Multicomponent molluscicide mixtures for zebra mussel control

Intuitively, it is reasonable to expect enhanced control of the biofouling zebra mussel through multicomponent molluscicide cocktails. In this study, the potential of combined potassium chloride, polyDADMAC, niclosamide ethanolamine salt and 2-(thiocyanomethylthio)benzothiazole (TCMTB) for zebra mussel mitigation was investigated. A series of mixtures of varying compositions was tested. First, the combination was considered in its entirety, and the nature of the biocides' joint toxicity was elucidated by adopting a structured classification system previously defined. Then, a central composite experimental design was employed to detail the contribution of each ingredient to the blend performance and ultimately derive an empirical model of mixture effects to optimise the formulation composition. Whilst the action of some of the toxins was synergised, the blend does not appear promising for zebra mussel control. Overall, the chemicals acted less than additively and, under some circumstances, antagonistic effects were observed. Although these results do not immediately lead to a new approach to pest mitigation, the study highlights aspects that are of practical relevance for the design of combined chemical treatments. In particular, this work recalls the funnel hypothesis from the field of ecotoxicology (blends tend to be additive as the number of ingredients increases), which may provide key guidance in the mixture design process. Furthermore, the study shows that multiple biocides do not necessarily ensure improved zebra mussel mitigation, and therefore the nature of their combined effects should always be carefully examined. The systematic procedure proposed here to critically design biocide blends is useful in this context.     

Heritability of Heat Tolerance in Zebra Mussel Veligers

Previous work has suggested that zebra mussel populations differ in temperature tolerance and researchers have hypothesized that these differences have been caused by natural selection. For selection to act on a phenotypic character such as heat tolerance, there must be additive genetic variation for the character within populations. Individuals must have different alleles at loci that contribute to measurable differences in the phenotypic character of interest. We tested the hypothesis that heat tolerance of zebra mussel veligers has an additive genetic component and is therefore heritable. We used a full-sib, half-sib design with a large number of full-sib families (20), half-sib families (5), and offspring (averaging 41) per family (estimated power, β = 0.75). We exposed family groups to a lethal temperature of 34°C and determined individual time-to-death. Our best estimate of heritability (which can range from 0 to 1) was −0.125 (±0.095), which implies that heritability of heat tolerance among veligers was extremely low. These results are surprising given the high genetic diversity reported for North American zebra mussel populations. A lack of additive genetic variation for heat tolerance within the studied population means that heat tolerance of the veliger stage is not likely to increase further via natural selection. Further studies examining both the veliger and adult stages, and examining populations in different watersheds are required to show whether this lack of variation is widespread among North American zebra mussel populations.     

Evaluation of candidate biocides to control the biofouling Asian clam in the drinking water treatment industry: An environmentally friendly approach

The biofouling Asian clam Corbicula fluminea greatly affects freshwater-dependent facilities in Europe and North America, including in the Great Lakes region. As chlorination has become increasingly restricted, finding alternative control agents is a priority. Due to the species' epifaunal nature, the body of knowledge on Dreissena polymorpha is larger than that on the Asian clam, and there is a tendency to assume that mitigation methods should work similarly for both species. However, this generalisation is inaccurate, and the optimisation of Asian clam control relies on species-specific toxicological data. This paper reports information on the potential of three candidate biocides for C. fluminea control: (i) the cationic polydiallyldimethylammonium chloride (polyDADMAC), (ii) potassium chloride and (iii) aluminum sulphate. While these chemicals may be employed in a range of contexts, they are particularly suitable for the highly regulated drinking water industry. LT₅₀ values ranging from 284.3 h, for polyDADMAC applied at 10 mg/L, to 855.1 h, for an aluminum sulphate concentration of 11 mg/L, were obtained. Ecotoxicological standard tests with Pseudokirchneriella subcapitata and Daphnia magna suggested that, amongst the three biocides, potassium chloride is the one representing lower potential environmental hazard (with, for example, 48 h-EC₅₀ higher than 1 g/L for Daphnia immobilisation, which compares to values of 20.2 and 112.5 mg/L for polyDADMAC and aluminum sulphate, respectively). The three chemicals are promising control agents at dosages compatible with waterworks' operational requirements with polyDADMAC having the highest biocidal activity, but also posing more environmental risks.     

