Effects of Artificial Filamentous Substrate on Zebra Mussel (Dreissena polymorpha) Settlement

Though a great deal of research focuses on the range expansion and presence of adult zebra mussels, there is still a need to understand the processes of larval settlement and how that relates to adult populations. There is evidence that marine bivalves preferentially settle on filamentous substrates such as hydroid colonies and algae; however, similar studies are rare in freshwater systems. We examined the importance of filamentous substrate for the settlement of the zebra mussel (Dreissena polymorpha) larvae by deploying PVC settlement plates with and without polypropylene filaments in the Bark River for a 6-week period. Larval supply was monitored weekly. Our results suggest that artificial filaments facilitated recruitment, primarily by increasing surface area available for attachment. Mussels on artificial filaments were significantly smaller in size than mussels attached to filamentous or control plate surfaces, providing some evidence that mussels may detach from filamentous substrate after initial settlement. This study adds to our general understanding about the role of filamentous substrates in the process of larval settlement and suggests that substrates colonized by filamentous epibionts may face increased risk of fouling by zebra mussels. An erratum to this article is available at .     

Inhibition of the reattachment of young adult zebra mussels by single-species biofilms and associated exopolymers

AIMS: To determine the effects of single-species bacterial films and their associated extracellular products on the reattachment of young adult zebra mussels. MATERIALS AND RESULTS: Ten strains of bacteria were isolated from surfaces where adult zebra mussels can be found attached in nature. Single-species biofilms were developed on both glass and polystyrene using these bacteria. The reattachment of zebra mussels (i.e. with byssal threads) was compared between surfaces with and without films. Although no differences were observed in mussel reattachment between glass surfaces with and without films (P > 0.05, anova), a reduction in mussel reattachment between polystyrene surfaces with and without films was observed for seven of the 10 strains (P < or = 0.05 to <0.001, anova). Bacterial extracellular products (BEP) were isolated from five bacterial films and tested for their effects on mussel reattachment. Four of the five sets of isolated extracellular products evoked the same effects as their respective intact biofilms. CONCLUSIONS: We conclude that depending on the substratum, individual strains of bacteria in biofilms can inhibit the reattachment of adult zebra mussels. In some cases, BEP were the source of the inhibitory effects. SIGNIFICANCE AND IMPACT OF THE STUDY: The nature of the substratum on which the biofilms develop affects properties of the biofilm and its extracellular components, which subsequently influences zebra mussel reattachment.     

The effects of natural biofilms on the reattachment of young adult zebra mussels to artificial substrata

This laboratory study examined the effects of natural biofilms on the reattachment of young adult zebra mussels, Dreissena polymorpha, in Petri dishes. Natural biofilms were developed in glass and polystyrene Petri dishes using water samples collected at various times of the year. Biofilms were developed over 1, 3, 8, and 14 d. Controls were clean glass and polystyrene Petri dishes. Zebra mussels collected from the field (< or = 10 mm, ventral length) were placed in the dishes and their reattachment by byssal threads was recorded after 1 d. Zebra mussels reattached to the dish surface or the shells of other mussels in the dish, or remained unattached. The data indicate that reattachment to clean glass was greater than to clean polystyrene (p < or = 0.05, ANOVA), but there were no consistent differences between reattachment to filmed polystyrene and filmed glass dish surfaces. Zebra mussels in control and filmed glass dishes reattached in higher percentages to the dish surface compared to the shells of other mussels (p < or = 0.05, ANOVA). There was no difference in mussel of reattachment between the dish surface and the shells of other mussels in most control polystyrene dishes (p > 0.05, ANOVA), whereas in filmed polystyrene the percentage of reattachment to the dish surface was greater than to the shells of other mussels (p < or = 0.05, ANOVA). These results indicate that substratum wettability and the presence of biofilms on some types of substrata can be factors in the reattachment of young adult zebra mussels.     

