Limnoperna fortunei versus dreissena polymorpha: population densities and benthic community impacts of two invasive freshwater bivalves.
Article Type: Report
Subject: Population biology (Research)
Invasive species (Research)
Invasive species (Environmental aspects)
Bivalvia (Research)
Bivalvia (Environmental aspects)
Zebra mussels (Research)
Zebra mussels (Environmental aspects)
Authors: Karatayev, Alexander Y.
Burlakova, Lyubov E.
Karatayev, Vadim A.
Boltovskoy, Demetrio
Pub Date: 12/01/2010
Publication: Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2010 National Shellfisheries Association, Inc. ISSN: 0730-8000
Issue: Date: Dec, 2010 Source Volume: 29 Source Issue: 4
Topic: Event Code: 310 Science & research
Geographic: Geographic Scope: Argentina Geographic Code: 3ARGE Argentina
Accession Number: 247523216
Full Text: ABSTRACT In this study, for the first time, using similar methods, we compared the population density and distribution across different substrate types of Limnoperna fortunei and Dreissena polymorpha, as well as their impacts on the composition of benthic communities. Data on L. fortunei were obtained in Rio Tercero Reservoir, Argentina, whereas studies on D. polymorpha were conducted in North America and Europe. We found that, similar to the zebra mussel, L. fortunei creates high densities on hard substrates in the littoral zone, and avoids soft substrates in the profundal zone; however, the overall population density of L. fortunei in a water body seems to be higher than that of zebra mussels. Additional studies on Limnoperna are needed to confirm this hypothesis. The effect of L. fortunei on macrobenthos is very similar to the effect of D. polymorpha and is associated with an increase in the overall diversity, density, and biomass of native macroinvertebrates in druses compared with bare sediments. The presence of L. fortunei druses in the littoral zones of Rio Tercero has increased the average species richness of native benthic invertebrates per sample by almost 70% and their density and biomass by threefold, positively affecting epifaunal organisms and negatively burrowing invertebrates and unionids. In the near future, the freshwaters of North America may be colonized by L. fortunei, resulting in strong impacts on entire invaded ecosystems and devastating impacts on native unionids, especially in the southern regions of the United States, which are not colonized with dreissenids.

KEY WORDS: invasive species, Limnoperna fortunei, zebra mussel, Dreissena polymorpha, distribution, density, impacts on zoobenthos

INTRODUCTION

The strong ecological and economic impacts of the zebra mussel (Dreissena polymorpha (Pallas, 1771)), well documented both in Europe and in North America, make this mollusc the most aggressive freshwater invader in the northern hemisphere (reviewed in Karatayev et al. 1997, O'Neill 1997, Karatayev et al. 2002, Karatayev et al. 2007b). Much less information is available on the other invasive byssate bivalve, the golden mussel (Limnoperna fortunei Dunker, 1857). L. fortunei is a freshwater mytilid native to mainland China that was introduced into Hong Kong, Taiwan, and Japan between 1965 and 1990 (Morton 1975, Nakai 1995). In 1989 to 1990, L. fortunei invaded South America (Pastorino et al. 1993), where it has already spread across Argentina, Uruguay, Paraguay, Bolivia, and Brazil, having significant economic and ecological impacts (Boltovskoy et al. 2006, Boltovskoy et al. 2009). The overall impacts of both species on the areas invaded are scale dependent and are determined by the number of water bodies colonized (regional scale) and the population density in each of them (water body scale). Different factors govern the spread and distribution of L. fortunei and D. polymorpha at different spatial scales, affecting their population densities and environmental impacts (Karatayev et al. 2007b). Both species have similar length (typically approximately 20-30 mm; maximum, about 42-46 mm), and usually live about 3-4 y (reviewed in Karatayev et al. 2007a). They are sessile suspension feeders with a planktonic larval stage and high reproductive capacity, which allows them to colonize large areas quickly, producing significant local and systemwide effects (reviewed in Karatayev et al. 2007a, Karatayev et al. 2007b). Although the local effects are chiefly associated with their ability to form aggregations (druses) and physically change substrates, providing shelter and food for other benthic organisms, the systemwide effects are associated with their filtering activities. Being powerful suspension feeders, they greatly enhance benthic-pelagic coupling in the ecosystems they invade (reviewed in Karatayev et al. 1997, Darrigran 2002, Karatayev et al. 2002, Vanderploeg et al. 2002, Boltovskoy et al. 2006, Karatayev et al. 2007a, Boltovskoy et al. 2009). Because both local and systemwide effects depend primarily on the presence and activity of individual organisms, the magnitude of the overall impact strongly correlates with their population densities. Therefore, information on their distribution and abundance is critically important for understanding and predicting their ecological impacts. Although for Dreissena such surveys are numerous (e.g., reviewed in Karatayev et al. 1998, Patterson et al. 2005, Burlakova et al. 2006), for Limnoperna, information on population densities over large areas is restricted to a single comprehensive survey (Boltovskoy et al. 2009), all other abundance data being isolated records of peak densities (Darrigran 2002, Boltovskoy et al. 2006). Although this constraint imposes limitations on comparisons between the 2 bivalves, the fact that we have had extensive experience with both species and used similar methods allows us to address several key issues involving parallels and contrasts between these mussels. In this study we compare the population density and distribution across different substrate types of L. fortunei and D. polymorpha, as well as their impacts on the composition of benthic communities.

METHODS

Limnoperna fortunei Distribution

Samples were collected in Embalse Rio Tercero, a mediumsize reservoir located in Cordoba Province, central Argentina (32[degrees]11'S, 64[degrees]13'W). The surface area of the reservoir is 47 [km.sup.2], its average depth is 10.1 m, and its volume is 0.48 [km.sup.3] (Boltovskoy et al. 2009). The reservoir was built in 1936 for hydroelectric power supply, and became a cooling reservoir for a 600-MW nuclear power plant in 1983. Large water-level fluctuations (up to 10 m) were typical for this water body before 1983. After the nuclear power plant was built, these fluctuations were substantially reduced, although during dry periods more than 17% of the reservoir bottom is occasionally exposed to air (Mariazzi et al. 1992). The littoral zone is dominated by rocks and sand, whereas deeper areas are mostly covered with mud. At the time of sampling, macrophytes were scarce, most likely because of the high water-level fluctuations.

