Zebra mussels (Dreissena polymorpha): a new perspective for water quality management

In the evaluation of the role of lake restoration programmesin situ measurements of the filtration rate of the freshwater musselDreissena polymorpha have been performed in Lake Wolderwijd, The Netherlands. The filtration rate mainly depends on the suspended matter content of the water, and shows an inverse exponential relationship with this factor. The filtration activity is temperature indifferent between approx. 5 and 20 °C. At low temperatures the filtration rate drops abruptly, at high temperatures gradual inhibition occurs. The filtration rate shows a sigmoidal relation with the length of the mussel. The largestD. polymorpha have a diminished filtration rate compared to animals of smaller size. This might be a degenerative feature of the oldest mussels. In Lake Wolderwijd a population density of 675 per m² is required to compensate phytoplankton growth by grazing. Manipulation of the size of the population can be executed by adding suitable substrates for the mussel.     

Influence of food size and food quantity on the feeding of the mussel Dreissena polymorpha

Summary In common with many other suspension feeders, the freshwater mussel Dreissena polymorpha has a maximum filtration rate at low food concentrations and a maximum ingestion rate at high food concentrations. These high rates, which reflect the potential maximum food uptake of the animal, are called the filtration capacity and the ingestion capacity respectively. The ingestion capacity was attained without forming pseudofaeces with Chlamydomonas reinhardii as food. The incipient limiting level could be calculated as the quotient of these two values. A decrease of the filtration rate at high food concentrations was correlated with changes in pumping activity, which showed more frequent interruptions, or a lower level of water transport. Dreissena can filter out particles of diameter greater than 0.7 m from the water. Retention reaches a plateau at about 5 m particle diameter. Scanning electron micrographs of the arrangement of the cilia on the gill filaments are given.     

Distribution and aspects of population dynamics of the zebra mussel,Dreissena polymorpha (Pallas, 1771), in the lake IJsselmeer area (The Netherlands)

Summary The distribution, settling and growth of the zebra mussel, Dreissena polymorpha Pallas, in Lake IJsselmeer and Lake Markermeer in The Netherlands were studied from 1980 to 1985. In these lakes D. polymorpha is the most important food source for wintering diving ducks. The study was part of an investigation into the carrying capacity of the lakes for these birds.     

Estimating the effective clearance rate and refiltration by zebra mussels, Dreissena polymorpha, in a stratified reservoir

1. The grazing impact of zebra mussels, Dreissena polymorpha Pallas, is often evaluated by applying the individual filtration rate measured in the laboratory to the field abundance and then by comparing the total volume of water filtered with the whole lake volume. Since this approach overlooks refiltration, it overestimates the grazing impact of zebra mussels. To deal with this problem, the present authors developed an in situ method for collecting faeces and pseudofaeces to measure the actual volume of water that is cleared of suspended particles by Dreissena in a unit time under a given set of environmental conditions. This is termed the effective clearance rate (ECR). 2. The experiment was conducted in Hargus Lake, OH, U.S.A., a small thermally stratified reservoir, to test the effects of spatial aggregation, mussel density and the concentration of particulate inorganic matter (PIM) on the effective clearance rate of Dreissena. 3. Over 40 measurements, the ECR values ranged from 15.3 to 68.6 mL ind^??1 h^??1. Much of the variation can be explained by colony form, mussel density and seston concentration. The effects of these variables were all statistically significant. The average ECR for isolated individuals was higher than that for those in clumps (40.4 versus 32.8 mL ind^??1 h^??1), which is attributed to increased refiltration in the cores of the clumps. The ECR decreased with increased zebra mussel density because of intensified competition for food particles within the group. The ECR increased with increased PIM concentration in the lake water, which may be interpreted as a result of enhanced water mixing which ultimately caused increases in both sediment resuspension and particle delivery to the mussels. 4. Taking the filtration rates for a 20-mm mussel to be between 116 and 234 mL ind^??1 h^??1, based on data from the literature, the clumped mussels under the present experimental conditions would have a refiltration ratio between 3.4 and 6.9. 5. The present authors developed an areal clearance model which predicts that seston removal by the Dreissena population is limited by the particle delivery from the ambient water to the mussel bed and will reach a maximum value beyond which no further increase will occur with increased population density. 6. It is concluded that the direct grazing impact of zebra mussel on phytoplankton in thermally stratified lakes is much less effective than predicted from simple filtration rate estimation.     