Zebra mussels decrease burrowing ability and growth of a native snail, <Emphasis Type="Italic">Campeloma decisum</Emphasis>

Invasive species can drive native organisms to extinction by limiting movement and accessibility to essential resources. The purpose of this study was to determine if zebra mussels (Dreissena polymorpha) affect the burrowing ability and growth rate of a native snail, Campeloma decisum. Snails with and without zebra mussels were collected from Douglas Lake, MI, and burrowing depths were studied in both the laboratory and Douglas Lake. Growth rates were calculated as the amount of shell growth from 2004 to 2005. Both the tendency of snails to burrow and the depth to which they burrowed into the substrate were significantly decreased by the presence of zebra mussels on snail shells in both laboratory and lake experiments. There was no difference in the percentage of snails that exhibited growth as a function of zebra mussel presence. However, for those snails that grew, there was a 50% higher growth rate for snails without zebra mussels compared to snails with zebra mussels. These negative effects of zebra mussels on growth and burrowing ability will likely lead to decreases in snail population densities in the future. Handling editor: S. Wellekens     

Testing a typology system of running waters for conservation planning in Hungary

The zebra mussel (Dreissena polymorpha) and its congener the quagga mussel (Dreissena rostriformis bugensis) are both invaders in freshwater, but have very different invasion histories, with zebra mussels attaining substantially faster rates of spread at virtually all spatial scales. However, in waterbodies where they co-occur, D. r. bugensis can displace D. polymorpha. To determine if the mechanisms for this displacement are associated with different survival and growth, we kept mussels in flow-through tanks for 289 days with two temperature regimes that mimicked the natural surface water (littoral zone) and hypolimnion conditions of Lake Erie. For the littoral zone regime, we used water directly from the surface of Lake Erie (range 4–25°C, average 11.9 ± 0.6°C). For the profundal zone treatment, Lake Erie surface water was chilled to about 6°C (range 5–8°C, average 6.2 ± 0.6°C) for the full duration of the experiment. For each of these temperature regimes, we used three replicate tanks with only zebra mussels present and three replicate tanks with only quagga mussels (150 ind./tank each), and three replicate tanks with both species (75 ind./tank of each species). Quagga mussels had higher survivorship and grew more than zebra mussels in all treatments. For both species, the size of the mussel entering the winter was critical for survivorship. Larger mussels had a higher survival over the winter in all treatments. For both species, there was a survivorship and growth tradeoff. In the warmer littoral zone treatment both species had higher growth, but lower survival than in the colder profundal zone treatment. Surprisingly, although quagga mussels outperformed zebra mussels, zebra mussel survivorship was better when they were faced with competition by quagga mussels than with just intraspecific competition. In addition, quagga mussels suffered size-specific mortality during the growing season only when facing interspecific competition with zebra mussels. Further experiments are needed to determine the possible mechanisms for these interspecific effects.     

Predator‐induced morphological defences in two invasive dreissenid mussels: implications for species replacement

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Zebra mussels (Dreissena polymorpha) limit food for larval fish (Pimephales promelas) in turbulent systems: a bioenergetics analysis*

We conducted a factorial experiment, in outdoor mesocosms, on the effects of zebra mussels and water column mixing (i.e., turbulence) on the diet, growth, and survival of larval fathead minnows (Pimephales promelas). Significant (P<0.05) larval mortality occurred by the end of the experiment with the highest mortality (90%) occurring in the presence of both turbulence and zebra mussels, whereas mortality was 37% in treatment with turbulence and 17% and 18% in the zebra mussels treatment, and the control, respectively. The size of individual fish was significantly different among treatments at the end of the experiment and was inversely related to survival. Levels of trophic resources (i.e., phyto and zooplankton) varied among treatments and were treatment specific. Turbulent mixing facilitated removal of phytoplankton by zebra mussels by making the entire water column of the tanks available to these benthic filter feeders. Early in the experiment (Day = 0 to 14) the physical process of turbulent mixing likely caused a reduction in standing stocks of zooplankton. The interactive effect of turbulence and mussels reduced copepod and rotifer stocks, through physical processes and through filtration by zebra mussels, relative to the turbulence treatment. The reductions in the number of total zooplankton in the turbulent mixing mesocosms and the further reduction of rotifer and copepod in the turbulence and mussels treatment coincided with a period of increased reliance of larval fathead minnows on these prey. Estimates of consumption from bioenergetics modeling and measured prey standing stocks indicated caloric resources of suitable prey in turbulence treatments during the early weeks of the experiment were insufficient to prevent starvation. Early mortality in the turbulence and mussels treatment likely released surviving fish from intense intraspecific competition and resulted in higher individual growth rates. A combination of high abundance of zebra mussels in an environment with a well-mixed water column can have significant effects on larval fish survival and growth.     

