Growth of Eurasian watermilfoil, a non-native, invasive aquatic plant, poses a threat to recreation at Waubee Lake in Kosciusko County.
But DNR officials say more watermilfoil is not necessarily bad given the scarcity of other, native plants in the 187-acre lake.
Based on sampling by DNR fisheries biologists, coverage of Eurasian watermilfoil in Waubee Lake has increased five-fold since 2010. It is now the second most abundant plant, up from ninth in 2010. Where present, it is also denser than in 2010.
Last summer several near-shore areas were off-limits to boaters and swimmers where watermilfoil formed thick mats on the surface. Concerns are that it could spread even more.
The good news is that watermilfoil in Waubee Lake may help keep the water clean by tying up nutrients that otherwise might go into producing algae blooms.
Clean water allows sunlight to penetrate deeper in a lake, producing more oxygen from photosynthesis and improving fish habitat.
Water clarity last year at Waubee Lake was the best it had been in years. Objects as deep as 13 feet were still visible in August. Enough oxygen for fish was present down to 26 feet, compared to 20 feet in previous years.
If habitat conditions continue to improve, biologists may consider restocking ciscoes into Waubee Lake. Ciscoes, a cold-water fish, were present in the lake until the early 1970s but died out as water clarity and oxygen levels declined.
“Waubee Lake may be unique in that more watermilfoil here is not necessarily a bad thing,” said Jed Pearson, DNR biologist. “Unlike other lakes where milfoil threatens to displace native plant communities, Waubee Lake doesn’t have many plants.”
According to Pearson, aquatic plants have never been abundant in Waubee Lake.
“There is no question that milfoil is now a nuisance in some areas of the lake, but overall the amount of vegetation is still low,” Pearson said.
Pearson said the increase in water clarity and watermilfoil last year may have been due to the weather: hot, dry summers and mild winters allow watermilfoil to flourish. That could change as a result of this spring’s colder and wetter weather.
“We can’t predict how much milfoil will grow this year,” Pearson said. “If it continues to increase, we can take steps to reduce it, but we don’t want to start a major control program that could have negative side effects.”
Based on this wait-and-see approach, the DNR postponed a request last month from local residents for funds to help spray watermilfoil with herbicides.
The DNR, however, will allow spot treatments of watermilfoil in specific areas along residential shorelines where use is impaired and near the boat ramp where the risk of transfer by boaters is greater.
It is illegal to transport Eurasian watermilfoil from one lake to another.
Meanwhile, DNR biologists will resample Waubee Lake’s plant community in August to monitor watermilfoil abundance.