Impact of the exotic invasive zebra mussel
By Jason Morrison
This paper was written for and Ecology class at Ohio Wesleyan University, Fall 2000. The paper had to be a general survey of the topic, but only scientific journal sources were allowed. I needed 20 journal articles to scrape together enough background for a 5-page paper.
Zebra mussels (Dreissena polymorpha) arrived in the Great Lakes in 1986 in the ballast of ships travelling from Eastern Europe. They are considered to be a nuisance species because of their rapid population growth and ability to bond to many man made surfaces (Marsden and Lansky 2000), but have also had a large impact on native species in North America. The small mollusks, each about the size of a fingernail, have multiplied and spread quickly throughout the Great Lakes, its tributaries, and even into parts of the Mississippi River basin. (Cope and Bartsch 1999).
Zebra mussels are just one example of a much larger problem-the introduction of exotic species by man into new habitats. In the Great Lakes alone, scientists have found the mussels, Eurasian ruffe, the round goby, the sea lamprey and other species effecting the ecosystem. These species often find themselves without natural predators so they are able to reproduce quickly and feed of native species. Exotic species may also compete with native species for food or other resources. And because many have moved in recently, it's still unclear what overall effects exotics will have on local ecosystems (Binder 2000).
Zebra mussels are a particularly interesting case. Not only have their introduction into the lakes had widespread and complex ecological effects, but they have been a costly nuisance for human populations as well. The mussels themselves do not directly harm people, but they do form in large aggregates that can block commercial, industrial and recreational water systems (Putchakayala S and Ram J 2000), causing millions of dollars in cleanup every year. They have been found attached to any hard surface, including docks and boats. They have even been known to sink buoys as more and more of the creatures pile on (Nichols 1995).
This ability to aggregate has effected other organisms as well. For example, they "pose another threat to unionid populations; unionid populations decline following invasion of a body of water by zebra mussels. Zebra mussel attachment may impair locomotion, interfere with valve closure and opening, generate toxic wastes, smother siphons or strip inhalent water of food. Laboratory studies indicate that unionids with attached zebra mussels are nutritively stressed either as a result of reduced food intake or increased metabolic costs." (Baker and Hornbach 2000).
The first biological effect that humans noticed, however, was the mussels' incredible filtering ability. The mussels filtered so much phytoplankton out of the water in some areas that they were able to lower the trophic level, making it difficult for small organisms near the bottom of the food chain to survive. In one case, "the years since zebra mussels became abundant in Oneida Lake have been characterized by high water clarity, low chlorophyll concentrations, long clearwater phases, and low Daphnia biomass compared with the previous 17 years." (Horgan and Mills, 1999).
Zebra mussels also filter non-living matter out of the water, including phosphorus, which has traditionally been thought of as a water pollutant. While governments worked in the late 80s and 90s to reduce the phosphorus pollution that caused huge algae blooms (and therefore huge fish kills), the mussels moved in and led some to fear there wouldn't be enough phosphorus left to support the rest of the ecosystem (Nichols, 1995). Zebra mussels are also so successful in filtering toxins out of the water that they have been used to test the overall water quality of the St. Lawrence River (Lafontaine et. al., 1999) as well as the Hudson and Niagara Rivers and Lakes Erie and Ontario, though some fear that they will reintroduce the toxins back into the food chain in much larger doses if they are eaten by predators. (Roditi H et. al. 2000) Additional studies have shown that the mussels affect the nitrogen cycle by increasing ammonium levels and effecting the food web (Lavrentyev et. al., 2000) and have linked zebra mussels to decreasing dissolved oxygen levels in the Hudson River (Caraco F et. al., 2000).
The impact of zebra mussels has not been totally negative, however. In a recent study in Lake Erie about a recent decline in zebra mussel population, ducks were found to be altering their diets and migration routes to take advantage of the zebra mussels as a food source. The mussels have also encouraged the growth of certain plants that have bolstered rare native duck species (Petrie and Knapton, 1999). Researchers in Put-In-Bay, on an island in Lake Erie, noticed several rare plant species return shortly after zebra mussels arrived (Nichols, 1995). Two more studies found no overall negative reaction to zebra mussels in yellow perch in Oneida Lake, New York (Mayer et. al., 2000) and that "shallow, productive lake ecosystems may either be resistant to many of the zebra mussel-induced changes experienced in deeper, less eutrophic lakes, or that such changes may take more than three years before becoming evident." (Mercer et. al., 1999). Overall, zebra mussels seem to have a dramatic and multifaceted influence on the ecosystems they are introduced into, but much more study needs to be done to determine what their ultimate influence will be. Many studies will also be strengthened over time as more data comes in and the native ecosystems adapt to the mussels.
Much of the current body of study is centered on getting rid of zebra mussels. Between large-scale water intake blocks and worry about the danger they may pose to the great lakes and surrounding rivers, humans seem to be preparing for war against these Eastern European immigrants. Approaches to the problem have come from all directions. Some scientists are studying areas the mussels have not moved into in an effort to determine why. Lake George, New York, for example, resisted the mussels for quite some time. When they finally moved in, they only survived in relatively calcium-rich areas in the usually calcium-poor pond (Revkin 1999). Some had hoped another exotic, the round goby, would be able to manage zebra mussel growth as a natural predator. The goby, however, eats native species as well and may turn out to be as damaging as the zebra mussels themselves (Kuhns and Berg, 1999). Some researchers have looked to native species to help control the mussels. The ducks mentioned above have led to some optimism that if zebra mussels cannot be wiped out, they can at least be managed by native species. Scientists have also hoped to use common carp as zebra mussel predators, though large carp populations can be damaging to other species as well (Howard and Van Boven, 1995). Another recent study showed fresh-water sponge overgrowth would weaken mussel shells, result in lower body weight, and have a considerable lethal effect (Lauer and Spacie, 2000).
Other attempts take a more direct approach, testing chemicals and poisons in an attempt to find something that will knock out the zebra mussels without much collateral ecological damage. One study has shown an increase in copper toxicity with an increase in water temperature (Prasada Rao and Khan, 2000) while another tested a molluscicide called metaldehyde but found it to be much less effective on zebra mussels than other mollusks (Putchakayala and Ram, 2000). Other studies test their physical limits, seeing how long they can stay alive out of water or in semi-wet conditions (Paukstis, 1999). All of the lines of research mentioned above have more or less just begun. Similar lines of inquiry will no doubt be ongoing.
It's very difficult to say what ultimate effect zebra mussels will have in North America, except that it seems likely they will continue to expand, from waterway to waterway, across the continent. None of the current techniques of managing their population, stopping their spread, or total eradication are fully successful, and some may be more dangerous than simply leaving the zebra mussels be. In fact, so little is known about their ultimate impact on the ecosystem that it is hard to be certain, as many people were in the early 90s, that they will truly damage anything. It is quite possible there's nothing humans can do, and that the most important developments will be in finding materials or engineering techniques to keep them from clogging water intakes.
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Mayer C, VanDeValk A, Forney J, Rudstam L and Mills E (2000) Response of yellow perch (Perca flavescens) in Oneida Lake, New York, to the establishment of zebra mussels (Dreissena polymorpha). Canadian Journal of Fisheries and Aquatic Sciences 57: 742-754.
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Copyright 2001 Jason Morrison