Predation on exotic zebra mussels by native fishes: effects on predator and prey

SUMMARY 1. Exotic zebra mussels, Dreissena polymorpha, occur in southern U.S. waterways in high densities, but little is known about the interaction between native fish predators and zebra mussels. Previous studies have suggested that exotic zebra mussels are low profitability prey items and native vertebrate predators are unlikely to reduce zebra mussel densities. We tested these hypotheses by observing prey use of fishes, determining energy content of primary prey species of fishes, and conducting predator exclusion experiments in Lake Dardanelle, Arkansas. 2. Zebra mussels were the primary prey eaten by 52.9% of blue catfish, Ictalurus furcatus; 48.2% of freshwater drum, Aplodinotus grunniens; and 100% of adult redear sunfish, Lepomis microlophus. Blue catfish showed distinct seasonal prey shifts, feeding on zebra mussels in summer and shad, Dorosoma spp., during winter. Energy content (joules g⁻¹) of blue catfish prey (threadfin shad, Dorosoma petenense; gizzard shad, D. cepedianum; zebra mussels; and asiatic clams, Corbicula fluminea) showed a significant species by season interaction, but shad were always significantly greater in energy content than bivalves examined as either ash-free dry mass or whole organism dry mass. Fish predators significantly reduced densities of large zebra mussels (>5 mm length) colonising clay tiles in the summers of 1997 and 1998, but predation effects on small zebra mussels (≤5 mm length) were less clear. 3. Freshwater drum and redear sunfish process bivalve prey by crushing shells and obtain low amounts of higher-energy food (only the flesh), whereas blue catfish lack a shell-crushing apparatus and ingest large amounts of low-energy food per unit time (bivalves with their shells). Blue catfish appeared to select the abundant zebra mussel over the more energetically rich shad during summer, then shifted to shad during winter when shad experienced temperature-dependent stress and mortality. Native fish predators can suppress adult zebra mussel colonisation, but are ultimately unlikely to limit population density because of zebra mussel reproductive potential.     

Indigenous microflora and opportunistic pathogens of the freshwater zebra mussel, Dreissena polymorpha

Freshwater fouling invertebrate zebra mussels (Dreissena polymorpha) harbor a diverse population of microorganisms in the Great Lakes of North America. Among the indigenous microorganisms, selective species are opportunistic pathogens to zebra mussels. Pathogenicity to zebra mussels by opportunistic bacteria isolated from the mussels was investigated in this study. Among the more than 30 bacteria isolated from temperature-stressed mussels, Aeromonas media, A. veronii, A. salmonicida subsp. salmonicida, and Shewanella putrefaciens are virulent pathogens to juvenile zebra mussels. Inoculation of a bacterial concentration of A. media, A. salmonicida subsp. salmonicida and S. putrefaciens at 10⁷ cells per zebra mussel resulted in 100% mortality within 5 days, and only 64.9% for A. veronii. In contrast, mortality was less than 12.3% following inoculation of a sterile phosphate buffer solution as a control. In addition, mortality was dependent on the size of the pathogen population used in inoculation and the incubation temperature, indicating the close relationship between the bacterial population and subsequent death. On the mussel tissue, a dense microbial population was evident from the moribund mussels viewed with Scanning Electron Microscope (SEM). Opportunistic bacteria invaded and destroyed the D. polymorpha tissue after 7 days of incubation when the bacterial inoculation was larger than 10⁵ per zebra mussel. Our results suggest that mussels are reservoirs of opportunistic pathogenic microorganisms to aquatic organisms and humans and a better understanding of the microbial ecology of the mussels will provide insights to the possible health hazards from these microorganisms.     

