General Impact
For a detailed account of the environmental impacts of Dreissena polymorpha please read: Dreissena polymorpha Impacts Information. The information in this document is summarised below.To date (2002) D. polymorpha has been the most aggressive freshwater invader worldwide (Karayayev et al. 2002). Once introduced, populations of zebra mussel can grow rapidly and the total biomass of a population can exceed 10 times that of all other native benthic invertebrates (Sokolova et al. 1980a; Karatayev et al. 1994a; Sinitsyna & Protasov 1994, in Karayayev et al. 2002) Ecosystem Change: Most of the impacts of zebra mussels in freshwater systems are a direct result of their functioning as ecosystem engineers (Karayayev, et al. 2002). An individual zebra mussel can filter one to two liters of water each day; as a result high densities of zebra may cause major shifts in the plankton communities of lakes and rivers. Reductions in phytoplankton numbers and biomass also limit food to fish larvae and other consumers further up the food chain (Birnbaum 2006). Modification of Natural Benthic Communities: The introduction of Dreissena is generally associated with increased benthic macroinvertebrate density and taxonomic richness (Ward & Ricciardi 2007). Biodeposition of organic wastes and dense colonization of the benthos by zebra mussels has also substantially altered benthic communities; many invertebrates benefit from the increased food resources and complex habitat, while benthic spawning and foraging fishes may be negatively impacted. Overall gastropod densities increased in the presence of Dreissena, but large-bodied snail taxa tended to decline (Ward & Ricciardi 2007). Habitat Alteration: The high consumption of phytoplankton by zebra mussels results in increased water clarity, changing habitat characteristics and ecosystem functions (DAISIE 2006). The dense colonization of soft substrates can impede fish foraging (Beekey et al. 2004), and colonization of hard substrates affects spawning fishes (Marsden & Chotkowski 2001). Predation: Zebra mussel populations significantly deplete plankton densities as a result of filter feeding. Competition: Suspension-feeding species may experience increased competition for resources in the presence of high zebra mussel densities, as was reflected in the declines of sphaeriid clams in the Hudson River (Strayer, et al. 1998). Modification of Nutrient Regime: Zebra mussels may influence ecosystem processes such as nitrogen (N) cycling by increasing denitrification rates (Bruesewitz et al. 2006). Threat to Endangered Species: Freshwater mussels (Order Unionoida) are the most imperiled faunal group in North America with 60% of the species considered endangered or threatened (Ricciardi et al. 1998). The zebra mussel represents a new stress to populations of these native mussels as it is a biofouling organism that smothers the shells of other molluscs and competes with suspension feeders for food (Ricciardi, et al. 1998). Biofouling: Other mussels serve as substrate for settlement by Dreissena, and are energetically stressed and eventually starve as filter feeding is disrupted (Böhmer et al. 2001, in Birnbaum 2006) Economic Impact: Negative economic impacts caused by D. polymorpha include those caused by fouling of intake pipes, ship hulls, navigational constructions and aquaculture cages; the zebra mussel may also reduce angling catches (Gollasch & Leppäkoski 1999; Minchin et al. 2002, in Birnbaum 2006) Bioaccumulation: Zebra mussels may bioaccumulate pollutants which may poison animals further up the food chain (DAISIE 2006).
Location Specific Impacts:Euphrates River (Asia) 
Fouling: Dreissena polymorpha damages and hinders the operation of dams by fouling their intakes and moving parts in the Euphrates basin (Bobat et al, 2004). Canada
Threat to endangered species: Dreissina polymorpha competes with and threatens 11 species of endangered freshwater molluscs in the Great Lakes Basin of Canada and the United States (Alan et al, 2006). Denmark
Competition: Competitive effects on other species due to the zebra mussel have been recorded in Denmark (NOBANIS 2009).
Ecosystem change: Dreissena consumes a lot of plankton in the water. In Lake Esrom, Denmark, 18% of the phytoplankton production was consumed by D. polymorpha (Hamburger et al. 1990, in Birnbaum 2006). This decrease in plankton may cause a decline in the populations of some fish species (Birnbaum 2006). Estonia
Competition: Competitive effects on other species due to the zebra mussel have been recorded in Estonia (NOBANIS 2009).
Economic/Livelihoods: Socio-economic impacts due to the zebra mussel have been recorded in Estonia (NOBANIS 2009).
Ecosystem change: Abiotic changes due to the zebra mussel have been recorded in Estonia (NOBANIS 2009). Finland
Competition: Competitive effects on other species due to the zebra mussel have been recorded in Finland (NOBANIS 2009).
Ecosystem change: Dreissena polymorpha facilitates and may even be the main cause of rapid growth of filamentous algae in the Neva estuary of Finland; this algae has become a serious environmental problem (Golubkov et al, 2003). Germany
Ecosystem change: In German freshwater the most important molluscan invader is D. polymorpha. Ireland
Competition: Competitive effects on other species due to the zebra mussel have been recorded in Ireland (Birnbaum 2006).
Economic/Livelihoods: Socio-economic impacts due to the zebra mussel have been recorded in Ireland (NOBANIS 2009).
