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Weather slows aquatic plant growth in lakes

Statement as issued Tuesday by the Indiana Department of Natural Resources:

This year’s late winter thaw and cool spring weather have slowed the growth of aquatic plants in northeast Indiana lakes. This should, according to DNR officials, delay and may even reduce the need to use chemical herbicides to control vegetation in lakes where there are such issues.

Each year residents at more than 125 area lakes hire commercial pesticide applicators to spray aquatic weeds where they interfere with boating and swimming. To legally do so, applicators must first obtain a permit from the DNR.

“The amount of weed control done each year has a lot to do with the weather,” said Jed Pearson, a DNR fisheries biologist. “Water temperature, the number of sunny days, and rainfall can have a big effect on aquatic plant growth.”

The seasonal timing of herbicide applications also depends on water temperature and various growing stages of the plants.

“We typically see fluctuations in plant growth from year to year,” Pearson said. “That makes it difficult for local lake associations and lake residents to plan and fund their weed control programs.”

It also can create some common misconceptions about plant ecology.

“Just because plants are more abundant one year and cover wider areas doesn’t mean a lake is getting polluted,” Pearson said. “And just because fewer plants are growing this year doesn’t mean they won’t be back next year.”

To show how much aquatic plants vary from year to year, Pearson cites data that he and other DNR biologists have collected at 47 lakes over the past four years.

On a region-wide basis, plant coverage was greatest in 2012, a year marked by an early spring and record warm temperatures with less rainfall. As a result, lakes were much clearer than normal. Aquatic plants depend on how deeply sunlight can penetrate. Clearer lakes tend to have more vegetation and at greater depths.

In 2012 leafy plants typically covered 75 percent of the area where they could potentially grow, an area biologists call the littoral zone.

In contrast, leafy aquatic plants covered an average of 65 percent of the littoral zone in lakes during 2013 after a cold and wet spring.

The number of species also varied. More species grew per site in 2012 than 2013. Moderate numbers of were present in 2010 and 2011.

“But not all species followed the same trend,” Pearson said. “Coontail, one of the most common species, was more abundant in 2013 even though overall plant coverage was less. Plants respond differently to different conditions.”

So what does all this mean?

“Lake residents need to take a long-term view of plant management and not over-react to short-term yearly changes,” Pearson said.

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