Body size and food thresholds for zero growth in Dreissena polymorpha: a mechanism underlying intraspecific competition

1. Dreissena polymorpha is an extraordinarily successful invasive species that shows high recruitment of small juvenile mussels on established mussel banks. Such juvenile settlement on, and overgrowth of, large adult mussels; however, leads to competition with adults, and often at high densities and low‐food concentrations. 2. The concept of food thresholds for zero growth has been a powerful approach to explaining size‐related exploitative competition in different zooplankton species. We applied it to investigate whether food threshold concentrations for zero growth (C₀) differ between juvenile and adult zebra mussels. 3. By determining body mass growth at various concentrations of a diet mixture (Nannochloropsis limnetica and Isochrysis aff. galbana) we demonstrate that the threshold food concentration for growth of juvenile mussels (C₀ = 0.08 mg C L⁻¹) is substantially lower than that for adults (C₀ = 0.36 mg C L⁻¹). 4. This indicates that, at low food availability, juvenile zebra mussels are competitively superior to their larger conspecifics. Within zebra mussel banks plankton food is substantially depleted and so the observed mechanism might ensure juvenile success and therefore the regeneration of mussel banks in nature.     

Significant fish predation on zebra mussels Dreissena polymorpha in Lake Champlain,U.S.A.

The objectives of this study were to: (1) determine which fishes were consistently eating zebra mussels Dreissena polymorpha in Lake Champlain and document their feeding behaviour and (2) quantify the diet composition of the fish predators that were found to consume zebra mussels. From 2002 to 2005, freshwater drum Aplodinotus grunniens, pumpkinseed Lepomis gibbosus, yellow perch Perca flavescens and rock bass Ambloplites rupestris all consumed zebra mussels at varying frequencies and amounts. Aplodinotusgrunniens and L. gibbosus chewed clumps of zebra mussels, expelling shells, whereas P. flavescens and A. rupestris swallowed small individuals whole. Lepomis gibbosus consumed zebra mussels at the highest frequency (65–89% of prey consumed) and zebra mussels comprised a large part of this fish’s diet (up to 40% by dry mass). Zebra mussels were also an important component of the diet of A. grunniens (up to 59% of the diet by dry mass, 40–63% frequency of consumption). The percentage of the diet comprising zebra mussels in P. flavescens and A. rupestris varied significantly from year to year but never exceeded 10%. Because A. grunniens and L. gibbosus crushed zebra mussels, the nutritional return from consuming zebra mussels would be similar to other prey; for P. flavescens and A. rupestris zebra mussels were only partially digested and the nutritional return would probably be low. As predation on zebra mussels is widespread and significant, it is possible that fish predators could contribute to regulating the population of zebra mussels in Lake Champlain.     

Effects of invasive zebra mussels on phytoplankton,turbidity, and dissolved nutrients in reservoirs

Few experiments have quantified the effects of invasive zebra mussels (Dreissena polymorpha) on man-made reservoirs relative to other aquatic habitats. Reservoirs, however, are the dominate water body type in many of the states that are at the current front of the zebra mussel invasion into the western United States. The objective of this research, therefore, was to determine how zebra mussels affected phytoplankton, turbidity, and dissolved nutrients in water that was collected from three Kansas reservoirs that varied in trophic state (mesotrophic to hypereutrophic), but all experienced frequent cyanobacterial blooms. Laboratory mesocosm experiments were conducted to document the effects of zebra mussels on cyanobacteria and general water quality characteristics in the reservoir water. Zebra mussels significantly reduced algal biomass, and the total biovolume of cyanobacteria (communities were dominated by Anabaena) in each reservoir experiment. The effects of zebra mussels on other major algal groups (diatoms, flagellates, and green algae) and algal diversity were less consistent and varied between the three reservoir experiments. Similarly, the effects of zebra mussels on nutrient concentrations varied between experiments. Zebra mussels increased dissolved phosphorus concentrations in two of the reservoir experiments, but there was no effect of zebra mussels on dissolved phosphorus in the mesotrophic reservoir experiment. Combined, our results strongly suggest that zebra mussels have the potential to significantly impact reservoirs as they continue to expand throughout the western United States. Moreover, the magnitude of these effects may be context dependent and vary depending on the trophic state and/or resident phytoplankton communities of individual reservoirs as has similarly been reported for natural lakes.     