Effects of the Zebra Mussel,Dreissena polymorpha,on Community Nitrogen Dynamics in Saginaw Bay,Lake Huron

The effects of the zebra mussel, Dreissena polymorpha, on chlorophyll and nutrient concentration changes and community ammonium uptake and regeneration rates were determined in bottle experiments on waters collected from a eutrophic site and an oligotrophic site in Saginaw Bay, Lake Huron in 1992. Our objectives were to estimate nitrogen cycling rates and to determine the direct (excretion) and indirect (foodweb) effects of the zebra mussel on these rates. Isotope labeling experiments with added ¹⁵NH₄⁺ were conducted on waters collected on five sampling dates between April and October. Direct effects of zebra mussels on ammonium regeneration and potential uptake were examined by comparing results from bottles incubated with (15 individuals in 4 L lake water) and without added zebra mussels. Indirect foodweb effects were examined by measuring regeneration and potential uptake rates in subsamples of water that had previously been incubated in the presence or absence of zebra mussels.Zebra mussels removed a large fraction of chlorophyll from the oligotrophic site on all sampling dates and from the eutrophic site in October, but had a negligible effect on chlorophyll levels in waters from the eutrophic site in June, July, August, and September when cyanophytes were abundant. Community ammonium regeneration rates and uptake rates both followed seasonal patterns resembling those for chlorophyll concentrations in control treatments at the eutrophic site. Rates for water from the oligotrophic site were low (usually not significantly different from zero) and are not reported here. Community ammonium regeneration rates were consistently enhanced in the presence of zebra mussels, indicating that zebra mussel excretion could have a dominant effect on nitrogen regeneration in regions where it is abundant. Zebra mussels appeared to decrease community uptake rates of ammonium in August and September but did not predictably affect nitrogen remineralization rates by other lower foodweb organisms (e.g. bacteria, protozoans, zooplankton).     

Effects of Zebra Mussel (Dreissena polymorpha) Colonization on Water Quality Parameters in Saginaw Bay,Lake Huron

A large-scale study of Saginaw Bay was initiated in 1990 and continued through 1993 to examine the effects of the zebra mussel colonization which began in summer/fall 1991. Saginaw Bay responded quickly to the zebra mussel colonization, as fall 1991 values of chlorophyll were similar to 1992 and 1993 values. In inner Saginaw Bay, where most zebra mussels were found, chlorophyll, kPAR, and total phosphorus values decreased, and Secchi disk depth increased during the study period, regardless of the presence or absence of zebra mussels at a specific station. At outer bay control stations no significant differences were found for chlorophyll, kPAR, and Secchi disk values. In order to examine longer-term trends, water quality data from 1979–1980 (STORET) were combined with our 1990 data (pre-zebra mussel period) and compared to values from the post zebra mussel period (fall 1991, all 1992 and 1993). At stations with high densities of zebra mussels, chlorophyll and total P decreased by 66% and 48%, respectively, and Secchi disk values increased 88%. At outer bay control stations no significant differences were found for chlorophyll or Secchi disk. When parameters were averaged throughout inner Saginaw Bay, zebra mussels caused a 59% and 43% decrease in chlorophyll and in total phosphorus and a 60% increase in Secchi disk transparency. Although zebra mussels significantly altered water quality parameters in the pelagic region of Saginaw Bay, they did not necessarily change system trophic state; rather they altered the spatial partitioning of resources.     

Behavioral and reproductive effects of the lampricides TFM and TFM:1% Niclosamide on native freshwater mussels

The lampricides TFM (3-trifluoromethyl-4′-nitrophenol) and Niclosamide (NIC, 2′, 5-dichloro-4′-nitrosalicylanilide) are used to control sea lamprey populations in the Great Lakes and associated tributaries. Niclosamide is often used as an additive to TFM to reduce the amount of TFM required to control sea lamprey. Concern is growing over the risk that lampricide treatments pose to native freshwater mussels residing in streams. Our objectives were to determine the acute toxicity of TFM and TFM:NIC to free glochidia (removed from the marsupial gills), compare the relative toxicity of TFM and TFM:NIC between free glochidia and brooded glochidia (within the marsupial gills), determine if glochidia age influences toxicity, and assess if exposure of gravid mussels to TFM and TFM:NIC alters behavior and reproduction. Three acute toxicity tests (2:TFM, 1:TFM : NIC) were conducted with glochidia and adults of the plain pocketbook mussel (Lampsilis cardium). In tests with glochidia, viability did not differ across TFM and TFM : NIC concentrations that encompassed typical stream treatments. Glochidia age influenced toxicity as glochidia obtained later in the brooding season were less viable than glochidia obtained earlier in the brooding season. Exposure of adults to elevated concentrations of lampricides often resulted in behavioral effects, but rarely affected reproductive endpoints. Because mussels are long-lived (30 to 100 y), even intermittent and short duration exposures may cumulatively affect mussels over their lifetime. The risks posed by lampricide treatments in the Great Lakes would be further informed by research on the sublethal effects of lampricides, particularly effects on non-target organisms such as mussels.     