Products of Chlorophyll a Transformation by Selected Benthic Organisms in the Odra Estuary (Southern Baltic Sea)

The transformation of chlorophyll a by two benthic bivalves – Dreissena polymorpha and Mytilus edulis and, for comparison, by the chironomid Chironomus plumosus – was examined. D. polymorpha is a suspension-feeding, freshwater but very salinity-tolerant species, M. edulis a suspension-feeding marine species, and C.␣plumosus a typical detritivorous freshwater species. All three are common in the Odra Estuary. Specimens of the three species were collected from this area in the period 1999–2003. The bivalves were transferred to a compartment on land and kept in tanks filled with water (filtered and unfiltered) collected simultaneously with the organisms, under light and temperature conditions similar to those obtaining in their natural environment. Pigments in the water samples, faeces and pseudofaeces from the tanks, and in the gut content from C. plumosus, were determined with HPLC. The digestion products of the three species were mainly phaeophorbides a, generally thought to originate mainly from zooplankton grazing, and pyrophaeophytin a in minor quantities. The results indicate that the main chlorophyll a derivatives in faeces and pseudofaeces and gut content of the three species were the same as those in the sediments of the Baltic Sea, including the Odra Estuary. Our previous studies have shown that the sediments in this area are richer in phaeophorbides a than sediments from other parts of the southern Baltic Sea. All this suggests that the benthos may play an important role in the transformation of chlorophyll a in the Odra Estuary area.     

Release of metals from polluted sediments in a shallow lake: quantifying resuspension

The contribution of erosion of bed sediment to the load of metals leaving Lake Ketelmeer, a shallow lake in the Netherlands fed by the IJssel branch of the River Rhine, is reported. Transport of suspended matter and associated trace metals was measured using both centrifuges and sediment traps at several locations in the lake. Mass balances of suspended matter and heavy metals were calculated using data from these field measurements.Metal/scandium-ratios were used to identify the source of the suspended matter in the lake. Since the bed sediment is more polluted than the suspended sediment entering the lake, higher metal/scandium-ratios were found for bottom sediment in the lake compared with those for suspended matter entering the lake from the River IJssel. Using the metal/scadium-ratio in suspended matter from the lake, it was calculated that bottom sediments made up 43% of the suspended matter leaving the lake. This implies an erosion flux of bottom sediment of 16 g m^–2 d^–1. For cadmium, mercury, chromium and zinc, this erosion process accounts for more than 50% of the pollutant load leaving Lake Ketelmeer and entering Lake IJsselmeer.     

Vegetative growth and productivity ofEichhornia azurea with special emphasis on leaf dynamics

The lifeform and the biological production of pure stands ofEichhornia azurea Kunth in three lakes in tropical Brazil were studied. The lifeform ofE. azurea is termed ‘semi-emergent’, because the plant has well developed trailing stems just under the water, and the aerial lamina emerges with the thick petiole. The density of shoot apices was 9.9, 17.2 and 17.1 m⁻² in Lake Dom Helvecio, Lake Jacaré and Lake Carioca, respectively. The mean daily increment of the apical shoot biomass was between 1.8 and 4.8 g m⁻² day⁻¹. The mean leaf life-span in Lake Dom Helvecio, Lake Jacaré and Lake Carioca was estimated to be 78, 49 and 64 days in the wet season and 73, 70 and 73 days in the dry season, respectively. The stem life-span was estimated to be about 28 months. Starch content in the current years' stem ranged from 24 to 118 mg g⁻¹ dry matter with fluctuations, the amplitude of which decreased with age. The differences for most of the growth parameters, such as density of shoot apices, daily increment of biomass and leaf life-span, between dry and wet season are smaller than those among the three lakes. Both the decrease in daily dry matter production and the increase in leaf life-span occurred in order from Lake Dom Helvecio to Lake Jacaré and Lake Carioca. The low productivity ofE. azurea is considered to be related to a low leaf area index, a long time interval for the emergence of new leaves, long leaf life-span and a low capacity for branching.     