Zebra mussels affect benthic predator foraging success and habitat choice on soft sediments

The introduction of zebra mussels (Dreissena spp.) to North America has resulted in dramatic changes to the complexity of benthic habitats. Changes in habitat complexity may have profound effects on predator-prey interactions in aquatic communities. Increased habitat complexity may affect prey and predator dynamics by reducing encounter rates and foraging success. Zebra mussels form thick contiguous colonies on both hard and soft substrates. While the colonization of substrata by zebra mussels has generally resulted in an increase in both the abundance and diversity of benthic invertebrate communities, it is not well known how these changes affect the foraging efficiencies of predators that prey on benthic invertebrates. We examined the effect of zebra mussels on the foraging success of four benthic predators with diverse prey-detection modalities that commonly forage in soft substrates: slimy sculpin (Cottus cognatus), brown bullhead (Ameirus nebulosus), log perch (Percina caprodes), and crayfish (Orconectes propinquus). We conducted laboratory experiments to assess the impact of zebra mussels on the foraging success of predators using a variety of prey species. We also examined habitat use by each predator over different time periods. Zebra mussel colonization of soft sediments significantly reduced the foraging efficiencies of all predators. However, the effect was dependent upon prey type. All four predators spent more time in zebra mussel habitat than in either gravel or bare sand. The overall effect of zebra mussels on benthic-feeding fishes is likely to involve a trade-off between the advantages of increased density of some prey types balanced against the reduction in foraging success resulting from potential refugia offered in the complex habitat created by zebra mussels.     

Predation by the invasive American signal crayfish, Pacifastacus leniusculus Dana, on the invasive zebra mussel, Dreissena polymorpha Pallas: the potential for control and facilitation

Non-indigenous crayfish often have major ecological impacts on invaded water bodies, and have contributed to the decline of native crayfish species throughout Europe. The American signal crayfish, Pacifastacus leniusculus, is the most widespread invasive crayfish in Great Britain, where the zebra mussel, Dreissena polymorpha, is similarly an invasive pest species. The potential for the American signal crayfish to regulate zebra mussel populations was investigated through a series of laboratory experiments. Crayfish were found to be highly size selective, consuming significantly more of the smallest size class of zebra mussels offered (7–12 mm), over medium (16–21 mm) and large (25–30 mm). Crayfish feeding rate on zebra mussels was not altered when mussels were presented clumped together in natural druses compared with mussels in a disassembled druse. Crayfish spent significantly more time foraging when mussels were unattached, and a greater proportion of attacks were on medium and large than on small mussels (83% of attacks were on medium and large mussels when unattached as opposed to 47% when on druses). Individual crayfish feeding rate decreased significantly at densities of > ~5 crayfish m⁻². Signal crayfish are, therefore, unlikely to be able to significantly impact established populations of zebra mussels in the wild, although zebra mussels have the potential to provide crayfish with a substantial food source.     

