The Arrival of Spiny Waterfleas

In August of this year, the spiny waterflea was found in Lake George, greatly increasing the possibility of its eventual arrival in Lake Champlain. Lake George has a direct connection to Lake Champlain via the LaChute River. Spiny waterfleas have also been found in the Champlain Canal which connects Lake Champlain to the Hudson River. The spiny waterflea has sat atop the list of least desirable invasive aquatic species for quite some time, but how did it earn such apprehension?

Spiny waterfleas are zooplankton, small animals that live in the water column and drift with the currents. While most zooplankton are microscopic, the spiny waterflea can be seen with the naked eye. Still, a dozen or more would fit on a fingernail. Zooplankton play two very important roles in the ecosystem. First, they provide food for fish, especially rainbow smelt and alewives, the principal prey of lake trout and salmon. Second, they eat algae, potentially keeping some blooms in check. While Lake Champlain hosts nearly 50 invasive aquatic species including fish, plants, snails, and mussels, so far we have no invasive zooplankton.

At a recent workshop on invasive species, Dr. Jason Stockwell of the University of Vermont offered four predictions about how spiny waterfleas could change the ecology of Lake Champlain. Stockwell is the director of the Rubenstein Ecosystem Science Lab on Lake Champlain. Prior to that, he was station chief at the Lake Superior Biological Station in Ashland, Wisconsin. He based his predictions on a review of the literature, which mostly compares lakes in the Canadian Shield with spiny waterfleas to those without, and his own experiences in the Great Lakes.

First, he felt that the spiny waterflea’s impacts would most widely be felt in the Main Lake and Mallets Bay. These areas of the lake have clear waters compared to the murkier waters of Missisquoi Bay and the South Lake. Spiny waterfleas use visual cues to find and catch prey, so they would have an easier time feeding in the clearer waters.  

Second, spiny waterfleas would compete with rainbow smelt and mysids (a group of small, shrimp-like crustaceans) for food, leading to drops in those two populations. Both rainbow smelt and mysids provide important forage for lake trout and salmon, and both populations have already seen declines. Rainbow smelt were the dominant forage fish in the lake until the arrival and spread of alewives about 2003. While rainbow smelt can eat spiny waterfleas, the large spines provide little nutrition and have been found to fill up fish stomachs without being digested. Mysids, also known as opossum shrimp, play an important role in the energy and food dynamics of the lake. They migrate vertically in the water column each day providing a transfer of nutrients and energy from sediment and deeper water up into the water column.

Third, the prey of rainbow smelt, mysids, and spiny waterfleas are cladocera, another, medium-sized form of zooplankton. Stockwell predicts their population would go down as a result of the new predator. The presence of spiny waterflea affects them in two ways. Most obviously, the cladocera become food. However, the waterfleas also affect the cladocera’s behavior. In the presence of spiny waterflea the cladocera spend less time near the water surface where their food is more plentiful, but so are their predators. As a result, they grow more slowly when spiny waterfleas are present.

Cladocera are a group of medium-sized plankton. While the overall numbers for the group as a whole may go down, some of the species that have behaviors or defense mechanisms that help them avoid spiny waterfleas could actually increase in number.

Fourth, if the cladocera numbers go down, then the things that the cladocera eat may increase. Rotifers are tiny zooplankton. They are too small to attract the attention of spiny waterfleas but constitute the main food for cladocera. Rotifers may just thrive in the presence of the new invasive species.

A fifth possible outcome has been predicted in other systems but not actually measured. Models suggest that spiny waterfleas in a system could lead to a five to ten percent increase in mercury concentrations in fish. Mercury, a potent neurotoxin accumulates with each step up the food chain, thus smaller organisms have lower concentrations than larger organisms. Spiny waterflea may add another step in a lake’s food chain, thus increasing opportunities for accumulating mercury.

In addition to the potential ecosystem effects that spiny waterflea may cause, they are also a nuisance for anglers. Two-thirds of the length of the animals’ bodies is a long spine that can become caught on fishing line. At very high densities the spiny waterfleas accumulate on the line and make it difficult to reel in.

Every invasive species wreaks changes in its new environs. Spiny waterfleas are the subject of great concern because they would become the first invasive zooplankton in the lake. The changes suggested by Stockwell could ripple through the food web leading to additional unpredictable alterations. The potential impacts of spiny waterflea and other invasives like them have led the Lake Champlain Committee to call for <link learn lake-look on-closing-the-champlain-canal _blank external link in new>physical barriers in the Champlain Canal that would limit opportunities for species to spread between waterbodies. We would certainly be better off if they never arrive.