Analysis of the Impacts of the Zebra Mussel, Dreissena polymorpha, on Nutrients,Water Clarity,and the Chlorophyll-Phosphorus Relationship in Lower Green Bay

Zebra mussels, Dreissena polymorpha, invaded Green Bay, Lake Michigan in the early 1990s. In 1986, prior to zebra mussel invasion, the Green Bay Metropolitan Sewerage District initiated a long-term water quality monitoring program involving 12 stations in three distinct zones along a trophic gradient in lower Green Bay. We analyzed this data set pre and post invasion using various regression models to determine the impacts of the zebra mussel on water clarity, nutrient concentrations, and the relationship between chlorophyll and phosphorus in this system. Following zebra mussel invasion, Secchi depths did not change in all three zones. Chlorophyll a concentrations decreased post zebra mussels in all zones. These differences were attributed to the filter feeding abilities of zebra mussels. Lower Green Bay exhibits a strong trophic gradient and zebra mussel impacts on the chlorophyll-phosphorus relationship differed between the three zones. We saw no changes in the chlorophyll-phosphorus relationship in zone 1, zone 2 appeared to be a transition zone with slight changes in the chlorophyll-phosphorus relationship, and in zone 3 there was evidence of an altered chlorophyll-phosphorus relationship post zebra mussels. These results indicate that the impact of zebra mussels on water quality parameters and on chlorophyll-phosphorus dynamics may differ depending on initial trophic status and on zebra mussel densities.     

Phytoplankton Productivity in Saginaw Bay,Lake Huron: Effects of Zebra Mussel (Dreissena polymorpha) Colonization

Phytoplankton photosynthesis-irradiance parameters, chlorophyll concentrations, underwater extinction coefficients (kPAR), and surface irradiance were determined at 8–10 sites on 27 occasions in Saginaw Bay from spring 1990 through fall 1993 corresponding to a period before and after the establishment of large zebra mussel populations (began in summer 1991). Similar measurements, with the exception of the photosynthetic parameter, α, had also been made in 1974/75 at eight sites on nine occasions. In inner Saginaw Bay where zebra mussels were primarily found, chlorophyll and kPAR values decreased, while the photosynthetic parameters, P_max and α, increased after zebra mussel colonization. At sites in the outer bay where no zebra mussels were found, chlorophyll and kPAR values did not change after zebra mussel colonization, whereas photosynthetic parameters increased. Decreases in chlorophyll and kPAR in the inner bay were related to the zebra mussel, but increases in photosynthetic parameters in both the inner and outer bay were not. Areal-integrated and volumetric phytoplankton productivity decreased by 38% and 37%, respectively, in inner Saginaw Bay after the establishment of zebra mussels; phytoplankton productivity at outer bay control sites was similar during the same period. Decreased phytoplankton productivity in the inner bay was attributable to the large decrease in chlorophyll as increases in underwater irradiance (increased kPAR) and photo synthetic parameters could not compensate for the chlorophyll effect. Increase in underwater irradiance produced a significant increase in light to the benthic region and contributed to increased benthic primary productivity; ratio of photic zone to station depth increased in inner Saginaw Bay, from 0.6–0.8 before the zebra mussel colonization (1974–1990) to 1.1–1.3 after colonization (1992–1993). Overall, primary productivity in the inner bay did not exhibit a notable change after zebra mussel colonization as decreases in phytoplankton productivity were accompanied by increases in benthic primary productivity. Thus, zebra mussels altered inner Saginaw Bay from a pelagic-dominated system to a benthic/pelagic system which will have long-term effects on food web structure and productivity at higher trophic levels.     