All samples used to determine the distribution and abundance of L. fortunei were collected in December 2006 along 18 transects as a part of a larger survey (Boltovskoy et al. 2009). All transects were initiated on the shore and ran perpendicularly to the shore toward the center of the reservoir. Transects were distributed based on bathymetry and types of bottom sediments to represent all major habitat types adequately (Boltovskoy et al. 2009). For each transect, samples were collected from 3-10 sites at depths ranging from 2-19 m. Shallower areas were not sampled because all mussels in depths of less than 3 m were dead as a result of a recent drawdown. Each sample was collected by a scuba diver who retrieved all the specimens encompassed by a 50 x 50-cm metal frame (0.25 [m.sup.2] area quadrat) randomly placed from the boat. On hard bottoms, all rocks with mussels encompassed by the quadrat were removed or, when they were immobile or too large to remove, all adhering mussels were detached manually. On soft bottoms, the surface sediments within each quadrate down to 5 cm were examined and all mussels removed. Within 48 h of sampling, mussels were counted and weighed to the nearest 0.01 g, after removing water from their mantle cavity (wet weight, soft tissue plus shell).

Dreissena polymorpha Distribution

To compare the population densities and distribution of L. fortunei with D. polymorpha statistically, we needed primary data from other water bodies colonized by D. polymorpha obtained with similar methods of collection. Therefore, we used data obtained during our previous study in Belarus with a very similar experimental design and collection methods (Karatayev 1983, Burlakova et al. 2006). For these studies, Dreissena samples were collected from the glacial lakes Lukomskoye (data collected in 1978), Naroch (1993 to 1995, 1997, 2002), Myastro (1993, 1995, 2002), Batorino (1993, 1995, 2002), and Reservoir Drozdy (1995; Table 1). Lake Lukomskoe was sampled in July 1978 along 14 transects. For each transect, samples were collected at 0.5, 1, 2, 3, 4, 5, 6 m using scuba gear, and on silt substrates at 8 m using an Eckman grab (Karatayev 1983). The Eckman grab was very effective for sampling silt sediments, where only occasionally small Dreissena druses were found. Lakes Naroch, Myastro, and Batorino were sampled each year in July or August. We sampled 8 transects in Lake Naroch and 5 transects in lakes Myastro and Batorino. For each transect in these 3 lakes, up to 10 replicate samples were collected at 0.5-, 1-, 1.5-, 2-, and 3-m depths, and then at an interval of 1 or 2 m down to the maximum depth where D. polymorpha was found. At less than 8 m, samples were collected by divers based on a 0.25-[m.sup.2] quadrat, whereas at deeper sites on silt substrates an Eckman grab was used. A detailed description of our Dreissena sampling protocol was published previously (Karatayev 1983, Burlakova et al. 2006). Similarly to the L. fortunei study, all D. polymorpha samples were washed through a 550-[micro]m mesh, and within 48 h of sampling all zebra mussels larger than 1 mm in shell length were counted, opened with a scalpel to remove water from the mantle cavity, and the entire sample was weighed to the nearest 0.01 g after blotting dry on absorbent paper (wet weight, soft tissue plus shell) (Burlakova et al. 2006).

Impact on the Benthic Community

The impact of L. fortunei on the benthic community was studied in Rio Tercero Reservoir in Cordoba, Argentina, in December 2006. Twenty samples were collected at 3.5 m depth on silty sand within the same 5 x 10-m area: 10 samples of L. fortunei aggregations (druses) and 10 samples of bare sediments (without L. fortunei druses). L. fortunei druses were collected with their substrates by a diver, placed in zip-locked bags, and brought to the surface. Benthic samples were collected with a tube dredge sampler 7.2 cm in diameter (surface area, 0.004 [m.sup.2]). All samples were washed through a 500-[micro]m sieve and fixed with 10% buffered formaldehyde. All organisms from all samples were identified to the lowest possible taxonomic level, counted, and weighed after blotting dry on absorbent paper (total wet weight). Druse surface area was estimated as the projection on the surface; the mean area of the druses analyzed was 0.0058 [+ or -] 0.002 [m.sup.2].

To compare the effect of L. fortunei on benthic invertebrates with those of D. polymorpha, we used our data obtained in June 2007 in Lower Nashotah Lake, Wisconsin. In this lake we collected 12 samples at 2.5 m depth on silty sand within a single 5 x 10-m area, including 6 samples of Dreissena druses and 6 samples of bare sediments. Samples were analyzed using the same protocol as for L. fortunei.

Statistical Analyses

The average mass of the mussels in a sample was calculated as the ratio between the total mass of the animals and their number in the sample. To compare the average mass, density, and biomass of D. polymorpha and L. fortunei, we used Kruskal-Wallis tests (because many samples contained no mussels; density = 0), separately for each substrate (Zar 1996), and multiple comparisons of mean ranks for all groups. All statistical tests were performed with the aid of Statistica software (STATISTICA version 6, StatSoft, Inc.). Effects were considered statistically significant at P < 0.05. When multiple tests were conducted on the same data, we used a sequential Bonferroni correction to adjust the critical alpha considered for statistical significance (Rice 1989). When appropriate, we present the critical alpha with the results of each statistical test.