Predicting habitat use and trophic interactions of Eurasian ruffe,round gobies,and zebra mussels in nearshore areas of the Great Lakes

The Laurentian Great Lakes have been subject to numerous introductions of nonindigenous species, including two recent benthic fish invaders, Eurasian ruffe (Gymnocephalus cernuus) and round gobies (Neogobius melanostomus), as well as the benthic bivalve, zebra mussel (Dreissena polymorpha). These three exotic species, or “exotic triad,” may impact nearshore benthic communities due to their locally high abundances and expanding distributions. Laboratory experiments were conducted to determine (1) whether ruffe and gobies may compete for habitat and invertebrate food in benthic environments, and (2) if zebra mussels can alter those competitive relationships by serving as an alternate food source for gobies. In laboratory mesocosms, both gobies and ruffe preferred cobble and macrophyte areas to open sand either when alone or in sympatry. In a 9-week goby–ruffe competition experiment simulating an invasion scenario with a limited food base, gobies grew faster than did ruffe, suggesting that gobies may be competitively superior at low resource levels. When zebra mussels were added in a short-term experiment, the presence or absence of mussels did not affect goby or ruffe growth, as few zebra mussels were consumed. This finding, along with other laboratory evidence, suggests that gobies may prefer soft-bodied invertebrate prey over zebra mussels. Studies of interactions among the “exotic triad”, combined with continued surveillance, may help Great Lakes fisheries managers to predict future population sizes and distributions of these invasive fish, evaluate their impacts on native food webs, and direct possible control measures to appropriate species.     

Seasonal effects of zebra mussels on littoral nitrogen transformation rates in Gull Lake, Michigan, U.S.A.

• 1 Zebra mussels (Dreissena polymorpha) are successful colonisers of lake littoral habitats and they interact strongly with littoral benthos. Previous research suggests that localised areas colonised by zebra mussels may be hotspots of nitrogen (N) cycling.
• 2 The effects of zebra mussels on nitrification and denitrification rates were examined approximately every other month for 1 year in Gull Lake, Michigan, U.S.A. Littoral sediment was collected from an area free of zebra mussels and distributed into shallow trays; rocks colonised with zebra mussels were placed in half of the trays, while uncolonised rocks were placed in the remaining trays. After an incubation period of 6–8 weeks in the lake, sediment and zebra mussels were collected from the trays, replaced with new sediment and zebra mussels, and placed in the lake for the next interval. In the laboratory, sediment nitrification and denitrification rates were measured for each tray.
• 3 Sediment nitrification rates did not increase in the presence of zebra mussels; instead nitrification rates were sensitive to changes in water temperature and increased with increasing exchangeable sediment ammonium. In contrast, denitrification rates increased in sediment trays with zebra mussels in the winter when nitrate (NO₃⁻) availability was high and when Chara did not grow in the trays.
• 4 Sediment denitrification was NO₃⁻‐limited in all seasons, regardless of zebra mussel treatment. However, sediment in the presence of zebra mussels responded less to NO₃⁻ addition, suggesting that NO₃⁻ limitation of denitrification can be reduced by zebra mussel activity. Zebra mussels have a seasonally variable impact on sediment denitrification rates, and this may translate into altered seasonal patterns of N cycling in localised areas of lakes where they are particularly abundant.

     

Experimental Pathogenicity of Aeromonas spp. for the Zebra Mussel, Dreissena polymorpha

Experiments were conducted to determine whether species of Aeromonas were pathogenic to the zebra mussel Dreissena polymorpha. A. jandaei, A. veronii, and A. media, identified with Biolog, were originally isolated from dead zebra mussels. When inoculated into living mussels, these bacteria resulted in the mortality of the bivalves. Two additional species, A. salmonicida salmonicida (ATCC 33678) and A. hydrophila (ATCC 7966), were also demonstrated to be pathogenic to the mussels. In addition to the pathogenicity, the data also suggest that the zebra mussels may be an important reservoir for these bacteria in freshwater environments. Received: 26 March 1997 / Accepted: 7 July 1997     

In situ survival and growth of zebra mussels (Dreissena polymorpha) under chronic hypoxia in a stratified lake