Benthic Invertebrate Community Responses to Round Goby (Neogobius melanostomus) and Zebra Mussel (Dreissena polymorpha) Invasion in Southern Lake Michigan

The round goby (Neogobius melanostomus Pallas), a fish native to eastern Europe, recently has become established in southwestern Lake Michigan. Because round gobies prey on zebra mussels (Dreissena polymorpha Pallas) and other benthic invertebrates, the effects of round gobies on invertebrates within zebra mussel colonies was investigated. Using a 2 × 3 factorial design, the effects of round gobies (present or absent) and zebra mussel densities (zero, low, and high) on non-mussel invertebrates was examined. Ten ceramic tiles of each mussel density were colonized in the laboratory and then anchored in Calumet Harbor, IL for 10 weeks. Round gobies had access to half the tiles while half were covered with coarse mesh screening that excluded round gobies, but allowed invertebrates to move into and out of the exclosures. Low and high zebra mussel density tiles supported significantly greater numbers of non-mussel invertebrates (p < 0.001) than zero density tiles, particularly amphipods (p < 0.001), hydroptilid caddisflies (p < 0.05), isopods (p < 0.05), and chironomids (p < 0.001). Chlorophyll a concentrations were highest (p < 0.001) at low zebra mussel densities. The presence of round gobies significantly reduced densities of total non-mussel invertebrates (p < 0.01) and leptocerid caddisflies (p < 0.05), resulting in a significant increase in chlorophyll a (p < 0.01) concentrations. A significant zebra mussel density x round goby interaction showed that total invertebrate biomass responded positively to the combined effect of high zebra mussel density and round goby absence. These results demonstrate that round gobies and zebra mussels are altering benthic invertebrate community structure and algal resources in nearshore rocky areas of southwestern Lake Michigan.     

Characteristics of a refuge for native freshwater mussels (Bivalvia: Unionidae) in Lake St. Clair

The Lake St. Clair delta (∼ 100 km²) provides an important refuge for native freshwater mussels (Unionidae) wherein 22 of the ∼ 35 historical species co-occur with invasive dreissenids. A total of 1875 live unionids representing 22 species were found during snorkeling surveys of 32 shallow (∼ 1 m) sites throughout the delta. Richness and density of unionids and zebra mussel infestation rates varied among sites from 3 to 13 unionid species, 0.02 to 0.12 unionids/m², and < 1 to 35 zebra mussels/unionid, respectively. Zebra mussel infestation of unionids in the delta appears to be mitigated by dominant offshore currents, which limit densities of zebra mussel veligers in nearshore compared to offshore waters (13,600 vs. 28,000/m³, respectively). Glycogen concentrations in the tissues of a common and widespread species in the delta (Lampsilis siliquoidea) suggest that zebra mussels may be adversely affecting physiological condition of unionids in a portion of the Lake St. Clair delta. Physiological condition and community structure of unionids within the delta may also be influenced by differences in food quantity and quality resulting from the uneven distribution of water flowing from the St. Clair River. The delta likely supports the largest living unionid community in the lower Great Lakes and includes several species that have been listed as Endangered or Threatened in Canada and/or the state of Michigan, making it an important refuge for the conservation of native unionids.     

Modeling the Role of Zebra Mussels in the Proliferation of Blue-green Algae in Saginaw Bay,Lake Huron

Between 1991 and 1993, Saginaw Bay experienced an invasion by zebra mussels, Dreissena polymorpha, which caused a significant perturbation to the ecosystem. Blooms of Microcystis, a toxin-producing blue-green alga, became re-established in the bay after the zebra mussel invasion. Microcystis blooms had all but been eliminated in the early 1980s with controls on external phosphorus loadings, but have re-occurred in the bay most summers since 1992. An apparent paradox is that these recent Microcystis blooms have not been accompanied by increases in external phosphorus loadings. An ecosystem model was used to investigate whether the re-occurrence of Microcystis could be due to changes caused by zebra mussels that impacted phytoplankton community structure and/or internal phosphorus dynamics. The model was first used to establish baseline conditions in Saginaw Bay for 1991, before zebra mussels significantly impacted the system. The baseline model was then used to investigate: (1) the composite impacts of zebra mussels with average 1991–1995 densities; (2) sensitivity to changes in zebra mussel densities and external phosphorus loadings; and (3) three hypotheses on potential causative factors for proliferation of blue-green algae. Under the model assumptions, selective rejection of blue-green algae by zebra mussels appears to be a necessary factor in the enhancement of blue-green production in the presence of zebra mussels. Enhancement also appears to depend on the increased sediment-water phosphorus flux associated with the presence of zebra mussels, the magnitude of zebra mussel densities, and the distribution of zebra mussel densities among different age groups.     