Stable isotope analysis reveals trophic segregation between the invasive zebra mussel Dreissena polymorpha and the native duck mussel Anodonta anatina in Lake Trasimeno (Italy)

Non-indigenous freshwater bivalves negatively affect invaded ecosystems through different mechanisms, including inter-specific competition for trophic resources. Here, we investigated in Lake Trasimeno (Central Italy) the diet of the invasive Dreissena polymorpha and the native Anodonta anatina. δ¹⁵N and δ¹³C stable isotopes were measured in winter and summer in bivalves, phytoplankton, and sedimentary organic matter (SOM); the relative dietary contributions of the two resources were determined using Bayesian mixing models. To elucidate the different carbon and nitrogen pools characterizing the study site, isotopic analyses were extended to zooplankton and to representatives of the benthic flora and macroinvertebrate fauna. Independently from the season, the two bivalves showed a limited trophic overlap, as mixing models indicated for D. polymorpha a diet based primarily on phytoplankton, while A. anatina relied mainly on SOM. Dietary differences were less marked in summer, when comparable isotopic values characterized phytoplankton and SOM. In winter, conversely, the trophic differentiation between the two species was more evident, and corresponded with a significant enrichment in SOM δ¹³C values, likely due to a substantial contribution of carbon deriving from decaying macrophytes. Whether differences in ecological and behavioral traits alone can explain the observed trophic segregation between the two species, or if they have actively shifted their diet to reduce competition for food is discussed. We conclude emphasizing the need of an advanced resolution of the influence of non-indigenous species on the flux of energy and matter in invaded lentic systems, including Lake Trasimeno.

     

The structure of <Emphasis Type="Italic">Dreissena polymorpha</Emphasis> (Pallas) biocenosis and the role of mollusk in roach <Emphasis Type="Italic">Rutilus rutilus</Emphasis> (Linnaeus) feeding in Pleshcheevo Lake

As a result of studies on the macrozoobenthos composition of zebra mussel biocenosis in Lake Pleshcheevo carried out in 1996, a total of 44 species of bottom macroinvertebrates were found. The richest in terms of species composition were chironomids (17 species), oligochaetes (13 species), and mollusks and leeches (5 species each). After the introduction of zebra mussel Dreissena polymorpha (Pallas) into the lake, a natural biophilter was formed in the waterbody at depths of 4.5 to 9 m. This biophilter intercepts a substantial part of degrading macrophytes and allochtonous organic matter. Thus, the filter prevents the income of this organic matter in the lake’s profundal zone. Two roach Rutilus rutilus (Linnaeus) groups had inhabited Lake Pleshcheevo before dreissena introduction. Comparatively fast growing roach fed predominantly on mollusks of g. g. Valvata and Bithynia, while slowly growing roach fed mostly on zooplankton. After the introduction and mass development of D. polymorpha in the lake, roach started to consume these mollusks. This resulted in increases in the fish growth rate and maximum body size. Roach starts to feed on dreissena and other mollusks after reaching a body length of 15.0 cm. This relates to the third and the last change in the pharyngeal teeth shape, making possible for the fish to consume mollusks and to crush their shells.     

Alien molluscs affect the composition and diversity of native macroinvertebrates in a sandy flat of Lake Neuchâtel,Switzerland

The spread of alien molluscs is a serious threat to native biodiversity in fresh waters. Alien freshwater molluscs may deplete the resources of native species and alter the physical structure of the habitat through their shell mass. These changes might have both positive and negative effects on native community members. We investigated the native macroinvertebrate community in relation to the densities of four alien mollusc species (Corbicula fluminea, Dreissena polymorpha, Potamopyrgus antipodarum and Lithoglyphus naticoides) in a sandy flat of Lake Neuchatel, Switzerland. The habitat examined was dominated by these alien mollusc species. The abundance of the alien molluscs did not directly impact the native community assembly. However, C. fluminea and D. polymorpha influenced the composition and diversity of native macroinvertebrates by transforming the sandy substratum into a partly hard substratum habitat. Substantial differences in community composition between shallow (<3.5 m) and (≥5 m) deep sites were recorded. At shallow sites, the abundance of D. polymorpha was significantly reduced as a result of depth-selective feeding of ducks. A controlled shell decay study revealed that shells of alien molluscs (C. fluminea, D. polymorpha) persist for a longer period in the sediment than those of native molluscs. Consequently, shells of alien molluscs have a long-lasting impact by modifying the sandy habitat. This form of ecosystem engineering favours the occurrence of several native taxa, but is disadvantageous for other taxa with specific habitat requirements, and thus can be regarded as an indirect impact of competition.     

