see a large amount of aquatic plants, low algal productivity and a large population of zooplankton.
“The lake wants to go one way or the other - a turbid state or a clear water state,” Bischoff said. “There are factors that drive it from one state to another.”
To get Lake Okabena from a turbid state to a clear water state will be a challenge and involves many processes. Bischoff said it may include a drastic measure such as a whole lake drawdown to rid the lake of fish and create an environment for aquatic plants to grow.
“We can’t focus just on the nutrients - we have a lot of pieces to the puzzle,” he said.
The results of the study, as reported last month in the Globe, pointed to the agricultural landscape as the primary contributor of both nutrients and sediment in Lake Okabena.
“It’s very episodic,” said Bischoff, adding that based on last year’s data, 68 percent of phosphorus loading and 65 percent of solids entered the lake from rural areas.
“Those very intense storms, that’s when you’re getting a lot of the nutrients (in the lake),” he shared.
Bischoff told board members they have done good work in the district to encourage crop producers to implement Best Management Practices (BMPs) on their land. Even with those conservation practices in place, there are still significant amounts of nutrients and sediment ending up in Lake Okabena.
“Other (practices) may need to be considered - cover crops or conservation tillage,” he recommended.
Additional opportunities for the watershed board to reduce excess nutrient and sediment loading could include sedimentation or treatment ponds on the Prairie View Golf Links property. The Okabena Creek flows through the golf course, and by creating a larger area for water to collect before moving into Lake Okabena, some of the nutrients and sediment coming from agricultural lands could settle out.
The report stated the city generates 15 percent of the total phosphorus and 8 percent of the total suspended solids that end up in the lake, while streambank erosion contributes about 7 percent of the total suspended solids. Other sources of nutrient and sediment loading include wind deposition.
“You have a big lake - a big surface area - so it can collect a lot of that sediment,” Bischoff said. “A five-mile radius is the typical area where you can expect that to come from.”
Based on the data collected, Bischoff told the board there is “still a lot of work to do” in the watershed.
From turbid to clear water
Among the things the watershed district has done right, aside from incentivizing farmers to plant buffers along streams and incorporate other conservation practices, was conducting a significant rough fish harvesting event in 2006, and disconnecting the channel that flowed from Herlein-Boote Slough into the Okabena-Ocheda Watershed District.
Both of those events dramatically reduced the amount of carp in Lake Okabena and created what Bischoff said is now a “pretty balanced fishery with predator and panfish.”
While rough fish levels are now manageable, there remains a lack of aquatic plant growth in the lake to move it toward a healthy, clear water state.
The district could consider an alum treatment, which involves releasing aluminum sulfate into the lake. The alum would latch onto the sediment and bind up the phosphorus, which could prevent an algal bloom from occurring, Bischoff explained.
“You’d be looking at half a million to a million dollars to do an alum treatment on Lake Okabena,” he said. “If you don’t get the plant response, we’re still going to be in a turbid water state.
“I wouldn’t tell you to go out and do an alum treatment right away,” he added. “It’s an expensive process for something that’s pretty uncertain.”
The most drastic measure would be a complete lake drawdown, allowing time for the lakebed to dry out, become reinvigorated and promote reestablishment of aquatic plants.
“It’s a big deal to do this,” Bischoff said, explaining that a drawdown would require 75 percent landowner approval.
“If the plants don’t respond, that’s a lot of liability,” he said, adding that he’s talked to people with Ducks Unlimited who have conducted drawdowns in the past and been told there’s never been an instance where aquatic plants didn’t grow after a drawdown. (Drawdowns were conducted on Fulda Lake and Lake Maria, both in Murray County, with successful plant development.)
Bischoff said having a clear water lake with aquatic plants can lead to challenges. Some communities have purchased harvesters to limit aquatic plant growth on the surface of the lake.
As OOWD board manager Jay Milbrandt pointed out, people have to mow their lawn and with aquatic plants growing in the lake, the city may have to mow the lake to allow for recreational use.
“It’s impossible to have a sandy bottom, no plants and clear water,” Bischoff said.
Milbrandt asked what could be expected as the optimal state of Lake Okabena. Bischoff replied that the Minnesota way is for lakes to be all things to all people, thus the lakes would be dominated by native aquatic species, but not so robust as to impact recreation.
No quick fix
Summarizing his presentation, Bischoff recommended the watershed district consider creating a large water treatment system (sedimentation pond) on the Prairie View Golf Links property to settle out the nutrients and sediment. Next, the board might consider the alum treatment or a lake drawdown to encourage aquatic plant growth.
Lake drawdowns were a natural occurrence before the construction of dams, and to improve the health of the lake the water bodies should be drawn down every five to seven years, Bischoff said.
“I wish the tools were easier,” he added. “You’ve got to keep at it and try things and hope that you don’t spend a lot of (time and money on projects) on a lake and it isn’t successful.”
He discounted dredging as an option on Lake Okabena, saying it would take 10 years and millions of dollars to dredge enough from the lake to improve water quality.
“I hope that I didn’t make it sound hopeless,” Bischoff said of fixing the impaired lake. “What we want you to know is that it is a challenge.”
During the watershed board meeting that followed the Wenck presentation, board members:
- Approved $5,000 for the purchase of floating mats, grasses, forbs and additional materials to construct floating islands to be launched in Sunset Bay this spring.
- Approved a $500 contribution for a soil health and cover crops workshop offered March 12 in Worthington and March 13 in Marshall.
- Appointed the following individuals to the Okabena-Ocheda Watershed District’s advisory committee: Ben Krohn, John Ahlers, Rich Fellows, Jim McGowan, Doug Anton, Eric Roos, Galen Gordon, Paul Langseth, Dwayne Haffield, Bob Demuth Jr., Robert Rohrer, Scott Rall and Keith Schroeder.
- Approved a letter of support for Gov. Mark Dayton’s proposal to seek 50-foot buffer strips on all Department of Natural Resources-protected waters.
- Discussed plans for the district’s property on the south side of Worthington. The St. John’s property will be planted to soybeans this year, with a buffer of native grasses and flowers around the perimeter. Plans are to plant cover crops after the soybeans have been harvested.
- Adopted the watershed district’s 2014 annual report. The report will be posted to the district’s website in the near future.