The Quagga Mussel Invades the Lake Superior Basin

Prior studies recognized the presence of a single dreissenid species in Lake Superior—the zebra mussel Dreissena polymorpha. However, taxonomic keys based on traditional shell morphology are not always able to differentiate dreissenid species with confidence. We thus employed genetic and morphological analyses to identify dreissenids in a major river-embayment of Lake Superior—the lower St. Louis River/Duluth-Superior Harbor—during 2005–2006. Our results revealed the presence of a second dreissenid species—the quagga mussel D. bugensis (alternatively known as D. rostriformis bugensis). Both species occurred in mixed clusters, in which zebra mussels outnumbered quagga mussels (20–160:1). The largest quagga mussel collected in 2005 was 26.5 mm long and estimated to be two years old, suggesting that the initial introduction occurred no later than 2003. Further monitoring is necessary to determine whether the quagga mussel will colonize Lake Superior. Our results indicate that the coupling of conventional morphological and molecular approaches is essential for monitoring dreissenid species.     

Stable isotopes in zebra mussels as bioindicators of river–watershed linkages

Concerns about biodiversity and sustainability extend to many ecological systems, including large river systems that are highly modified by human activities. The Mississippi River is one such system that is currently regulated for navigational and flood control purposes, bears a large agricultural nutrient load, and has experienced rapid spread of the exotic zebra mussel, Dreissena polymorpha, since 1991. Human development of the extensive watershed system of the Mississippi River is ongoing and is expected to lead to further changes in river ecology. This study tested whether stable isotope compositions of the zebra mussel could help identify watershed and tributary loading of carbon (C), nitrogen (N), and sulphur (S) to the mainstem river. Zebra mussels were collected seasonally in the lower Mississippi River at Baton Rouge in 1997, and along most of the length of the river in a north–south transect from Minnesota to Louisiana during August 1998. Results showed substantial seasonal variations in C, N and S isotopic compositions of 2‰ or greater, but also that seasonal changes appeared regular and linked to changing watershed inputs and chemistry of the river water. Nitrate was the dominant N nutrient in the Mississippi River, but isotope analyses showed that food webs based on ammonium rather than nitrate were likely important for the zebra mussel. Results from the north–south transect followed expectations based on mixing of mainstem river water with tributaries that had different chemistries, so that local zones of influence were detectable in the combined CNS zebra mussel isotopes downstream of major tributary confluences for the Illinois, Missouri and Ohio Rivers. Overall, the study supports use of stable isotopes to help monitor watershed development and downstream effects on aquatic food webs. Copyright © 2003 John Wiley & Sons, Ltd.     

The Effects of Zebra Mussels on the Lower Planktonic Foodweb in Lake Champlain

Selective grazing by zebra mussels has altered phytoplankton communities in many North American lakes, but the specific changes are not the same in each ecosystem. Because of this variation in response, we investigated the impacts of zebra mussels on the plankton community of Lake Champlain with two objectives: first to determine whether zebra mussels increased the dominance of potentially toxic cyanobacteria in the phytoplankton, and second to explore the impact of zebra mussels on protozoans, rotifers, copepod nauplii, and other microzooplankton in the lower food web. Experiments were conducted in 200-L mesocosms filled with Lake Champlain water filtered through a 150-μm sieve to remove macrozooplankton. Zebra mussels were added to half of the mesocosms while the others were maintained as controls. Over a 96-hour experimental period, we tracked nitrogen and phosphorus concentration, chlorophyll a, microcystin concentration, and both phytoplankton and microzooplankton composition and abundance. We found an increase in SRP and total nitrogen concentration and a decrease in the ratio of TN:TP in the zebra mussel treatments over time. Microcystin was undetectable throughout the experiment using the ELISA assay. Phytoplankton biovolume, including cyanobacteria biovolume, declined significantly in the zebra mussel treatments, as did rotifer, protozoan and nauplii abundance. By both direct (consumption) and indirect (altered nutrient availabilities and increased competition) means, zebra mussels clearly seem capable of strongly influencing the lower planktonic foodweb in the many shallow water habitats of Lake Champlain.     

