Lake algae hangs around
WORTHINGTON — As colder temperatures worked to create a sheet of ice across Lake Okabena last week, passersby may have noticed something on the lake they hadn’t seen before — ice formations in hues of blue and green.
The coloring is the result of the continued presence of blue-green algae colonies in the lake. Okabena-Ocheda Watershed District Administrator Dan Livdahl said he’s never seen algae remain in the lake as long as it has this year.
“It really surprised me that for the last month, we’ve had algae from Sailboard Beach to the boat landing (in Ehlers Park),” he said.
Lake Okabena isn’t the only water body in southwest Minnesota experiencing the seemingly never-ending algal bloom of 2013.
“A lot of our lakes, and some of the Jackson County lakes, are greener than we would normally see this time of year,” said Ryan Doorenbos, area supervisor for the Minnesota Department of Natural Resources’ Windom Area Fisheries Office. “Some of our lakes have a real issue with being green.”
Doorenbos said the reason waters like Lake Okabena continue to suffer from algal blooms is that water levels remain a lot lower than normal.
“The idea of, the solution to pollution is dilution, if you have a lot less volume, your nutrients are more concentrated,” he explained.
High levels of nutrients such as nitrogen and phosphorus have long been known to fuel algal blooms in the prairie lakes of southwest Minnesota.
Thick mats of filamentous algae began gathering on Lake Okabena and Whiskey Ditch by mid-July this year — just two months after Livdahl announced water clarity in the local lake was the best it had been since testing began in 1998.
On May 23, he measured a maximum visual depth of 5 feet, 3 inches in the middle of the lake. By the time sailboarders were arriving in town for the annual Regatta celebration in early June, however, water clarity was already on the decline.
Improved water clarity, despite being something good for the lake, has a drawback in that it tends to lead to algal blooms.
“Clearer water means more light penetration and more algae growth,” said Livdahl. “When you have a bunch of nutrients and sunlight, something’s going to grow.”
The Okabena-Ocheda Watershed District has 16 years of water monitoring data. While it shows the various times over the years that algal blooms have surfaced, there’s no clear pattern. Sometimes they surface in June; other times a bloom may not occur until September — if at all.
Doorenbos said the late presence of algae this fall provides no indication of what may happen next year.
“Each year is different,” he said. “It does tell you there are plenty of nutrients available in the water to promote algae growth.”
The nitrogen and phosphorus getting into Lake Okabena and other prairie lakes comes from a multitude of sources, but agriculture gets much of the blame.
“We’re an agricultural-dominated market down here — that’s the livelihood,” Doorenbos said.
Despite soils that are naturally higher in nitrogen and phosphorus, farmers add more of the nutrients to grow crops that produce higher yields.
“We’ve seen a lot of tiling going in in recent years, and we have a lot of those nutrients finding their way into our waterways and then into our lakes,” Doorenbos said.
Livdahl agrees, but both said agricultural practices aren’t the only contributor to the nutrients that end up in the lake.
Lake Okabena, because it is within a municipality, is also impacted by water runoff into city storm drains, as well as lawn fertilizers and other chemicals that get washed into city streets, flow through storm sewers and end up in the lake.
“It is true that everybody contributes, but most of the land is ag land,” Livdahl said. “There’s probably a greater amount of pollution coming off urban land, but there are fewer acres.”
Finding ways to reduce nitrogen and phosphorus levels in Lake Okabena is an ongoing task of the Okabena-Ocheda Watershed District. Livdahl said the city conducts more street sweeping to keep excess nutrients from getting into the storm sewer system, and Worthington Public Utilities mails information with utility bills, asking residents to keep storm sewer grates free of debris, while discouraging vehicle washing on driveways, as the nutrients in soaps can impact lake health.
The city has also added a couple of rain gardens along the lake shore, and the E.O. Olson storm water pond constructed on the Minnesota West campus a year ago is capturing water runoff from 70 acres of agricultural land and impervious surfaces within city limits, filtering it before it reaches Lake Okabena.
As for farmers, Livdahl said the watershed district continues to provide incentive payments — in addition to what is offered through conservation programs — to encourage farmers to keep marginal land along Okabena Creek, north of Worthington, planted in grasses to act as filter strips.
“Just about every place we want a filter strip … has done that,” he said, adding that willing landowners who restored temporary wetlands through the Conservation Reserve Program are also helping to improve water quality as it flows south toward Lake Okabena.
Moving forward, the watershed district will encourage landowners to leave more residue in the fields and plant cover crops, which will utilize excess nutrients in the soil and better protect the land from wind and water erosion that can occur during the winter and into spring.
“If we want to reduce pollution, that’s what we need to do,” Livdahl said. “Fields are bare six to nine months of the year. Without protection, we have more runoff. It degrades the land, and we have more problems with water quality.”