Establishment of the Baikalian endemic amphipod Gmelinoides fasciatus Stebb. in Lake Ladoga

Gmelinoides fasciatus Stebb., a small amphipod from the Lake Baikal basin, was discovered in July 1988 in Lake Ladoga, the largest European lake. G. fasciatus likely invaded Lake Ladoga as a consequence of its intentional introduction, aimed at enhancing fish production, in some Karelian Isthmus lakes close to Lake Ladoga's western shore in the early 1970's. Benthos studies conducted in 1989 and 1990 revealed that G. fasciatus was well established in littoral communities along the western and northern shores of Lake Ladoga. G. fasciatus was the dominant species in these littoral communities and contributed over 70% of the macroinvertebrate biomass. The species was abundant in different macrophyte beds and stony littoral areas, both in heavily polluted and undisturbed sites. The maximum abundance and biomass of G. fasciatus was about 54,000 ind m^–2 and 160 g (wet wt.) m^–2. Negative impacts of G. fasciatus on native species have been observed. Studies are needed to identify the effects of G. fasciatus on the functioning of littoral communities and to predict and control the spread of this amphipod.     

Temporal variation in zebra mussel (Dreissena polymorpha) density structure the benthic food web and community composition on hard substrates in Lake Constance, Germany

Invasive species often influence existing biocenoses by altering their environment or facilitating the ecology of other species. Here we combined stable isotope analysis with quantitative benthic community sampling to investigate temporal variation in the influence of biodeposition of organic material (biodeposits) by the zebra mussel (Dreissena polymorpha) on the benthic food web in hard substrate habitats of Lake Constance, Germany. The accumulation of organic material excreted by zebra mussels (faeces and pseudofaeces) is hypothesised to implement a biodeposition based food web. By means of stable isotope analyses, we found that the feeding strategy of amphipods was dependent on the availability of zebra mussel biodeposits. The proportion of pelagic resources contributing to the diet (by ??¹³C) and trophic position (by ??¹⁵N) of amphipods were significantly and positively correlated to the production of zebra mussel biodeposits. Also, the benthic community in Lower Lake Constance showed a shift towards higher densities of the mayfly Caenis spp., Chironominae, and the caddisfly Ecnomus tenellus, which might act as links to higher trophic levels when more zebra mussel biodeposits were available. These results demonstrate that temporal variation in zebra mussel density, and thus in organic biodeposition, have a strong impact on the benthic communities and food web structure associated with hard substrates to the extent that there may be dependence on zebra mussel occurrence in such habitats in Lake Constance.     

The filtration rate of the zebra mussel Dreissena polymorpha used for water quality assessment in Dutch rivers

The effects of contaminated river water on the filtration rate of zebra mussels from a clean reference site were studied. After a 48-h exposure period to filtered water from the rivers Rhine, Meuse and Amstel (The Netherlands), the filtration rate was measured. It was demonstrated that water from contaminated locations inhibited the filtration rate. Inhibition was higher during low water levels in the rivers Rhine and Meuse than during high water levels, suggesting that contaminants are diluted during high water levels. It is concluded that the shortterm filtration assay with D. polymorpha can be used for assessing water quality.     

Invasive mussels induce community changes by increasing habitat complexity

Habitat complexity is among the most important factors affecting the diversity, structure, and density of natural communities. The invasive byssate bivalves zebra mussel (Dreissena polymorpha (Pallas, 1771)) and golden mussel (Limnoperna fortunei (Dunker, 1857)) are sessile suspension feeders that form aggregations (druses), physically changing sediments and increasing habitat complexity, and providing shelter and food for other benthic organisms. In this study, we explored the impact of the change in habitat complexity on benthic community created by druses of L. fortunei and D. polymorpha, formed on various sediments. D. polymorpha was studied in Europe (Naroch Lake, Belarus) and in North America (Glen Lake and Lower Nashotah Lake, USA), and L. fortunei was studied in South America (Río Tercero Reservoir, Argentina). Druses of D. polymorpha or L. fortunei and samples of bare nearby sediment (without druses of exotic mussels) were collected at each sampling site. We found significant changes in species richness, density, biomass, taxonomic, and trophic structure of communities formed in druses compared to the nearby bare sediments. Community taxonomic richness increased threefold, and density increased sevenfold with increasing complexity of habitat from sand to druse. The feeding functional group approach indicated that the impact of increased complexity was reinforced by an increase in food supply in D. polymorpha and L. fortunei druses. Along with increasing species richness and densities, byssate bivalves homogenized benthic communities.     

Effect of filtering activity of Dreissena polymorpha (Pall.) on the nutrient budget of the littoral of Lake Mikołajskie

The effect of filtering activity of D. polymorpha on nutrient dynamics in Lake Mikoajskie depended mostly on population density: D. polymorpha was important in N and P cycling in periods when its population density was medium or high. The amounts of N and P accumulated in mussel populations at medium density may be similar to those in emergent and submerged macrophytes. However, mussels remove nutrients from cycling matter for a much longer time than do macrophytes. The amounts of nutrients accumulated in a mussel population are several times lower than the quantities which flow through it.     