Macroinvertebrate community structures were assessed using macroinvertebrate abundance (density, measured in individuals per square meter; and biomass, measured in grams per square meter) and diversity indices. PRIMER 6 version 6.1.6, Primer E-Ltd.) was used to analyze differences in benthic communities. To assess and visualize differences between macroinvertebrate community composition, we used nonmetric multidimensional scaling, which calculates a set of metric coordinates for samples, most closely approximating their nonmetric distances (Legendre & Legendre 1998). The sample-to-sample similarity of macroinvertebrate community composition (density and biomass) was assessed with the aid of the Bray-Curtis index (Bray & Curtis 1957, Clarke 1993) based on square root transformed abundance data. Differences between assemblages were assessed by analysis of similarities (ANOSIM). ANOSIM is a resampling technique that uses permutation/randomization methods on Bray-Curtis similarity matrices to identify differences among groups of samples with subsequent pairwise comparisons (Clarke 1999). The SIMPROF test routine was used to test for structure in the data. To characterize diversity in druse and sediment communities, we used the univariate Margalefs index of species richness; to describe the variability in the multivariate structure of these communities, we used relative multivariate dispersion. The comparative index of multivariate dispersion (Warwick & Clarke 1993) was calculated as a measure of increased variability between druse and sediment communities; this index varies between 0 (no difference) and 1 (maximum difference).

RESULTS

Distribution Within a Water Body

L. fortunei was found in 36 of the 68 samples obtained. Densities and biomass of L. fortunei varied significantly with substrate type (P < 0.001, Kruskal-Wallis test), being lowest on silt, medium on sandy substrates, and very high on rocks and gravel (Tables 1 and 2). The rocky and sandy substrates were most common at 4-8.5 m, resulting in the highest densities and biomass of L. fortunei at these depths. Because of the recent drawdown, all L. fortunei found above 3 m were dead. Silty bottoms were generally barren of L. fortunei, except for isolated druses formed around solid objects lying on the sediment, such as wood debris, bottles, and so forth, that occasionally were very large. In fact, one of the largest druses recorded was found on silt below 10 m, on a plastic bottle. On sandy substrates we found numerous L. fortunei druses that used sand grains or small pebbles glued together with byssal threads as a substrate for their attachment (Fig. 1). At the time of our sampling, the population consisted of subadult and adult individuals. The smallest L.fortunei found was 2 mm; the largest was 50.5 mm long (Fig. 1).

Comparison of the distributional data for L. fortunei in Rio Tercero Reservoir with that of D. polymorpha in 5 European lakes indicates that in both cases, the largest densities and wet biomass were usually found on rocks and gravel, whereas the lowest densities were always found on silt (Table 1). We found that on gravel and sand, L. fortunei densities and biomass were significantly higher than those of zebra mussels (Table 1). We also found that the average mass of L. fortunei was higher than that of D. polymorpha on rocks, gravel, and silty sand (Table 2).

Impacts on the Benthic Community

We found a total of 20 taxa (species and higher taxa) of macroinvertebrates in druses of L. fortunei (excluding Limnoperna), and 16 taxa in the bare sediments near the druses (Table 3). Nine taxa were found both in druses and in the sediments. The average diversity of native benthic invertebrates per sample was significantly higher (P < 0.0003, t-test) in L. fortunei druses than in the sediments. Communities in bare sediments were also characterized by a much larger variability between samples; the very high index of multivariate dispersion values (0.9-1.0, Table 3) allows rejecting the null hypothesis of no differences in the variability between these communities.

[FIGURE 1 OMITTED]

The average number of species, total density, and biomass of benthic communities were significantly higher in L. fortunei druses (P < 0.001, 2-sided t-test, Table 3). Nearly all taxa were much more abundant in druses than in bare sediments (Table 4). Differences were especially large for gastropods, leeches, caddisflies, mayflies, and chironomids. Only oligochaetes were more abundant in the sediments than in Limnoperna druses.

Similarly, a significant increase in the diversity, density, and biomass of bottom invertebrates (diversity and density, P < 0.001; biomass, P = 0.0023; 2-sided t-test) was found in Dreissena druses compared with bare sediments (Tables 3 and 4). Burrowing mayflies (Hexagenia sp.) were found in bare sediments but not in Dreissena druses.

Assemblages of native benthic invertebrates associated with the druses differed significantly from those found in the nearby bare substrate (global R = 0.46, P = 0.001, ANOSIM; P = 0.01, SIMPROF test; Fig. 2). These conclusions held when we aggregated data from species to higher taxonomic levels (genus, family, order, and class, all P = 0.001, ANOSIM).

DISCUSSION

Population Density and Potential Systemwide Effect

The ecological impact of both L. fortunei and D. polymorpha is associated with their role as biofilters and is therefore determined by their filtration rate and the overall population density in a given water body. Being powerful suspension feeders, both species filter large volumes of water, transferring energy and material from the water column to the bottom, greatly enhancing benthic-pelagic coupling, and inducing major changes in the colonized ecosystems (reviewed in Karatayev et al. 2007a). Although for L. fortunei our comparisons are based on distribution and population density data from a single water body, abundant previous information indicates that, similar to D. polymorpha, the golden mussel dearly favors the hard substrates of the littoral zone and avoids the soft bottom of deeper areas (see review in Boltovskoy et al. 2006) (Table 1). Thus, the general pattern of distribution within a water body is generally similar for the 2 bivalves. However, because the numerical density of L. fortunei across all substrates combined appears to be higher than that of the zebra mussels, the overall population density of L. fortunei in a water body may be also higher. Furthermore, because filtration rate is tightly coupled with biomass, and because L. fortunei is larger than D. polymorpha, at similar densities it may attain higher biomass levels and is therefore a more powerful "biofilterer" than the zebra mussel. As a result, the time required for L. fortunei to filter a volume of water equivalent to that of the water body is often substantially shorter than that of D. polymorpha (Fig. 3). Thus, although further studies are needed, we anticipate that L. fortunei may have a stronger systemwide effect than D. polymorpha.

