Environmental impact of zebra mussels
Zebra mussels (Dreissena polymorpha) are invasive freshwater bivalves originally from Europe, which have had a significant environmental impact in North America since their introduction in the late 1980s. They rapidly proliferate in aquatic ecosystems, outcompeting native mussels and reducing biodiversity by displacing species such as native unionid mussels. Their ability to attach to various surfaces, including boats and water supply infrastructure, leads to blockages in pipes, impacting water supply systems such as hydroelectric plants and public utilities. Zebra mussels reproduce prolifically, with females capable of producing over 40,000 eggs, contributing to their dense populations, which can reach upwards of 700,000 individuals per square meter.
While zebra mussels can improve water clarity by filtering phytoplankton, their presence can disrupt local food webs by depleting resources available for zooplankton and fish, potentially shifting ecosystems from phytoplankton-dominated to macrophyte-dominated systems. Moreover, their filtering process can concentrate toxic substances in their bodies, posing risks to higher trophic levels in the food web. Despite threats to native ecosystems and infrastructure, the zebra mussel's dual role as both a filter feeder and a biofouler complicates its overall ecological impact. As their populations stabilize, ongoing monitoring and management efforts are essential to mitigate their negative effects on North American freshwater systems.
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Environmental impact of zebra mussels
DEFINITION: Endemic European freshwater bivalves that occur in North America as an exotic species
Zebra mussels are a source of concern in North America, where they are an invasive or exotic species, because of the mussels’ potential influence on aquatic systems. When zebra mussels colonize a body of water, they reduce the abundance and species diversity of native mussels, and they also reproduce in such density that they restrict water flow in pipes.
The zebra mussel (Dreissena polymorpha) is native to southern Russia and is thought to have been introduced into Lake Saint Clair—which lies between Michigan and Ontario, Canada—in 1986 via discharged ballast water. Since its introduction, the zebra mussel has become widely dispersed, occurring in all of the Great Lakes by 1990. By 2011, according to the US Fish and Wildlife Service, it had appeared in or adjacent to thirty US states. This rapid dispersal is largely the result of the mussels’ ability to attach to boats that navigate these waters, as well as their ability at all life stages to survive overland transport (for example, living on the hulls of boats as the boats are transported between lakes). It is expected that the species will continue to disperse and increasingly colonize North American inland lakes. However, in the second and third decades of the twenty-first century, native ecosystems and the invasive species have begun to reach an emerging equilibrium that has slowed their growth.
The shell sizes of zebra mussels average 25 to 35 millimeters (1 to 1.4 inches). Zebra mussels typically live three to five years, and females usually reproduce during their second year; each female can produce more than forty thousand eggs in one reproductive cycle. After fertilization, veliger larvae emerge within three to five days and are free-swimming for up to one month. Dispersal during this time is primarily caused by water currents. Larvae then settle to the bottom, where they crawl via foot, looking for suitable substrate (preferred to be hard or rocky). To attach themselves to the substrate, they secrete proteinaceous byssal threads from a byssal gland located in the foot.
From an ecological and environmental perspective, one of the most important concerns related to zebra mussel colonization of a body of water is that it reduces the abundance and species diversity of native unionid mussels. Because native unionid mussel beds provide the type of hard substrate that zebra mussels prefer, zebra mussels readily colonize such areas, negatively influencing the feeding, growth, locomotion, respiration, and reproduction of native unionids.
In addition, zebra mussels have an important influence on the because of their role as biofoulers. When they colonize pipes, they can restrict water flow, negatively affecting water supply to hydroelectric facilities, nuclear power plants, and public water supply plants. Zebra mussels can also attach to the hulls of boats, leading to increased drag, and their weight can navigational buoys. Because zebra mussel densities have been measured as high as 700,000 mussels per 1 square meter (10.8 square feet), they clearly can cause serious problems.
Despite their negative effects on aquatic systems, zebra mussels can have positive effects on water quality through their role as biofilters. An adult zebra mussel can filter the phytoplankton from as much as 1 liter of water per day and can significantly alter water quality. However, even this influence has potentially negative consequences in that it reduces the amount of food available for zooplankton and eventually for recruiting fish. This can also lead to a change in food webs from phytoplankton-dominated systems to macrophyte-dominated systems. Given their filtering ability, zebra mussels also tend to bioaccumulate substances, possibly increasing the of toxic substances that are passed up the food web.
Bibliography
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