Ecosystem change: Zebra mussels have changed the trophic structure of lakes (Minchin et al., 2005). Abiotic changes due to the zebra mussel have been recorded in Ireland (NOBANIS 2009)
Human nuisance: Zebra mussels have had some impacts on recreational activities (Minchin et al., 2005). Latvia
Ecosystem change: Abiotic changes due to the zebra mussel have been recorded in Latvia (NOBANIS 2009). Lake St. Clair (North America)
Reduction in native biodiversity: The introduction of Dreissena polymorpha in 1988 resulted in the extirpation of native mussels in most of Lake St. Clair by 1991. However, communities of native bivalves have continued to survive in refuge sites in various near-shore areas of the lake. Surveys from 1998-2001 found 22 species of native unionids at 33 sites giving hope for their preservation (Hunter & Simons, 2004; Schloesser et al, 1996; Zanetta et al, 2002). Lake Erie (North America)
Competition: Dreissena polymorpha competes with and fouls native bivalves in Lake Erie. The burrowing amphipod Diporeia hoyi declined from 38 to 1.8% of the total benthic biomass from 1979 to 1993 since the introduction of D. polymorpha, and its density declined from 1840×m2 in 1979 to 220×m2 in 1993 (Bially & MacIsaac, 2000; Dermott & Kerec, 1997).
Ecosystem change: Dreissena polymorpha is believed to have caused an increase in non-burrowing amphipods in Lake Erie due to increased habitat complexity that results from zebra mussel cover of substrate, possibly by reducing predation risk (Gonzalez & Downing, 1999).
Modification of nutrient regime: Results of a study (Conroy et al. 2005) which primarily tested for differences in ammonia-nitrogen and phosphate-phosphorus excretion rates of D. polymorpha and D. bugensis the two invasive molluscs in Lake Erie, and also compared dreissenid ammonia and phosphate excretion with that of the crustacean zooplankton, demonstrated that mussels add to nitrogen and phosphorus remineralisation, shortening nitrogen and phosphorus turnover times, resulting in the modification of the nitrogen and phosphorus cycles in Lake Erie. The authors conclude that this increased nutrient flux may facilitate phytoplankton growth and cyanobacterial blooms in well-mixed and/or shallow areas of western Lake Erie. There have been reports of recent increases in phytoplankton biomass and the recurrence of cyanobacterial blooms in western Lake Erie. Lake Michigan (North America)
Reduction in native biodiversity: Dreissena polymorpha has resulted in significant reduction in fingernail clams (Sphaeriidae spp.) in Lake Michigan. A study from 1992-1997 found that approximately half of the fingernail clams shells between I and 2 mm in size were used as substrate for attachment by juvenile and adult zebra mussels, while over 90% of the clams > 2 mm showed attachment by zebra mussels. Overall median densities of Sphaeriidae decreased from 832/m² to 13/m² at the 15 m depth and from 234/m² to 0/m² at the 10 m depth during the study. It appears that zebra mussel colonization caused a dramatic reduction of Sphaeriidae density by 1997 that may eventually result in their loss from the area (Lauer & McComish, 2001).
The decline in native amphipod Diporeia spp. occurred progressively from shallow to deep regions, and was temporally coincident with the expansion of Dreissena polymorpha (Nalepa et al, 2009). Lake Ontario (North America)
Modification of natural benthic communities: The establishment of Dreissena polymorpha and D. bugensis coincided with a drastic decline of native amphipod Diporeia spp. (Owens & Dittman, 2003).
Reduction in native biodiversity: In response to the disappearance of Diporeia spp., believed to have resulted from the introduction of Dreissena polymorpha and D. bugensis, populations of two native benthivores, slimy sculpin and lake whitefish, collapsed in eastern Lake Ontario, perhaps due in part to starvation, because Diporeia was their principal prey (Owens & Dittman, 2003). St. Lawrence River (North America)
Fouling: Locations of St. Lawrence River where Dreissena polymorpha occurred in densities of 4,000-20,000/m2 resulted in 90-100% native unionid mortality. D. polymorpha colonize unionids bivalves in high densities. Data suggests that once Dreissena mass is equal to or exceeds that of the bivalve, it will be extirpated (Ricciardi et al, 1996).
Modification of natural benthic communities: Dreissena polymorpha colonization alters macroinvertebrate communities on hard substrata by enhancing conditions for deposit-feeding organisms, small gastropods, and small predatory invertebrates, and by displacing large gastropods and certain large filterers. In the St. Lawrence River, these effects were associated with zebra mussel densities of 1500–4000 individuals/m2 (Ricciardi et al, 1997) Poland
Competition: Competitive effects on other species due to the zebra mussel have been recorded in Estonia (NOBANIS 2009; DAISIE 2006).