Impacts on the Benthic Community

Both European and North American studies have shown that aggregations of zebra mussels create new 3-dimensional habitats for different invertebrates, whereas their pseudofeces and feces provide an abundant food supply for detritivores (reviewed in Karatayev et al. 1997, Karatayev et al. 2002). Shelters created by D. polymorpha have been shown to be the primary mechanism for increased abundance of macroinvertebrates, especially snails and amphipods (Botts et al. 1996, Stewart et al. 1998). Similar to the zebra mussel, L. fortunei transforms a 2-dimensional surface of hard substrate into a 3-dimensional structure, altering the habitat and providing shelter and food for other benthic invertebrates (Sylvester et al. 2007, Sardina et al. 2008). Therefore, the mechanism by which L. fortunei affects benthic assemblages is very similar to the one described for D. polymorpha (reviewed in Karatayev et al. 2002, Burlakova et al. 2005; Karatayev et al. 2007a, Karatayev et al. 2007b, Ward & Ricciardi 2007).

Dreissena druses have positive effects on the majority of native bottom invertebrates, including turbellarians, leeches, gastropods, some oligochaetes, and chironomids (Karatayev et al. 1983, Botts et al. 1996, Karatayev et al. 1997, Stewart et al. 1998). On the other hand, negative effects have been reported for several species of oligochaetes (Afanasiev 1987), and devastating effects on native unionid bivalves (reviewed in Burlakova et al. 2000). Our results show that the effects of L. fortunei on native benthic organisms are very similar to those of the zebra mussel (Table 3). The density of turbellarians, molluscs, leeches, mayflies, and chironomids was from 3-20 times higher in druses than in nearby sediments (Table 3). Crustaceans and caddisflies were found exclusively in L. fortunei druses. On the other hand, oligochaete densities were 25 times higher in the sediments than in L. fortunei druses (Table 3). Although we did not find any amphipods in Rio Tercero, they were extremely abundant in L. fortunei druses collected in Rio de la Plata (authors' unpublished data). Similarly to D. polymorpha, L. fortuneis overgrowth may cause the mortality of native unionids (Darrigran & Drago 2000, Darrigran 2002). The only 3 unionid specimens (1 alive, 2 dead) found in our Rio Tercero survey were heavily overgrown by L. fortunei (Fig. 1).

Although we used a different sampling protocol, our data largely agree with recent South American studies on the effects of L. fortunei on the associated fauna (Darrigran et al. 1998, Darrigran & Drago 2000, Sylvester et al. 2007, Sardina et al. 2008). However, an interesting difference is that, in contrast with these reports, our data indicate that the effect of the mussel on oligochaetes is negative (rather than positive). This difference suggests a species-specific effect, whereby different species are affected differently. Freshwater oligochaetes comprise both infaunal (burrowing), and epifaunal species; it is conceivable that the oxygen depletion associated with the large amounts of organic matter derived from the mussels' feces and pseudofeces (Sardina et al. 2008) inhibits the development of burrowing species, while still providing an advantageous medium for the epifaunal forms. Incidentally, in Rio Tercero Reservoir, the burrowing oligochaete Branchiura sowerbyi was found in the nearby sediments, but not in L. fortunei druses (Table 4). Similarly, in Lake Lower Nashotah, the burrowing mayfly Hexagenia was found in bare sediments, but not in Dreissena druses. Facilitation by habitat modifiers in general, and by filter-feeding bivalves in particular, is a well-known phenomenon, but effects on the benthic fauna are often modulated and even reversed by specific makeups (particularly in oligochaetes associated with mussels (Afanasiev 1987, Spooner & Vaughn 2006)), suspended sediment concentration (Norkko et al. 2006), geographical location (Buschbaum et al. 2009), season (Spooner & Vaughn 2006), sediment characteristics (Radziejewska et al. 2009), and sometimes by more intricate relationships (multispecies interactions, nonlinear biotic/abiotic interactions, threshold effects) that elude straightforward generalizations (Cummins et al. 2001). In this case, both L. fortunei and D. polymorpha, being powerful ecosystem engineers, physically alter benthic substrates and change dramatically associated macroinvertebrate communities.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Potential Effect on North America

Although on the continental scale, as a result of temperature limits, L. fortuneis northward spread in northern America may be limited when compared with that of D. polymorpha, in central and southern regions of North America, Limnoperna may colonize many more water bodies than zebra mussels (Ricciardi 1998, Karatayev et al. 2007a, Karatayev et al. 2007b, Oliveira et al. 2010). It was shown that L. fortunei has a much wider tolerance to several key abiotic factors than D. polymorpha, including upper temperature limit (35[degrees]C for L. fortunei vs. 33[degrees]C for D. polymorpha), salinity (15[per thousand] for L. fortunei vs. 6[per thousand] for D. polymorpha), low pH values (5.5 for L. fortunei vs. 7.3 for D. polymorpha), calcium (around 3 mg/L for L. fortunei vs. 25 mg/L for D. polymorpha), and dissolved oxygen (0.5 mg/L for L. fortunei vs. 1.8 mg/L for D. polymorpha) (reviewed in Karatayev et al. 2007a, Karatayev et al. 2007b). Thus, on a regional scale, L. fortunei has a clear advantage in spreading among water bodies that are too warm and/or too acidic for D. polymorpha. Hence, in the near future, the freshwaters of North America may be colonized by another invader that, in certain regions, may be even more aggressive than the zebra mussel. The ecological consequences of this invasion may be similar to or even stronger than those of zebra mussels, including strong positive effects on epifaunal benthos (e.g., exotic amphipods and gastropods), negative effects on burrowing organisms, and devastating impacts on native unionids. The negative effect on unionids may be especially strong in southern regions of the United States, particularly if L. fortunei invades soft-water habitats that serve as refuges for threatened unionids against zebra mussels infestation (Ricciardi 1998).

ACKNOWLEDGMENTS

This work was supported by the following grants: Faculty Research grant no. 114123 from Stephen F. Austin State University (to A. K., L. B., D. P., D. B., and D. P. Molloy, 2006-2007); EX-096 (University of Buenos Aires), Fundacion Williams and PICT 2004 25275 (ANPCyT) (to D. B.), and by Research Foundation of SUNY.

