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Field Notes: Spiny Water Fleas

Spiny water fleas
Carol Warden
Spiny water fleas

Do you remember that 1979 horror movie, Alien? The alien from the title, the Xenomorph, was a creepy helmeted creature with a long tail, and nasty intentions. If you squint, it kind of looks like one of the threats to our local lakes - the spiny water flea. In fact, all kinds of arthropods, that is small invertebrates with jointed legs, are quite monstrous if blown up to human size. Many have murderous mouthparts and bristly knuckles. Many, especially the parasitic ones, have complicated life histories, like the Xenomorph, that erupted out of the belly of one of the astronauts after incubation.

Spiny water fleas are aquatic arthropods called zooplankton. Zooplankton are small shrimp-like creatures, and most of them survive by eating phytoplankton, the microscopic photosynthetic algae at the bottom of the food chain in lakes. In turn, zooplankton are eaten by small fish and other animals. Spiny water fleas upset the usual order of aquatic ecosystems because, although they are zooplankton, they are large enough to prey on other zooplankton that usually feed on phytoplankton. So, spiny water fleas can unleash a bloom of algae that would normally be eaten by the smaller zooplankton. It appears that the long tail on a spiny water flea makes it difficult for small fish to eat them.

Bithotrephes longimanus, the scientific name of spiny water fleas, are native to brackish Northern European and Asian waterbodies where they inadvertently slipped into ocean-going ship holds along with ballast water and were then released into the Great Lakes when the ship’s ballast tanks were emptied. They are present in Lake Superior and Lake Michigan, and from there have spread to a few of our inland lakes including Trout, Plum, Ike Walton, Stormy and Star Lakes in Vilas County. Generally, they are thought to flourish only in large, deep, cold lakes, but are also found in the Gile Flowage in Iron County which is neither deep nor cold.

Spiny water fleas were first found in Stormy Lake in 2009 and in Trout Lake in 2014. Trout Lake is part of the Long-Term Ecological Research project at Trout Lake Station and zooplankton are sampled and quantified every two weeks during open water and monthly in winter. However, because SWF are relatively rare in the water column and are large relative to other zooplankton, biologists sample spiny water fleas using a special large net with larger mesh holes than is used for regular zooplankton sampling. Using this special net, Stormy Lake was sampled every two weeks from 2009 through 2016 and numbers were high in 2013 and lower in all other years. In Trout Lake, spiny water fleas numbers were highest in 2014, the first year they were found. Since then, their numbers in Trout have been low, or none have been found at all.

So, are spiny water fleas demons like the Xenomorphs in Aliens? Mostly not. In some lakes, such as Lake Mendota in Madison, WI, spiny water fleas have caused large changes to the aquatic ecosystem, including reduced water clarity. However, Lake Mendota is eutrophic, meaning it has very high nutrient levels, has poor water clarity, has a mostly urban and agricultural watershed and, along with spiny water fleas, has invasive zebra mussels. (Jeesh – what a mess!). Our northern lakes are different; most are nestled in forested watersheds, are not eutrophic and so are clearer, and have few other invasive animals.

One other difference is that Trout Lake has cisco, a deep and cold-water fish that apparently adapted quickly to eating spiny water fleas. While not all our lakes with spiny water fleas have cisco, cisco are common in Trout and Plum Lake and present in Stormy and Star. So, while some “typical” zooplankton predators cannot eat spiny water fleas, some fish can and may be able to keep their numbers low. Notably, cisco are extremely rare in Madison’s Lake Mendota. Work on Trout Lake by Ben Martin and Tyler Butts of the UW-Madison Center for Limnology suggests that spiny water fleas can affect the whole food web, especially when their numbers are high, but native fishes within the food web, like cisco, may be successfully controlling their population, at least temporarily. However, the only way to control spiny water fleas is to never let them into the lake. Please make every effort to keep them out. This means consistently draining your live well and other water reserves in your boats and cleaning your gear before moving to a new lake. If you see spiny water fleas accumulate on your fishing line, be sure to let the WDNR know.

At least currently, spiny water fleas may not be the Xenomorphs of our northern lakes. But it is our responsibility to do everything to keep them out.

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Susan Knight works for the University of Wisconsin-Madison’s Center for Limnology at Trout Lake Station and collaborates closely with the Wisconsin Department of Natural Resources. She is involved in many aspects of aquatic plants, including aquatic plant identification workshops and research on aquatic invasive plants. She is especially fond of bladderworts.
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