Bad bloom: Why is Lake Okabena green?
WORTHINGTON -- Mats of blue-green algae mimicked the appearance of an oil spill along the shores of Lake Okabena and into Whiskey Ditch Monday morning, bringing with it the rotten-egg-like odor caused as the bacteria decay.
Recent days of hot, humid weather and little wind have caused a major algal bloom on the lake. From a distance, it appears the lake has taken on a greenish hue, but an up-close inspection shows billions of filamentous algae floating in the water.
Both the Okabena-Ocheda Watershed District and Bioverse, a local company that works to treat algal growth in ponds, are encouraging people and their pets to stay out of the water until conditions improve.
"People should be aware that (algal blooms) are potentially poisonous to animals," said OOWD Administrator Dan Livdahl on Monday. They are harmful to people too, especially if the water is ingested. Most likely, people could suffer irritation due to skin rashes or respiratory problems.
Livdahl said the concentration of algae in the lake began a couple of weeks ago. What people are seeing on top of the water is cyanobacteria, which is the bacteria that forms a floating mat and smells really bad as it decays.
Cooler temperatures and wind often helps to break up the algae and cause it to disappear to the bottom of the lake. With cooler temperatures in the forecast for the remainder of this week, the hope is for the decaying bacteria to leave the shoreline.
Livdahl has been monitoring algal blooms on Lake Okabena for about 20 years. This year, the watershed district is working in partnership with Bioverse, a relatively new company to Worthington, that manufactures biodegradable products to help keep algae from forming in contained water bodies like golf course ponds and ornamental ponds.
While Lake Okabena is far from being a contained water body, Bioverse Chief Technical Officer Connie Schmidt said they are conducting new research to find ways in which Bioverse products can be successful in the local lake setting.
"The issues we have are that the lake is so large," Schmidt said, adding that Bioverse products at this time aren't applicable for the size of Lake Okabena. As a result, the company is doing much of its testing in Whiskey Ditch, as well as in stormwater ponds in the community. Bioverse products used in the Glenwood Heights stormwater pond, for example, are meeting the company's expectations.
"We don't make any algae claims as a company," Schmidt cautioned. "We capture the particles that are creating accumulated organic matter ... and we're trying to convert that bacteria."
Monitoring the bloom
Bioverse employees began monitoring Whiskey Ditch on July 1 for pH and dissolved oxygen levels. The ongoing algal bloom has significantly depleted the oxygen in the ditch, particularly in the area between the lake and 10th Avenue. Near the 10th Avenue bridge, a boom collects the floating bacteria, helping to keep most of it from flowing farther north in the ditch, closer to the residential area.
Schmidt said Bioverse is trying a new formulation of its Biosphere products, which are all-natural and environmentally friendly, to see if the amount of slime and scum can be reduced in the ditch, thereby also reducing the odor caused by decaying algae.
Despite the work being done, Schmidt said there's a "very low probability that we're highly successful."
The problem is that the ditch collects water from the lake, as well as water flowing in from the north. Water volume is constantly changing, and water volume is what the company uses to determine how much product is needed to treat a water body.
"When we have a ditch like this, we can't tell the bacteria to stay in one area," said Bioverse CEO Glenn Thuringer. "We probably can't dose it high enough because of the connection to the lake."
Both Schmidt and Thuringer are grateful for the partnership with the watershed district and the ability to test their natural products on the algal blooms. The watershed district is contributing financially to the products, although a large part of the funding is covered by Bioverse because of the research the company is conducting.
While algae blooms on Lake Okabena typically happen in late summer, a mid-July bloom is not uncommon.
"It's pretty normal when you get hot, still conditions, they will appear," said Livdahl, adding that they often appear in August and remain until the end of the season.
In addition to working with Bioverse to reduce the severity of algal blooms in Lake Okabena, the watershed district is also testing its floating islands.
In place in the E.O. Olson Stormwater Pond on the campus of Minnesota West Community and Technical College, the four floating islands are thriving this summer, with numerous grasses and forbs growing on them.
"Last week we had the swamp milkweed flowering and the black-eyed Susans are flowering," said Livdahl.
During this year of trial and error, he said it's difficult to say whether the islands are doing as they are intended -- to reduce the amount of nutrients in stormwater runoff before it flows into Lake Okabena. The four islands cover just half a percent of the total area of the stormwater pond, and it is recommended to have 10 percent coverage in order to see measurable water quality benefit.
"That said, the plants are using nutrients as they grow," Livdahl said.
In the first few months since the islands were installed, the stormwater pond has attracted wildlife, including two pairs of geese that raised goslings on the pond and a booming frog habitat. Livdahl said he's also seen numerous schools of minnows along the edge of the pond, but unfortunately those could be young carp.
"We are going to work with the DNR to stock something in there that will prey on carp eggs," he said, adding that the carp likely got into the stormwater pond earlier this summer, when high rain events increased the flow of water into the lake. The carp used that high water to swim upstream and into the pond.
"My opinion is a carp could swim up a fire hose, given enough attempts," Livdahl said.
Plans are to catch some of the minnows and identify the species. As for stocking the stormwater pond, Livdahl said the DNR could put any kind of predator fish in the pond to feed on carp eggs, such as sunfish, bluegills and others.
Meanwhile, the islands will remain anchored in place on the pond through the fall and winter, with hopes that the plants growing from the floating mat will emerge again in the spring.
"If they survive the winter and come back next spring, I think we'll look at adding additional islands," Livdahl said.
At an expense of $24 per square foot of floating island -- without factoring in the cost for plants, seed and fencing to keep the geese out -- it may prove too costly.
"If we were to scale this up, we would need to find a way to do it more cheaply," Livdahl said, adding that to get 10 percent coverage by floating islands on the stormwater pond, it would cost about $140,000.
"(That) sounds like a lot of money, but when you look at removing nutrients from ponds, it's fairly inexpensive," he said.