Composition and properties of the sexual agglutinins of the flagellated green alga Chlamydomonas eugametos

Sexual interaction between gametes of opposite mating type (mt) of the unicellular green alga Chlamydomonas eugametos starts with agglutination of the cells via particular glycoproteins on the flagellar surface. Purification of these socalled agglutinins was achieved by a three-step procedure consisting of, successively, gel filtration, anion-exchange chromatography, and high-performance gel filtration. The amino-acid and sugar compositions of both agglutinins showed a high degree of similarity; the most prominent amino acids were hydroxyproline, serine and glycine, and the main sugars were arabinose and galactose. The carbohydrate portions represented about half of the molecular mass of both agglutinins. Using high-performance gel filtration, a calibration curve was constructed for high-molecular-mass compounds from which the Stokes' radius of the sexual agglutinins could be estimated. The mt ⁺ agglutinin had a Stokes' radius of 39 nm and a sedimentation coefficient of 9.3 S. From these data its molecular mass was estimated to be 1.2·10⁶. The corresponding data for the mt ^- agglutinin were 38 nm, 9.7 S and 1.3·10⁶, respectively. The biological activity of both agglutinins was destroyed by mild periodate treatment. Treatment with specific glycosidases had a differential effect on the biological activity of the agglutinins. These observations indicate that carbohydrate side-chains are needed for biological activity and perhaps are responsible for the specifity of the sexual agglutinins. A comparison of both agglutinins is given and their possible structure is discussed in relation to their amino-acid and sugar compositions.Abbreviations HP high performance - mt mating type - SDS sodium dodecyl sulfate     

Physiological response of Spisula subtruncata (da Costa, 1778) to different seston quantity and quality

Individuals of the bivalve Spisula subtruncata were fed a mixed diet comprising of sea water enriched with the diatom Phaeodactylum tricornutum and ashed silt within a range of concentrations, simulating natural conditions above pseudofaeces threshold. The designed ranges for total particulate matter were between 10 and 30 mg l^–1and organic content of seston 15–40%. Filtration rate, rejection rate, ingestion rate and absorption rate were measured at those different conditions. Filtration rate and rejection rate were significantly correlated to total particulate matter and percentage of organic matter, with higher rates at higher values of total particulate matter and lower values of percentage organic matter. Ingestion rate was maintained at similar levels in all the treatments and organic enrichment of the ingested food occurred due to preingestive selection of the filtered material. A differential absorption rate occurred at different levels of organic matter in the diet with high rates at high values of the organic content of the diet. S. subtruncata showed different physiological responses to changes of the food conditions: (1) Increase of pseudofaeces production at increasing levels of particulate matter, (2) preingestive selection of organic material which enriched the organic fraction of ingested food, (3) stabilized ingestion rate and (4) increase of the absorption rate at high organic levels of the seston.     

Accumulation of cadmium ions in the methylotrophic yeast Hansenula polymorpha

Intracellular cadmium (Cd²⁺) ion accumulation and the ability to produce specific Cd²⁺ ion chelators was studied in the methylotrophic yeast Hansenula polymorpha. Only one type of Cd²⁺ intracellular chelators, glutathione (GSH), was identified, which suggests that sequestration of this heavy metal in H. polymorpha occurs similarly to that found in Saccharomyces cerevisiae, but different to Schizosaccharomys pombe and Candida glabrata which both synthesize phytochelatins. Cd²⁺ ion uptake in the H. polymorpha wild-type strains appeared to be an energy dependent process. It was found that Δgsh2 mutants, impaired in the first step of GSH biosynthesis, are characterized by increase in net Cd²⁺ ion uptake by the cells, whereas Δgsh1/Δmet1 and Δggt1 mutants impaired in sulfate assimilation and GSH catabolism, respectively, lost the ability to accumulate Cd²⁺ intracellularly. Apparently H. polymorpha, similarly to S. cerevisiae, forms a Cd-GSH complex in the cytoplasm, which in turn regulates Cd²⁺ uptake. Genes GSH1/MET1 and GGT1 are involved in maturation and metabolism of cellular Cd-GSH complex, respectively. Transport of [³H]N-ethylmaleimide-S-glutathione ([³H]NEM-SG) conjugate into crude membrane vesicules, purified from the wild-type cells of H. polymorpha appeared to be MgATP dependent, uncoupler insensitive and vanadate sensitive. We suggest that MgATP dependent transporter involved in Cd-GSH uptake in H. polymorpha, is similar to S. cerevisiae Ycf1-mediated vacuolar transporter responsible for accumulation of organic GS-conjugates and Cd-GSH complex.     

