Skimming the surface
WORTHINGTON -- Rain and cooler temperatures have nearly cleared up the visible blue-green algae on Lake Okabena in the last week, but with temperatures predicted to reach the upper 80s and 90s as early as next week, it seems only a matter of time before the green goo makes another appearance.
With the cause of the unwelcome algal blooms coming from potentially numerous sources, the Okabena-Ocheda Watershed District continues to look for ways to either reduce the severity of the blooms or eliminate them entirely.
That goal led several OOWD members to Plymouth a week ago, where they viewed an eco-harvester in action on Schmidt Lake. With a rolling, cutting drum at the front of the harvester, plant growth is clipped at the surface and carried up a conveyor belt, ultimately getting dumped in a box and removed from the lake. Video of the eco-harvester in action can be seen at weedersdigest.com.
The harvester is also marketed as an algal skimmer, which is why OOWD members wanted to view the machine.
"Two years ago we had some fairly substantial floating algae blooms and the question was, could those be skimmed off the top of the water," explained OOWD Administrator Dan Livdahl. "This equipment can skim weeds, but we don't know if it can skim algae."
During last week's demonstration by Mike Faber, of Weeders Digest -- a retailer of the eco-harvester -- Livdahl, along with OOWD board members Rolf Mahlberg and Jay Milbrandt, were told the machine was used to harvest algae on a lake in New York state recently with success. Video was taken of the process, and the watershed district is waiting to review it.
"The question I have is whether the algae we have -- that doesn't stick together, it's mostly water -- whether you'd be able to pick that up with a skimmer," Livdahl said. "If it was shown to work, the machine itself is pretty reasonably priced."
At $45,000, Milbrandt said due diligence will be done before making a decision to invest in a machine.
"We'd have to be confident before we purchase one," Milbrandt said. "If it does what we think it might do to harvest the algae, I think it's something worthwhile to consider further."
Milbrandt said the technology is valuable enough for the watershed district to explore, although it won't be the total solution to Lake Okabena's algal issues.
"It isn't going to solve the ultimate problem ... but it could be something that helps bridge the gap," he said. "We send people out to mow the grass. It's similar to that process -- we need to send someone out to mow the lake.
"Our algae gets pretty thick on Lake Okabena," Milbrandt said. "There are times you can put a stick in the water and pull up a big glob of algae."
The filamentous algae, which gathers in large mats and can now be seen in Whiskey Ditch, could be cleaned off with the weed harvester, but Livdahl said the challenge would be getting the equipment into the ditch.
"It would need to make it under the foot bridge, and under normal conditions you wouldn't be able to take this particular equipment into Whiskey Ditch," Livdahl said.
The water clarity in Schmidt Lake, with the use of both the eco-harvester and a total-lake aeration system, is significantly better than the clarity in Lake Okabena. Aerators have been used in the lake for the past two years.
The clarity, however, may not be due entirely to the aeration and eco-harvester, Livdahl cautioned, saying that the 37-acre Schmidt Lake is considerably smaller than Lake Okabena's 750-acre expanse.
"We have a secchi disk reading of about half a meter and they get secchi disk readings of about two meters," he said. "There's also about one-third less phosphorus in their lake."
Lake depth is another variable. Schmidt Lake has a maximum depth of about 27 feet, while Lake Okabena has a max depth of 12.5 to 13 feet where the lake was dredged years ago. The average depth is closer to 7 feet.
"(Schmidt Lake) is also in a more sheltered area -- it doesn't have the wind action of Lake Okabena," Livdahl shared.
The OOWD had hoped to begin a water quality monitoring program on Lake Okabena this year in cooperation with the Minnesota Pollution Control Agency.
Developed by Wenck & Associates, the plan calls for monitoring the internal load on the lake -- whether nutrients in the lake are coming from bottom sediment or another source -- as well as water quality coming into the lake and going out.
Contributors of sediment -- such as farm fields, stream banks and wind-blown dust -- will also be considered, as well as the materials that get into the city's stormwater system.
"The MPCA will use the data that's collected when they do the TMDL (Total Maximum Daily Load) study for this part of the state," Livdahl said.
The study, estimated at approximately $70,000, could be partially funded by the MPCA, and Livdahl said that is the delay at this point. He anticipates a $20,000 to $25,000 commitment from the agency.
While some watershed board members and city employees have said they don't need another study, Livdahl said the information could prove valuable to addressing the water quality issues in the lake.
"I think these are really important questions -- questions we didn't answer 20 years ago ... with the Clean Water Partnership Study," he said.