Fouling: Presents a problem in that it fouls ships, barges and other water vessels (DAISIE 2006). Russian Federation
Economic/Livelihoods: Socio-economic impacts due to the zebra mussel have been recorded in Russia (NOBANIS 2009) Turkey
Fouling: Fouling problems caused by the Driessena polymorpha were observed in Kovada I Hydroelectric Power Plant (HEPP) situated in the south western Mediterranean region of Turkey in 1964 (Bobat et al, 2004). England (United Kingdom (UK))
Economic/Livelihoods: Dreissena polymorpha has resulted in many water treatment works experiencing blockage of microstrainers and pumps, the occlusion of pipes, and the compromising of filter bed efficiency in Britain (Aldridge et al, 2006).
Ecosystem change: A fishing lake in Lancashire has experienced increased water clarity and reduced fish biomass coincidental with the arrival of Dreissena polymorpha (Aldridge, 2004).
Reduction in native biodiversity: A newly recorded population of Dreissena polymorpha in Barden Lake, Kent, appears to be having a deleterious impact on native unionid mussels, particularly the swan mussel (Anodonta cygnea) and, more, recently the painters mussel (Unio pictorum) (Aldridge et al, 2004). Northern Ireland (United Kingdom (UK))
Modification of natural benthic communities: The invasion of Dreissena polymorpha resulted in an increase of zooplankton in Lough Erne (Maguire et al, 2006). Great Lakes (USA) (United States (USA))
Competition: Since the introduction of Driessena polymorpha and Driessena bugensis Diporeia hoyi has declined in all Great lakes except Lak Superior (Dermott et al, 2005).
Threat to endangered species: Dreissina polymorpha competes with and threatens 11 species of endangered freshwater molluscs in the Great Lakes Basin of Canada and the United States (Alan et al, 2006). Hudson River (United States (USA))
Ecosystem change: After the introduction of Dreissena polymorpha studies found that the median decrease in abundance of open-water species was 28%, whereas the median increase in abundance of littoral species was 97%. Populations of open-water species shifted downriver away from the zebra mussel population, whereas those of littoral species shifted upriver. Median apparent growth rates fell by 17% among openwater species and rose by 12% in the single littoral species studied. Researchers found that the influence of zebra mussels on fish should vary widely across ecosystems as a function of system morphology, factors that limit primary production, and diets of the fish species (Strayer et al, 2005).
Modification of natural benthic communities: The invasion of Dreissena polymorpha resulted in an 85% decrease of phytoplanktron in the Hudson River. However, bacterial abundances in the Hudson have increased roughly two times since the D. polymorpha arrived, making it clear that direct consumption by Dreissena is a minor process, but they do dramatically reduced flagellated protozoans, the major predator of bacteria. Another study found a 778- fold decrease in cyanobacteria and a 2.5- times fold decrease in diatoms in the Hudson River after the introduction of D. polymorpha. (Caraco et al., 1997; Findlay et al., 1998; Smith et al., 1998).
Reduction in native biodiversity: Native bivalves of the Hudson experienced a 65-100% decline in the Hudson River after the introduction of Dreissena polymorpha. By 2000, their populations seemed to level off and are believed to remain at 4-22% of preinvasion densities (Strayer & Malcom, 2007). Illinois (United States (USA))
Reduction in native biodiversity: Significant declines of native mussels have occurred in Illinois rivers as a result of the establishement of Dreissena polymorpha (Ricciardi et al, 1998). Michigan (United States (USA))
Reduction in native biodiversity: A sampling of the Detroit River in 1982-1983 before the introduction of Dreissena polymorpha and Dreissena bugensis resulted in 97% live unionids of 20 different species, whereas a sampling in 1992 after the introduction of the two Dreissena spp. resulted in only 10% live unionids of only 13 different species. Surveys of native freshwater mussels along main channels of the Detroit River from 1992-1994 showed that unionids had been extirpated from all but four sites in the upper reaches of the river due to impacts of dreissenid mussels (Schloesser et al, 1998; Schloesser et al, 2006). Mississippi River (United States (USA))
Threat to endangered species: Over 60 endemic mussel species are threatened with extinction by impacts from Dreissena polymorpha. Specifically it threatens federally-endangered Higgins eye pearlymussel (Lampsilis higginsii) (Ricciardi et al, 1998; Wege, 2005). New York (United States (USA))
Ecosystem change: Researchers report a significant increase in water clarity of Oneida Lake, New York as a result of the introduction of Dreissena polymorpha. Its filtration of particles in the water column increases water clarity, allowing deeper light penetration which in turn enhances benthic photosynthesis (Zhu et al, 2006). Ohio (United States (USA))
Ecosystem change: Colonization of Dreissena polymorpha population in Hargus Lake, a small thermally stratified reservoir in Ohio, US., caused a significant increase in water clarity and a remarkable decrease in phytoplankton biomass during the period from 1993 to 1995. Increased light penetration and reduced organic matter loading to the meta-and hypolimnion were reflected in the lake stratification patterns, particularly in the temperature and oxygen profiles in the metalimnion (Yu & Culver, 2000).
Reduction in native biodiversity: Significant declines of native mussels have occurred in Ohio rivers as a result of the establishment of Dreissena polymorpha (Ricciardi et al., 1998). Pennsylvania (United States (USA))
Fouling: Dreissena polymorpha fouls native unionid mussels in these lakes (Butkas & Ostrofsky, 2006).
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