Undergraduates Dive in to Mats of Milfoil and the Mechanics of Invasion

Dense beds of Eurasian watermilfoil carpet the surface of Hancock Lake, a few miles east of Tomahawk, W.I., where undergraduate researchers Linden Taylor and Brigid Doyle have come to take samples of the invasive aquatic plant.

The plant’s rapid spread has fascinated biologists and frustrated homeowners, who have seen it out-compete native plants, change freshwater ecosystems, hinder navigation and lower property values.

But Linden recognizes that studying the mechanisms of an invasion are just as important as studying its consequences.

Brigid scans the lake for Eurasian watermilfoil. The invasive plants’ stems are often visible above the surface of the water. Photo: S. Widell

A few recent studies have shown that a compound produced by the Eurasian watermilfoil can hinder the growth of some algae by releasing a compound that interferes with algae’s ability to photosynthesize, or to convert sunlight into usable energy. That characteristic might be one of several that make the invasive species so successful in Wisconsin’s lakes, Linden said. Biologists call this phenomenon allelopathy, or a plant’s potential to chemically impede the growth of another organism.

Some of those studies were conducted in Europe, where Eurasian watermilfoil is not considered invasive. Linden wants to know if the Northern watermilfoil,a species that is native to Wisconsin, interacts with algae in a similar way.

To find out, she is creating plant extracts from both species and testing them against bluegreen algae samples. Her research is one of many independent projects being lead by undergraduate researchers at Trout Lake Station this season.

So much milfoil! Linden collects the rope-like plant from mats on the surface of the water. Photo: S. Widell

“I’m more interested in these species-to-species interactions,” Linden said. “It is important to know how different species compete, because ultimately, competition affects ecosystem structure and ecosystem stability.”

Linden steers the boat into the middle of the lake, and doesn’t have to look hard to find what she’s looking for. Scraggly red stalks poke up through the water and our motor chokes against the weeds suddenly caught in its blades.

Linden reaches over the side of the boat and pulls on a strand of milfoil. As she tugs, more and more of the plant comes loose, until her hands are full of tangled masses of weed.

“There’s so much!” she laughs, as she sorts her samples into plastic bags.

Linden needs two bags of milfoil this size to create her plant extract, or, as she says, “make some tea.” Photo: S. Widell

When she’s confident she has enough for her experiment, Linden eases the boat closer to shore, so she and Brigid can scan the sediment-rich shallows for Northern watermilfoil.

This one is harder to find. In order to collect the sample, Linden will have to snorkel. Getting wet is pretty common practice in the field, and Linden and Brigid know to pack their swimsuits when they head out for a day of sampling.

Eurasian and Northern watermilfoil look fairly similar, but Linden and Brigid’s trained eyes are quick to find their differences. Brigid explains that the Eurasian species has many more leaflets per leaf than the Northern variety, but that the Northern’s leaves grow closer together.

Northern waterfoil has fewer leaflets per leaf than its Eurasian cousin. The leaflets are the small, feathery shoots that grow off of the plant’s stem. Photo: S. Widell

The differences are not only structural. On a chemical level, the Eurasian species contains a higher concentration of the bluegreen algae growth-inhibiting compounds.

Linden believes having a higher concentration of that compound will give Eurasian watermilfoil a greater potential to stunt algae growth than the Northern or hybrid varieties when she runs her experiment back in the lab.

In the next few weeks, she’ll create milfoil extract by drying her samples in an oven. Then, she will use a chemical solution to extract the allelopathic compounds from the plants. Finally, she will apply the extract to the bluegreen algae she has been growing for the last month.

Linden back at the lab, stirring the algae samples she’ll be using once she completes her extracts. Photo: S. Widell

For Linden, getting to design and conduct her own experiment is an experience she called “incredibly important.”

“Knowing how to independently plan and conduct a project is a good skill to have,” Linden said. “If you can do that, you’ve really shown that you can put a lot of time and effort into a question that you are really curious about, with the help of many others.”

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