The good,the bad,and the algae: Perceiving ecosystem services and disservices generated by zebra and quagga mussels

Dreissenid (zebra and quagga) mussels are widely recognized as having strong, adverse ecological and economic impacts, e.g., biofouling and loss of water column primary production. We assessed perceptions and values associated with two less often considered ecological outcomes of dreissenid mussel influences on coastal ecosystems along Lake Ontario and the western St. Lawrence River in New York State. One, the generation of water clarity through filtration, we define as an ecosystem service; the other, the production of large amounts of nuisance algae (e.g., Cladophora and Microcystis) is defined as an ecosystem disservice. Surveys of business owners and homeowners quantified their preferences and the formation of values regarding these products of zebra mussel influence. Water clarity increased greatly, particularly in the eastern portion of Lake Ontario, and algal problems increased throughout. Businesses attributed increases and decreases in revenues associated with water clarity and algae; homeowners reported analogous changes in property values. Water clarity was positively associated, and algae negatively associated, with changes in revenues and property values. Threshold responses of costs as functions of filamentous algae were evident. Given the likely continued influx of invasive species due to human activities, further development of the ecosystem service concept should consider potential “goods” and “bads” of invasives and their influence on ecosystem and social system resiliency.     

Changes in the Dreissenid Community in the Lower Great Lakes with Emphasis on Southern Lake Ontario

A field study was conducted in the lower Great Lakes to assess changes in spatial distribution and population structure of dreissenid mussel populations. More specifically, the westward range expansion of quagga mussel into western Lake Erie and toward Lake Huron was investigated and the shell size, density, and biomass of zebra and quagga mussel with depth in southern Lake Ontario in 1992 and 1995 were compared. In Lake Erie, quagga mussel dominated the dreissenid community in the eastern basin and zebra mussel dominated in the western basin. In southern Lake Ontario, an east to west gradient was observed with the quagga mussel dominant at western sites and zebra mussel dominant at eastern locations. Mean shell size of quagga mussel was generally larger than that of zebra mussel except in western Lake Erie and one site in eastern Lake Erie. Although mean shell size and our index of numbers and biomass of both dreissenid species increased sharply in southern Lake Ontario between 1992 and 1995, the increase in density and biomass was much greater for quagga mussels over the 3-year period. In 1995, zebra mussels were most abundant at 15 to 25 m whereas the highest numbers and biomass of quagga mussel were at 35 to 45 m. The quagga mussel is now the most abundant dreissenid in areas of southern Lake Ontario where the zebra mussel was once the most abundant dreisenid; this trend parallels that observed for dreissenid populations in the Dneiper River basin in the Ukraine.     

Biometry,shell resistance and attachment of zebra and quagga mussels at the beginning of their co-existence in large European lakes

In invasive dreissenid communities, the zebra mussel usually appears earlier and then is displaced by the quagga mussel. We analysed length-weight allometric relationships, attachment strength (2 days, 1 week and 1 month of exposure), shell crushing resistance and glycogen content across the entire size range of both species in large shallow European lakes where this displacement has recently occurred. In Lake Balaton (Hungary) and Ijsselmeer (The Netherlands), the soft tissue dry weight increment of zebra mussels per unit length decreased after the quagga mussel invasion and became lower than that of quagga mussels. In Lake Markermeer (the Netherlands), having relatively worse environmental conditions, dry weight increment per unit length was always higher in quagga mussels than in zebra mussels, but no negative change in dry weight increment occurred in zebra mussels during the quagga mussel invasion. Small zebra mussels had more resistant shells and stronger attachment than quagga mussels. These differences were reduced (shell hardness) or reversed (long-term attachment) in larger individuals. Zebra mussels had lower glycogen content than quagga mussels across the entire size range. Thus, the quagga mussel advantage over zebra mussel likely consists in the faster dry weight increment per unit length and higher storage product contents of the former, due to its lower investments in attachment strength and shell crushing resistance.     

