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Natural reaction

August 16, 2019 by Jacob Friede

Spiny water fleas have been found in Plum Lake in Vilas County. Two of the microscopic, invasive zooplankton were discovered July 24 by Ben Martin, a PhD student from the University of Wisconsin-Madison’s Center for Limnology.  

So what does the finding mean and how will it impact the lake? It’s too early to tell and there are still a lot of questions. However, the timing of the discovery, which Martin described as a needle in a haystack, could not have come at better time to eventually answer some of them.     

“Really the key is with Plum Lake we have sort of the very early detection of this, when they’re at such a low abundance and so right now is really when I’m collecting my key data,” said Martin, who is studying how spiny water fleas affect food webs in lakes.  

Particularly, since fish eat zooplankton, he is looking at how the diets of fish will be impacted. To do this he studies the condition factor of fish which is a like a Bio Mass Index for humans.

“So it’s your weight over your respective length,” Martin said. “That gives sort of an indication of how well fed you are. So often when there’s a change in the ecosystem we can potentially see a change in the condition factor.”

Because Martin found the spiny water fleas early, he is able to gauge the condition of the fish right now and will be able to use that as a comparison if the infestation takes off.

“I did collect fish samples, so this will be sort of an early part of the invasion that I’ll have data on,” Martin said. “Later on down the road, if spiny water fleas do take off in that lake, I’ll have that data to rely on to know bluegill were weighing this much at a given length — has that changed?”

Whether or not the spiny waters flea population takes off is another question that only time will answer, however, conditions in Plum Lake are not necessarily ideal for them.

Spiny water fleas eat other zooplankton, such as Daphnia, which feed on algae. Since Plum Lake, due to its low levels of nutrients, such as phosphorus, does not have a lot of algae there is not the food base for spiny water fleas to flourish. 

“That’s true of much of our Northwoods lakes,” Martin said. “There’s not farmland up here so we don’t have the nutrients coming into the lakes and so our baseline algae levels up here make it so that spiny water fleas likely aren’t going to take off too strongly. There would need to be a lot of food for spiny water fleas to take off and simply theres not that many zooplankton in a lot of these lakes.”

Lake Mendota, part of the Madison chain of lakes in the southern part Wisconsin, is an example of what can happen with spiny water fleas when there are high levels of nutrients in the water.

There the algae was plentiful and so was the Daphnia that eats it. In 2008, a colder than usual year, warm water was not able to kill off the spiny water flea egg bank and there was a population explosion. They feasted on the Daphnia, the key algae grazer in the lake, and the result was a loss of three feet of water clarity because Daphnia were not there to eat the algae.

Spiny water fleas, which require cold water, can be a ticking time bomb. They were present in Lake Mendota for years before it took a climatic anomaly like the strange, cold year in 2008, to really trigger their explosion. 

“They just operate at these low abundances and then something, some sort of disturbance, some change that makes the food web, the whole ecosystem vulnerable, that is what sparks often these impactful invasive species,” Martin said.

Spiny water fleas can reproduce asexually and their eggs are very hard to kill. Even if a pregnant flea is eaten by a fish.

“If a fish eats a spiny water flea that has eggs those eggs will actually pass through the intestines, come out the other end, end up in the mud and they’re still viable,” Martin. said

Because those almost indestructible resting eggs are in the mud, their prime source of travel from lake to lake is on anchors and anchor lines, which is why it is so important for boaters to decontaminate their muddy gear.

“The recommended amount to bleach your boat isn’t quite enough to kill them,” Martin said. “Drying them is really the key. If you can dry it for as long as possible, that’s the best way to kill them off.”

However, there is not an effective strategy to control their population once they’re already in a particular lake.

Fish could be the answer and Martin is studying which fish can effectively feed on spiny water fleas. They are not harmful for fish but only some fish can eat them because of the flea’s large size and awkward shape. A fish must be large enough and have a wide enough gape to get them down the hatch.

“We’re trying to verify which ones are actually eating them and which ones are avoiding them by looking at their stomach content,” Martin said.

Additionally he will be doing tests in the lab.

“I will be raising fish in tanks and doing feeding trials to see which fish are most effective at eating spiny water flea,” Martin said. “And does that sort of agree with what I see in the food web.”

This summer Martin collected samples from 20 northern Wisconsin lakes which he classified as vulnerable due to their depth and exposure to boats. Of those 20 lakes, only Plum Lake was found to have spiny water fleas.

In years past, spiny water fleas have also been found in Trout Lake, Star Lake, Stormy Lake, Ike Walton Lake, Butternut Lake, and the Gile Flowage.  

It is now up to boaters in the Northwoods to be diligent about cleaning and contaminating their vessels and gear to ensure that another lake is not added to this list. 

Jacob Friede may be reached at [email protected] or [email protected]

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