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ALEXANDER Y. KARATAYEV, (1) * LYUBOV E. BURLAKOVA, (1,2) VADIM A. KARATAYEV (3) AND DEMETRIO BOLTOVSKOY (4)

(1) Great Lakes Center, Buffalo State College, 1300 Elmwood Avenue, Buffalo, NY 14222; (2) The Research Foundation of The State University of New York, Buffalo State College, Office of Sponsored Programs, 1300 Elmwood Avenue, Bishop Hall B17, Buffalo, NY 14222-1095; (3) City Honors School, 186 East North Street, Buffalo, NY 14204; (4) Department of Ecology, Genetics and Evolution, University of Buenos Aires, C1428EHA Buenos Aires; Consejo Nacional de Investigaciones Cientificas y Tecnicas, and Museo Argentino de Ciencias Naturales "Bernardino Rivadavia," Argentina

* Corresponding author. E-mail: karataay@buffalostate.edu
TABLE 1.
Limnoperna fortunei and Dreissena polymorpha densities (per square
meter), and total wet biomass (grams per square meter) across
different substrates in water bodies in Argentina and in Belarus.

                                           Substrate

Water Body                                   Rocks

Limnoperna fortunei (Argentina)
  Rio Tercero Reservoir
    Density                        3,909 [+ or -] 917 (14)
    Biomass                        2,302 [+ or -] 508 (100)
Dreissena polymorpha (Belarus)
  Lake Lukomskoye
    Density                                   NR
    Biomass
  Lake Naroch
    Density                        2,206 [+ or -] 378 (22)
    Biomass                          804 [+ or -] 163 (100)
  Lake Myastro
    Density                        1,644 [+ or -] 806 (4)
    Biomass                        1,024 [+ or -] 567 (100)
  Lake Batorino
    Density                                   NR
    Biomass
  Drozdy Reservoir
    Density                        4,048 [+ or -] 1,552 (2)
    Biomass                          845 [+ or -] 303 (100)
  All water bodies
    Density                        2,257 [+ or -] 339 (28)
    Biomass                          839 [+ or -] 147 (100)
  P Kruskal-Wallis test
    Density                                 0.31
    Biomass                                 0.055

                                           Substrate

Water Body                                  Gravel

Limnoperna fortunei (Argentina)
  Rio Tercero Reservoir
    Density                        3,938 [+ or -] 1,121 (6)
    Biomass                        2,859 [+ or -] 945 (100)
Dreissena polymorpha (Belarus)
  Lake Lukomskoye
    Density                                   NR
    Biomass
  Lake Naroch
    Density                        1,456 [+ or -] 296 (6)
    Biomass                          378 [+ or -] 148 (100)
  Lake Myastro
    Density                          248 [+ or -] 25 (3)
    Biomass                           72 [+ or -] 11 (100)
  Lake Batorino
    Density                                   NR
    Biomass
  Drozdy Reservoir
    Density                        2,281 [+ or -] 954 (6)
    Biomass                        1,065 [+ or -] 466 (100)
  All water bodies
    Density                        1,545 [+ or -] 427 (15)
    Biomass                          591 [+ or -] 214 (100)
  P Kruskal-Wallis test
    Density                                 0.073
    Biomass                                 0.024 *

                                           Substrate

Water Body                                   Sand

Limnoperna fortunei (Argentina)
  Rio Tercero Reservoir
    Density                        1,302 [+ or -] 1,046 (7)
    Biomass                          576 [+ or -] 456 (71)
Dreissena polymorpha (Belarus)
  Lake Lukomskoye
    Density                          267 [+ or -] 90 (32)
    Biomass                           55 [+ or -] 16 (69)
  Lake Naroch
    Density                          119 [+ or -] 39 (215)
    Biomass                           31 [+ or -] 9 (25)
  Lake Myastro
    Density                          699 [+ or -] 340 (12)
    Biomass                          402 [+ or -] 231 (58)
  Lake Batorino
    Density                          965 [+ or -] 517 (6)
    Biomass                          215 [+ or -] 97 (50)
  Drozdy Reservoir
    Density                        5,829 [+ or -] 2,808 (6)
    Biomass                        2,310 [+ or -] 891 (69)
  All water bodies
    Density                          307 [+ or -] 85 (271)
    Biomass                          105 [+ or -] 30 (34)
  P Kruskal-Wallis test
    Density                                 0.030
    Biomass                                 0.017 *

                                           Substrate

Water Body                                 Silty Sand

Limnoperna fortunei (Argentina)
  Rio Tercero Reservoir
    Density                         1,007 [+ or -] 596 (10)
    Biomass                           554 [+ or -] 298 (90)
Dreissena polymorpha (Belarus)
  Lake Lukomskoye
    Density                         3,620 [+ or -] 1,091 (16)
    Biomass                         1,055 [+ or -] 255 (100)
  Lake Naroch
    Density                                    NR
    Biomass
  Lake Myastro
    Density                                    NR
    Biomass
  Lake Batorino
    Density                                    NR
    Biomass
  Drozdy Reservoir
    Density                          808 [+ or -] 664 (4)
    Biomass                          470 [+ or -] 403 (100)
  All water bodies
    Density                        3,058 [+ or -] 913 (20)
    Biomass                          938 [+ or -] 221 (100)
  P Kruskal-Wallis test
    Density                                 0.035
    Biomass                                 0.159

                                          Substrate

Water Body                                   Silt

Limnoperna fortunei (Argentina)
  Rio Tercero Reservoir
    Density                           29 [+ or -] 27 (31)
    Biomass                           24 [+ or -] 23 (7)
Dreissena polymorpha (Belarus)
  Lake Lukomskoye
    Density                          140 [+ or -] 57 (37)
    Biomass                           69 [+ or -] 27 (41)
  Lake Naroch
    Density                          116 [+ or -] 60 (42)
    Biomass                           26 [+ or -] 14 (10)
  Lake Myastro
    Density                           91 [+ or -] 91 (22)
    Biomass                           56 [+ or -] 56 (5)
  Lake Batorino
    Density                           43 [+ or -] 41 (29)
    Biomass                           13 [+ or -] 13 (7)
  Drozdy Reservoir
    Density                                  0(7)
    Biomass
  All water bodies
    Density                           97 [+ or -] 29 (137)
    Biomass                           38 [+ or -] 13 (16)
  P Kruskal-Wallis test
    Density                                 0.177
    Biomass                                 0.190

* Significant at P < 0.025 (Kruskal-Wallis test with Bonferroni
correction).

