Fall 2008


Spiny water flea, unknown location. Couresy of US Fish and Wildlife Service

UW Sea Grant Outreach

Tiny waterfleas may have BIG CONSEQUENCES

By Carolyn Rumery Betz

Standing aboard the research vessel Neeskay in Lake Michigan, UW–Milwaukee Associate Professor Craig Sandgren oversees an oceanographic-sized net being pulled up from 100 meters. Considering the enormous size of the net and the large volume of water that is being collected, it’s surprising that what he and fellow biology professor John Berges are anxiously looking for are tiny water fleas barely visible to the naked eye. What’s more, these tiny species have the capability of significantly changing the existing food web.

The critters are the carnivorous zooplankton Bythotrephes cederstroemi (spiny water flea) and Cercopagis pengoi (fishhook water flea). Like the zebra and quagga mussels, both are invaders from Eastern Europe that have hitched their way into the Great Lakes through ballast water on ships.

Every other week this summer, the two researchers collected samples off the coast of Milwaukee and several miles out in the open water.

“In their native environment, there are all kinds of checks and balances so the zooplankton are not a problem,” said Sandgren during a research cruise in August. “But when they come to a new place, they may become superabundant because they have left behind their natural enemies.”

What concerns these scientists is the fact that the two invasive species occupy the same critical spot in the middle of the food web. While two similar species might compete against each other, these tiny fleas may in fact divide and conquer, each devouring separate types of zooplankton. That would leave less food for fish to eat during their critical initial growth periods. Additionally, if they eat zooplankton that normally consume algae, prolific algae blooms may increase in frequency.

“Adding invasives changes the dynamics of the food web in very unpredictable ways,” John Berges said. “The more invasive species there are, the greater the problems become.”

The scientists are able to make comparisons with the data Sandgren collected in the 1990s when only the spiny water flea was on the scene, but before the fishhook water flea and zebra and quagga mussels were introduced to Lake Michigan, further complicating the system. Sandgren and Berges are also conducting hundreds of experiments in their labs under controlled conditions. The two can manipulate light, temperature, and nutrients to see how the primary producers, zooplankton grazers, and invasive species respond.

The researchers are developing a basic understanding of what the two water fleas eat to better predict their impacts on the food web. Unlike fish, which can be cut open to examine the contents of their stomachs, these carnivorous water fleas feed like spiders, sucking out the contents of their prey and leaving the hard parts behind.

“If you imagine the tiny animal they eat is a can of soup, they squeeze out the soup and leave the can behind,” explained Sandgren. “We’re trying to read the label without having the can.”

Berges reports that thus far they have developed specific antibodies for six prey ­species that will allow them to identify those species in the “soupy” gut contents of the invasive water fleas.

Next summer, the two biologists will examine the spatial distribution of the ­zooplankton to see if the nearshore and offshore environments that they have already sampled are representative of all of Lake Michigan.

As Sandgren examined the water sample raised from the depths of Lake Michigan, he observed the tiny creatures swimming around.

“This isn’t the mixture of species I had expected to see at all,” he said, shaking his head and chuckling. “It’s really exciting. We keep realizing that whenever we think we know what’s going on, we find something different. It’s just such a complicated system.”









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