Invasive zebra mussel (Dreissena polymorpha) threatens an exceptionally large population of the depressed river mussel (Pseudanodonta complanata) in a postglacial lake

1. Freshwater mussels are in decline worldwide, with the depressed river mussel Pseudanodonta complanata being one of the rarest and most endangered species in Europe. Invasive mussels are suspected to be an important factor of decline, but there is little information on their interaction with native species.
2. This study analyzed densities, depth distribution, and individual sizes and weights in one of the largest known populations of P. complanata in Europe in relation to the co‐occurring invasive zebra mussel Dreissena polymorpha and other mussel species, using a systematic transect analysis.
3. Pseudanodonta complanata was the dominant unionid species in Lake Siecino reaching densities of up to 26 ind/m², with half of the specimens found at a water depth of 2.0–4.0 m. Densities were highest on sandy substrates in areas of underwater currents. In contrast, 67% of native Unio tumidus were found at depths < 1 m, indicating different habitat preference.
4. In the study area, 91% of P. complanata, 92% of U. tumidus, and all Anodonta individuals were fouled by D. polymorpha. The dreissenid:unionid mass ratio (mean ± SD; maximum) was 0.43 ± 0.56; 4.22 and 0.86 ± 1.87; 8.76 in P. complanata and U. tumidus, respectively. Pseudanodonta complanata fouled with D. polymorpha were impaired in their anchoring capability and had shell deformations potentially affecting shell closing and filtration activity. Fouling intensity was negatively correlated with unionid density, potentially leading to accelerated population declines.
5. The observed adverse effects of invasive zebra mussels on the depressed river mussel and the difficulties in eradicating established populations of invasive mussels suggest that D. polymorpha should be considered a serious threat to P. complanata. Therefore, the further spread of zebra mussels into habitats with native unionids needs to be avoided by all means.

     

Transformation of the offshore benthic community in Lake Michigan: recent shift from the native amphipod Diporeia spp. to the invasive mussel Dreissena rostriformis bugensis

1. The native amphipod Diporeia spp. was once the dominant benthic organism in Lake Michigan and served as an important pathway of energy flow from lower to upper trophic levels. Lake‐wide surveys were conducted in 1994/1995, 2000 and 2005, and abundances of Diporeia and the invasive bivalves Dreissena polymorpha (zebra mussel) and Dreissena rostriformis bugensis (quagga mussel) were assessed. In addition, more frequent surveys were conducted in the southern region of the lake between 1980 and 2007 to augment trend interpretation. 2. Between 1994/1995 and 2005, lake‐wide density of Diporeia declined from 5365 to 329 m⁻², and biomass (dry weight, DW) declined from 3.9 to 0.4 g DW m⁻². The percentage of all sites with no Diporeia increased over time: 1.1% in 1994/1995, 21.7% in 2000 and 66.9% in 2005. On the other hand, total dreissenid density increased from 173 to 8816 m⁻², and total biomass increased from 0.4 to 28.6 g DW m⁻². Over this 10‐year time period, D. r. bugensis displaced D. polymorpha as the dominant dreissenid, comprising 97.7% of the total population in 2005. In 2007, Diporeia was rarely found at depths shallower than 90 m and continued to decline at greater depths, whereas densities of D. r. bugensis continued to increase at depths greater than 50 m. 3. The decline in Diporeia occurred progressively from shallow to deep regions, and was temporally coincident with the expansion of D. polymorpha in nearshore waters followed by the expansion of D. r. bugensis in offshore waters. In addition, Diporeia density was negatively related to dreissenid density within and across depth intervals; the latter result indicated that dreissenids in shallow waters remotely influenced Diporeia in deep waters. 4. With the loss of Diporeia and increase in D. r. bugensis, the benthic community has become a major energy sink rather that a pathway to upper trophic levels. With this replacement of dominant taxa, we estimate that the relative benthic energy pool increased from 17 to 109 kcal m⁻² between 1994/1995 and 2005, and to 342 kcal m⁻² by 2007. We project that previously observed impacts on fish populations will continue and become more pronounced as the D. r. bugensis population continues to expand in deeper waters.