Cell values are means [+ or -] SE of sample size (top number) and
percent of quadrats with zebra mussels (bottom number in parentheses).
Tests of significance (Kruskal-Wallis test) compared the density and
biomass of D. polymorpha and L. fortunei. NR, not recorded.

TABLE 2.
Limnoperna fortunei and Dreissena polymorpha average individual mass
(total wet mass of body and shell, measured in grams, mean [+ or -]
SE) across different substrates.

                                          Substrate

Water Body                       Rocks             Gravel with Sand

Limnoperna fortunei
  Rio Tercero Reservoir   0.640 [+ or -] 0.076   0.638 [+ or -] 0.067
Dreissena polymorpha
  Lake Lukomskoye                  NR                     NR
  Lake Naroch             0.452 [+ or -] 0.139   0.242 [+ or -] 0.043
  Lake Myastro            0.633 [+ or -] 0.071   0.291 [+ or -] 0.033
  Lake Batorino                    NR                     NR
  Drozdy Reservoir        0.211 [+ or -] 0.006   0.466 [+ or -] 0.029
P, Kruskal-Wallis test           0.001 *                0.005 *

                                          Substrate

Water Body                        Sand                Silty Sand

Limnoperna fortunei
  Rio Tercero Reservoir   0.504 [+ or -] 0.030   0.594 [+ or -] 0.088
Dreissena polymorpha
  Lake Lukomskoye         0.290 [+ or -] 0.056   0.357 [+ or -] 0.041
  Lake Naroch             0.373 [+ or -] 0.033            NR
  Lake Myastro            0.549 [+ or -] 0.143            NR
  Lake Batorino           0.256 [+ or -] 0.051            NR
  Drozdy Reservoir        0.431 [+ or -] 0.057   0.469 [+ or -] 0.096
P, Kruskal-Wallis test           0.064                 0.0162 *

                              Substrate

Water Body                        Silt

Limnoperna fortunei
  Rio Tercero Reservoir   0.770 [+ or -] 0.066
Dreissena polymorpha
  Lake Lukomskoye         0.402 [+ or -] 0.075
  Lake Naroch             0.207 [+ or -] 0.025
  Lake Myastro                   0.618
  Lake Batorino           0.253 [+ or -] 0.053
  Drozdy Reservoir                 NR
P, Kruskal-Wallis test           0.060

* Significant at P < 0.0167 (Kruskal-Wallis test with Bonferroni
correction)/

Tests of significance (Kruskal-Wallis test) compared the average
individual mass of D. pohmorpha and L. fortunei.

TABLE 3.
Total and average species richness (number of taxa found), density
(per square meter), wet biomass (grams per square meter), and the
coefficient of variation of density (CV) of native macroinvertebrates
(excluding L. fortunei and D. polymorpha) in Rio Tercero Reservoir,
Argentina, and Lake Lower Nashotah, Wisconsin.

                                    Rio Tercero Reservoir

Parameters                      Sediments         Limnopevna Druses

Total species recorded             16                     20
Species per sample           5.6 [+ or -] 0.9      10.3 [+ or -] 0.6
Density/[m.sup.2]          4,650 [+ or -] 123    15,313 [+ or -] 284
CV of density (%)                  11                     8
Biomass (g/[m.sup.2])       8.91 [+ or -] 0.25    30.75 [+ or -] 0.50
CV of biomass (%)                  11                     7
Relative multivariate             1.449                 0.551
  dispersion
IMD substrate vs. druse                                 0.908
  communities
Margalef's species                1.776                 1.972
  richness

                           Lake Lower Nashotah

Parameters                      Sediments          Dreissena Druses

Total species recorded              9                     32
Species per sample           2.7 [+ or -] 0.8      16.5 [+ or -] 1.2
Density/[m.sup.2]            833 [+ or -] 21     18,165 [+ or -] 156
CV of density (%)                   7                     4
Biomass (g/[m.sup.2])       2.71 [+ or -] 0.13    33.03 [+ or -] 0.30
CV of biomass (%)                  14                     5
Relative multivariate             1.484                 0.516
  dispersion
IMD substrate vs. druse                                  1.0
  communities
Margalef's species                1.19                  3.161
  richness

Cell values are means [+ or -] SE. Diversity indices (calculated on
densities), including relative multivariate dispersion, index of
multivariate dispersion (IMD), and Margalefs species richness are
given for each community.

TABLE 4.
Density per square meter (average  [+ or -]  SD) and occurrence
(percent of samples with the taxon, in parentheses) of
macroinvertebrates (excluding L. fortunei and D. polymorpha) in Rio
Tercero Reservoir, Argentina, and Lake Lower Nashotah, Wisconsin.