Experiments and field surveys were conducted in Hargus Lake (Ohio, U.S.) to investigate the effect of lake stratification on the survival, growth and distribution of zebra mussels. During the lake stratification period, relatively stable temperature and dissolved oxygen (DO) gradients persisted across the water column, allowing us to examine the chronic effect of hypoxia on zebra mussels. Zebra mussels were incubated in cages and suspended at different depths in the water column at both pelagic (max. depth = 12 m) and littoral (max. depth = 3.5 m) sites from April 18 to September 28, 1994. No mussel survived to the end of the experiment in cages ≥ 5.5 m, whereas the highest survival rate (76%) occurred at 5 m depth where temperature and DO remained fairly stable for at least 3 months. The threshold oxygen level for survival was between 1.0–1.7 mg l^-1 when water temperature was at about 17–18 °C. While zebra mussels′ survival rate was not affected under the sublethal hypoxic conditions, their growth was greatly retarded by poor water quality. The field survey showed that the zebra mussels and macrophytes had about the same distribution and their biomasses were positively related. The percentage of mussels in aggregates increased towards their maximum distribution depth. The maximum distribution depth of the naturally occurring zebra mussels was only 2.8 m, whereas the adult mussels could survive the entire stratification period when being artificially placed on the 3.5 m bottom, and young mussels could colonize the 3.5 m bottom if solid substrates were provided. We conclude that lack of substrate, rather than hypoxia, was the limiting factor of zebra mussel distribution above 5 m depth in Hargus Lake. This revised version was published online in July 2006 with corrections to the Cover Date.     

The zebra mussel (<Emphasis Type="Italic">Dreissena polymorpha</Emphasis>) impacts European bitterling (<Emphasis Type="Italic">Rhodeus amarus</Emphasis>) load in a host freshwater mussel (<Emphasis Type="Italic">Unio pictorum</Emphasis>)

The European bitterling, Rhodeus amarus, is a non-indigenous fish species in British fresh waters. It lays its eggs in unionid mussels which themselves are vulnerable to fouling by the non-indigenous zebra mussel, Dreissena polymorpha. Observations from an unmanipulated natural system showed that only 27% of zebra mussel-fouled Unio pictorum hosted bitterling, while 47% of unfouled U. pictorum hosted bitterling. We conducted a field experiment in the River Great Ouse catchment, Cambridgeshire, England in May–June 2007 and 2008 to quantify the impact of zebra mussels on bitterling load in host mussels. Zebra mussel-fouled unionids were significantly less likely to host bitterling than unfouled unionids. The number of unionids hosting bitterling did not differ significantly whether the zebra mussels fouling the unionid were alive or dead. Bitterling appeared to discriminate against zebra mussel-fouled unionids less as the 2007 breeding season advanced, potentially because preferred unfouled unionids had a higher bitterling load, and were therefore relatively lower quality hosts than at the start of the breeding season.     

Soluble nitrogen and phosphorus excretion of exotic freshwater mussels (Dreissena spp.): potential impacts for nutrient remineralisation in western Lake Erie

1. Recent increases in phytoplankton biomass and the recurrence of cyanobacterial blooms in western Lake Erie, concomitant with a shift from a community dominated by zebra mussels (Dreissena polymorpha) to one dominated by quagga mussels (D. bugensis), led us to test for differences in ammonia‐nitrogen and phosphate‐phosphorus excretion rates of these two species of invasive molluscs. 2. We found significant differences in excretion rate both between size classes within a taxon and between taxa, with zebra mussels generally having greater nutrient excretion rates than quagga mussels. Combining measured excretion rates with measurements of mussel soft‐tissue dry weight and shell length, we developed nutrient excretion equations allowing estimation of nutrient excretion by dreissenids. 3. Comparing dreissenid ammonia and phosphate excretion with that of the crustacean zooplankton, we demonstrated that the mussels add to nitrogen and phosphorus remineralisation, shortening nitrogen and phosphorus turnover times, and, importantly, modify the nitrogen and phosphorus cycles in Lake Erie. The increased nutrient flux from dreissenids may facilitate phytoplankton growth and cyanobacterial blooms in well‐mixed and/or shallow areas of western Lake Erie.