Lake Okabena's odd odor
WORTHINGTON -- Unseasonably warm temperatures and clear skies have been a perfect combination for farmers working to get this year's crop into the bins, but they have also lead to a later-than-usual algae bloom on Lake Okabena.
Since Saturday, visitors to the eastern shore of Worthington's lake have encountered a foul smell and a layer of slime that, except for its color, could mirror an oil slick. The large mats of bubbly brown, turquoise and milky white hues are dead and decaying cyanobacteria, otherwise known as blue-green algae.
Water clarity on Lake Okabena typically begins to improve by mid- to late-September as the cooler weather moves in and water temperatures drop into the lower to mid-50s. On Sunday, the water temperature was in the upper 60s to lower 70s, helping to fuel the late-season bloom. Another likely contributor was the nearly 5-inch rainfall in late September.
"The rainfall brought all sorts of nutrients into the lake," said Okabena-Ocheda Watershed District Administrator Dan Livdahl Monday morning. Those nutrients come from a variety of sources -- the lawns along the lakeshore, leaves and grass clippings carried through the city's storm drains and sediment washing off of farm fields.
"We don't have a measure of how much soil washed into the lake," he said. "The crops were still in the field, which prevented a lot of soil from washing in."
Sediment that washes into the lake carries with it phosphorus, which in essence feeds algae present in the lake. One pound of phosphorus can produce 10,000 pounds of algae, according to one Soil and Water Conservation District website.
While the current algae bloom on Lake Okabena is the worst it has been all year, Livdahl said there seemed to be more algae present in the lake throughout the summer months.
"This year, like last year, we had some excellent water clarity from June through August," he said.
The increased water clarity leads to improved light penetration, which helps to fuel algae growth. It's basically a catch-22 -- people want improved water clarity, but they don't want the algae blooms.
Livdahl said the eastern third of the lake was "considerably greener" than the western shores over the weekend. The southwesterly winds are to thank for that -- they blew the filaments easterly, making it possible for algae clusters to form and gather in mats as they floated toward the eastern shoreline.
People and their pets should stay out of the water when floating algae is present, as it has the potential to be toxic, Livdahl said. Toxic blooms can cause skin and eye irritation, and could lead to allergy flare-ups.
Keeping it clean
Though it's late in the season for algae blooms, Livdahl said it's never too late to implement practices that reduce the amount of sediment getting into the lake.
"The only thing citizens can really do is try to keep nutrients (grass clippings and leaves) in their yards," he said. That includes keeping soil from getting into the streets during landscaping or construction projects. Anything that gets into the lake via storm drains and runoff this time of year will add fuel to next year's algae blooms.
As for the current bloom, the decaying algae will eventually get mixed back in the lake through wind and wave action and settle as lake-bottom sediment.
The Okabena-Ocheda Watershed District continues to evaluate studies to reduce algae blooms in Lake Okabena. In mid-September, an Associated Press story detailed a project on McKusick Lake in Stillwater, where bales of barley straw had been placed in the lake this spring to combat algae blooms. An enzyme in the barley straw inhibits new algae growth, the article said.
Livdahl said Lake Okabena is approximately 17 times larger than McKusick Lake, and would require about 12,000 pounds of barley straw for just one treatment. It is suggested the bales be replaced every three to six months. Also, they would need to be submerged and anchored in place, creating water hazards for recreationists on the lake, he added.