                                        Rio Tercero Reservoir

Taxon                                         Sediments

Turbellaria
  Dugesia tigrina                                 NR
  Planaria sp.                                    NR
  Turbellaria total                               NR
Gastropoda
  Amnicola limosus                                NR
  Biomphalaria sp.                       25 [+ or -] 79 (10)
  Gundlachia moricandi                            NR
  Gyraulus circumstriatus                         NR
  Physella sp.                                    NR
  Stenophysa marmorata                            NR
  Gastropoda total                                25
Bivalvia
  Pisidium sp.                                    NR
  Sphaerium sp.                                   NR
  Bivalvia total                                  NR
Oligochaeta
  Branchiura sowerbyi                    50 [+ or -] 105 (20)
  Stylaria lacustris                              NR
  Oligochaeta sp.                     2,050 [+ or -] 1,707 (100)
  Oligochaeta total                             2,100
Hirudinea
  Glossiphonia sp.                       75 [+ or -] 121 (30)
  Helobdella brasiliensis               225 [+ or -] 558 (20)
  Helobdella fusca                                NR
  Helobdella stagnalis                  325 [+ or -] 528 (60)
  Hirudinea total                                625
Decapoda
  Aegla scamosa                                   NR
Amphipoda
  Hyalella azteca                                 NR
Ceratopogonidae
  Culicoides sp.                         50 [+ or -] 105 (20)
Chironomidae
  Chironomus sp.                                  NR
  Cricotopus sp.                                  NR
  Cryptochironomus sp.                  100 [+ or -] 129 (40)
  Dicrotendipes tritomus                          NR
  Dicrotendipes sp.                      50 [+ or -] 158 (10)
  Harnischia sp.                        150 [+ or -] 394 (20)
  Larsia sp.                            300 [+ or -] 350 (60)
  Microtendipes gr. chloris                       NR
  Microtendipes pedellus                          NR
  Nilothauma sp.                                  NR
  Polypedilum halterale                           NR
  Polypedilum illinoense                          NR
  Polypedilum sp                        275 [+ or -] 702 (30)
  Procladius sp.                         75 [+ or -] 121 (30)
  Pseudochironomus sp.                            NR
  Tanytarsus sp.                        750 [+ or -] 957 (80)
  Tribelos jucundus                               NR
  Chironomidae total                            1,700
Coleoptera
  Stenelmis sp.                                   NR
Ephemeroptera
  Caenis sp.                             25 [+ or -] 79 (10)
  Ephemeroptera sp.                               NR
  Hexagenia sp.                                   NR
  Maccaffertium mexicanum integrum                NR
  Ephemeroptera total                             25
Megaloptera
  Sialis sp.                                      NR
Odonata
  Argia sp.                                       NR
  Chromagrion sp.                                 NR
  Epitheca princeps princeps                      NR
  Odonata total                                   NR
Trichoptera
  Agraylea sp.                                    NR
  Cyrnellus fraternos                             NR
  Cyrnellus sp.                                   NR
  Mystacides sp.                                  NR
  Oecetis sp.                           125 [+ or -] 317 (20)
  Orthotrichia sp.                                NR
  Limnephiloidea sp.                              NR
  Trichoptera total                              125

                                        Rio Tercero Reservoir

Taxon                                     Limnoperna Druses

Turbellaria
  Dugesia tigrina                                 NR
  Planaria sp.                               21 + 67 (10)
  Turbellaria total                               21
Gastropoda
  Amnicola limosus                                NR
  Biomphalaria sp.                                NR
  Gundlachia moricandi                  172 [+ or -] 109 (80)
  Gyraulus circumstriatus                         NR
  Physella sp.                                    NR
  Stenophysa marmorata                  289 [+ or -] 322 (70)
  Gastropoda total                               461
Bivalvia
  Pisidium sp.                                    NR
  Sphaerium sp.                          12 [+ or -] 36 (10)
  Bivalvia total                                  12
Oligochaeta
  Branchiura sowerbyi                             NR
  Stylaria lacustris                              NR
  Oligochaeta sp.                        82 [+ or -] 147 (30)
  Oligochaeta total                               82
Hirudinea
  Glossiphonia sp.                       43 [+ or -] 135 (10)
  Helobdella brasiliensis               547 [+ or -] 448 (100)
  Helobdella fusca                                NR
  Helobdella stagnalis                4,181 [+ or -] 2,132 (100)
  Hirudinea total                               4,771
Decapoda
  Aegla scamosa                         136 [+ or -] 295 (40)
Amphipoda
  Hyalella azteca                                 NR
Ceratopogonidae
  Culicoides sp.                                  NR
Chironomidae
  Chironomus sp.                                  NR
  Cricotopus sp.                         14 [+ or -] 43 (10)
  Cryptochironomus sp.                            NR
  Dicrotendipes tritomus                          NR
  Dicrotendipes sp.                   3,744 [+ or -] 1928 (100)
  Harnischia sp.                                  NR
  Larsia sp.                          1,932 [+ or -] 846 (100)
  Microtendipes gr. chloris             205 [+ or -] 319 (40)
  Microtendipes pedellus                          NR
  Nilothauma sp.                                  NR
  Polypedilum halterale                           NR
  Polypedilum illinoense                          NR
  Polypedilum sp                         43 [+ or -] 135 (10)
  Procladius sp.                                  NR
  Pseudochironomus sp.                            NR
  Tanytarsus sp.                        661 [+ or -] 586 (80)
  Tribelos jucundus                               NR
  Chironomidae total                            6,599
Coleoptera
  Stenelmis sp.                                   NR
Ephemeroptera
  Caenis sp.                          2,382 [+ or -] 1,576 (100)
  Ephemeroptera sp.                      13 [+ or -] 42 (10)
  Hexagenia sp.                                   NR
  Maccaffertium mexicanum integrum                NR
  Ephemeroptera total                           2,395
Megaloptera
  Sialis sp.                                      NR
Odonata
  Argia sp.                                       NR
  Chromagrion sp.                                 NR
  Epitheca princeps princeps                      NR
  Odonata total                                   NR
Trichoptera
  Agraylea sp.                                21 + 66 (10)
  Cyrnellus fraternos                   781 [+ or -] 907 (100)
  Cyrnellus sp.                                   NR
  Mystacides sp.                                  NR
  Oecetis sp.                                     NR
  Orthotrichia sp.                                NR
  Limnephiloidea sp.                     35 [+ or -] 75 (20)
  Trichoptera total                              837