Phytoplankton community composition of Saginaw Bay,Lake Huron,during the zebra mussel (Dreissena polymorpha) invasion: A multivariate analysis

The colonization of the zebra mussel (Dreissena polymorpha) in Saginaw Bay dramatically altered the phytoplankton community composition resulting in exclusion of light sensitive species and dominance of species with oligotrophic preferences and light resistance. In 1990, the NOAA Great Lakes Environmental Research Laboratory initiated a 7-year survey program to monitor changes in the lower food web of Saginaw Bay, where zebra mussels became established in the fall of 1991. To investigate shifts in the phytoplankton community composition over the 7-year period from 1990 to 1996 we searched for clusters of similar composition using multivariate principal component analysis (PCA) on proportions of 22 taxonomic groupings of the total phytoplankton density (cells per milliliter). We then used an agglomerative hierarchical clustering analysis of the PCA scores. We identified five characteristic phytoplankton communities in configurations that allowed recognizing four distinct periods in Saginaw Bay linked to the zebra mussel invasion. Significant changes were indicative of increased water clarity and eutrophic conditions being replaced by more oligotrophic conditions as clusters dominated by light sensitive species, such as the cyanobacteria Oscillatoria redekii, became immediately rare and clusters dominated by diatoms such as Cyclotella spp. became common. Microcystis spp., a light tolerant cyanobacteria not grazed by zebra mussel, dominated assemblages after 1994. The shifts in phytoplankton composition confirm that zebra mussels effects on phytoplankton communities are mediated by both direct (filtration) and indirect (nutrient cycling) mechanisms and also suggests that increased light penetration is an important mechanism behind some changes.     

Ecosystem-Level Effects of Zebra Mussels (Dreissena polymorpha): An Enclosure Experiment in Saginaw Bay,Lake Huron

We examined the short-term effects of zebra mussels (Dreissena polymorpha) on ecosystem processes in late August 1991 in Saginaw Bay, Lake Huron. Four 1,600-L enclosures, made of Fabreen with a diameter of 1 m, a depth of 2 m, and closed at the bottom, were used to enclose natural plankton communities. These communities were dominated by diatoms with some chlorophytes, chrysophytes, and cyanophytes. Phytoplankton growth was limited by P-availability. Two enclosures were held as controls, and zebra mussels encrusting unionid shells were suspended in two of the enclosures: one enclosure (HZ) contained approximately four-fold greater numbers of mussels than the other (LZ). The concentration of suspended particles, chlorophyll, and algal biomass in HZ and LZ declined over a 6-day interval. Diatom numbers declined more than other taxa. Phytoplankton growth rates in HZ and LZ increased to near μ_max; there was no apparent change in photosynthetic parameters a or P_max scaled for chlorophyll. Soluble reactive P (SRP) increased significantly (p < 0.05) in HZ but not LZ. Dissolved organic P (DOP) and ammonium ion were elevated; dissolved organic carbon (DOC) was unchanged in HZ and LZ. The rate of phosphate uptake by bacteria and algae declined to less than 2% of controls; this rate decrease could not be explained simply by grazing losses or isotope dilution. The rate of ammonium regeneration by the plankton and the potential rate of ammonium uptake by the plankton did not differ significantly in HZ or LZfrom the control enclosures. Our findings indicate that the zebra mussel can have significant short-term effects on phytoplankton abundance, water transparency, water chemistry and phosphorus dynamics. We propose a model of zebra mussel effects that suggests high densities of zebra mussels may indirectly alter and control those processes that are rate-limited or concentration-limited by nutrient availability.     

In situ reciprocal transplants reveal species-specific growth pattern and geographic population differentiation among zebra and quagga mussels

For bivalves, somatic growth is often inferred from shell measurements alone. However, shell growth may not always reflect changes in soft tissue due to confounding factors such as seasonal or ontogenetic asynchrony between shell and tissue, flexible energy allocation, or population differences. This study compares the relationship between shell growth, changes in soft tissue mass, and RNA/DNA ratio in the zebra mussel (Dreissena polymorpha) and quagga mussel (Dreissena bugensis) from contrasting riverine and brackish estuarine environments. Reciprocal transplantation indicated that shell growth in late summer was consistently lower for the estuarine source zebra mussels while the RNA/DNA ratio was highest for zebra mussels independent of either geographic source or destination. Shell growth of the river source quagga mussels was almost two times greater than zebra mussels at the river site, but both shell growth and final tissue mass were lower in the estuarine environment. While there were no differences in final RNA/DNA ratios between zebra and quagga mussels from the same source, the RNA/DNA ratio of zebra mussels from the estuary and transplanted to the estuary was higher than that of all other zebra mussel treatments. This study suggests that shell growth does not always accurately reflect tissue growth and that the shell and tissue growth of quagga mussels is greater than that of zebra mussels in fresh but not brackish waters, and that physiological plasticity can have a fixed geographic component.