                                       Lake Lower Nashotah

Taxon                                       Sediments

Turbellaria
  Dugesia tigrina                              NR
  Planaria sp.                                 NR
  Turbellaria total                            NR
Gastropoda
  Amnicola limosus                             NR
  Biomphalaria sp.                             NR
  Gundlachia moricandi                         NR
  Gyraulus circumstriatus                      NR
  Physella sp.                                 NR
  Stenophysa marmorata                         NR
  Gastropoda total                             NR
Bivalvia
  Pisidium sp.                        42 [+ or -] 102 (17)
  Sphaerium sp.                                NR
  Bivalvia total                               42
Oligochaeta
  Branchiura sowerbyi                          NR
  Stylaria lacustris                           NR
  Oligochaeta sp.                              NR
  Oligochaeta total                            NR
Hirudinea
  Glossiphonia sp.                             NR
  Helobdella brasiliensis                      NR
  Helobdella fusca                             NR
  Helobdella stagnalis                         NR
  Hirudinea total                              NR
Decapoda
  Aegla scamosa                                NR
Amphipoda
  Hyalella azteca                              NR
Ceratopogonidae
  Culicoides sp.                               NR
Chironomidae
  Chironomus sp.                      167 [+ or -] 204 (50)
  Cricotopus sp.                               NR
  Cryptochironomus sp.                         NR
  Dicrotendipes tritomus                       NR
  Dicrotendipes sp.                            NR
  Harnischia sp.                               NR
  Larsia sp.                                   NR
  Microtendipes gr. chloris                    NR
  Microtendipes pedellus                       NR
  Nilothauma sp.                               NR
  Polypedilum halterale              125 [+ or -] 209 (33)
  Polypedilum illinoense                       NR
  Polypedilum sp                               NR
  Procladius sp.                               NR
  Pseudochironomus sp.                         NR
  Tanytarsus sp.                      83 [+ or -] 129 (33)
  Tribelos jucundus                      208 + 292 (50)
  Chironomidae total                           625
Coleoptera
  Stenelmis sp.                                NR
Ephemeroptera
  Caenis sp.                             42 1- 102 (17)
  Ephemeroptera sp.                            NR
  Hexagenia sp.                       83 [+ or -] 129 (33)
  Maccaffertium mexicanum integrum             NR
  Ephemeroptera total                          125
Megaloptera
  Sialis sp.                          42 [+ or -] 102 (17)
Odonata
  Argia sp.                                    NR
  Chromagrion sp.                              NR
  Epitheca princeps princeps                   NR
  Odonata total                                NR
Trichoptera
  Agraylea sp.                                 NR
  Cyrnellus fraternos                          NR
  Cyrnellus sp.                                NR
  Mystacides sp.                               NR
  Oecetis sp.                                  NR
  Orthotrichia sp.                             NR
  Limnephiloidea sp.                           NR
  Trichoptera total                            NR

                                         Lake Lower Nashotah

Taxon                                      Dreissena Druses

Turbellaria
  Dugesia tigrina                       840 [+ or -] 467 (100)
  Planaria sp.                           29 [+ or -] 72 (17)
  Turbellaria total                              869
Gastropoda
  Amnicola limosus                       27 [+ or -] 66 (17)
  Biomphalaria sp.                                NR
  Gundlachia moricandi                            NR
  Gyraulus circumstriatus               184 [+ or -] 157 (67)
  Physella sp.                          966 [+ or -] 1,034 (100)
  Stenophysa marmorata                            NR
  Gastropoda total                               1177
Bivalvia
  Pisidium sp.                           66 [+ or -] 105 (33)
  Sphaerium sp.                                   NR
  Bivalvia total                                  66
Oligochaeta
  Branchiura sowerbyi                             NR
  Stylaria lacustris                     27 [+ or -] 66 (17)
  Oligochaeta sp.                       570 [+ or -] 388 (100)
  Oligochaeta total                              597
Hirudinea
  Glossiphonia sp.                                NR
  Helobdella brasiliensis                         NR
  Helobdella fusca                           39 +95 (17)
  Helobdella stagnalis                            NR
  Hirudinea total                                 39
Decapoda
  Aegla scamosa                                   NR
Amphipoda
  Hyalella azteca                     2,668 [+ or -] 1422 (100)
Ceratopogonidae
  Culicoides sp.                         31 [+ or -] 75 (17)
Chironomidae
  Chironomus sp.                         27 [+ or -] 66 (17)
  Cricotopus sp.                                  NR
  Cryptochironomus sp.                            NR
  Dicrotendipes tritomus               1,569 [+ or -] 661 (100)
  Dicrotendipes sp.                              N R
  Harnischia sp.                                  NR
  Larsia sp.                            158 [+ or -] 140 (67)
  Microtendipes gr. chloris                       NR
  Microtendipes pedellus              2,148 [+ or -] 953 (100)
  Nilothauma sp.                         27 [+ or -] 66 (17)
  Polypedilum halterale                 690 [+ or -] 353 (100)
  Polypedilum illinoense                 86 [+ or -] 210 (17)
  Polypedilum sp                                  NR
  Procladius sp.                         29 [+ or -] 72 (17)
  Pseudochironomus sp.                  347 [+ or -] 620 (50)
  Tanytarsus sp.                      2,417 [+ or -] 1,383 (100)
  Tribelos jucundus                   3,197 [+ or -] 2,613 (100)
  Chironomidae total                            10,697
Coleoptera
  Stenelmis sp.                          26 [+ or -] 65 (17)
Ephemeroptera
  Caenis sp.                          1,257 [+ or -] 1,083 (100)
  Ephemeroptera sp.                               NR
  Hexagenia sp.                                   NR
  Maccaffertium mexicanum integrum      163 [+ or -] 399 (17)
  Ephemeroptera total                           1,419
Megaloptera
  Sialis sp.                            152 [+ or -] 203 (50)
Odonata
  Argia sp.                             167 [+ or -] 144 (67)
  Chromagrion sp.                        60 [+ or -] 93 (33)
  Epitheca princeps princeps             56 [+ or -] 88 (33)
  Odonata total                                  283
Trichoptera
  Agraylea sp.                                    NR
  Cyrnellus fraternos                             NR
  Cyrnellus sp.                          27 [+ or -] 66 (17)
  Mystacides sp.                         26 [+ or -] 65 (17)
  Oecetis sp.                                     NR
  Orthotrichia sp.                       87 [+ or -] 151 (33)
  Limnephiloidea sp.                              NR
  Trichoptera total                              141

NR, not recorded.
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