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Distribution of Native Mussel (Unionidae) Assemblages in Coastal Areas of Lake Erie, Lake St. Clair, and Connecting Channels, Twenty-five Years After a Dreissenid Invasion
David T. Zanatta, Jonathan M. Bossenbroek, Lyubov E. Burlakova, Todd D. Crail, Ferenc de Szalay, Traci A. Griffith, Douglas Kapusinski, Alexander Y. Karatayev, Robert A. Krebs, Elizabeth S. Meyer, Wendy L. Paterson, Trevor J. Prescott, Matthew T. Rowe, Don W. Schloesser, and Mary C. Walsh

Northeastern Naturalist, Volume 22, Issue 1 (2015): 223–235

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Northeastern Naturalist Vol. 22, No. 1 D.T. Zanatta, et al. 2015 223 2015 NORTHEASTERN NATURALIST 22(1):223–235 Distribution of Native Mussel (Unionidae) Assemblages in Coastal Areas of Lake Erie, Lake St. Clair, and Connecting Channels, Twenty-five Years After a Dreissenid Invasion David T. Zanatta1,*, Jonathan M. Bossenbroek2, Lyubov E. Burlakova3, Todd D. Crail2, Ferenc de Szalay4, Traci A. Griffith1, Douglas Kapusinski4, Alexander Y. Karatayev3, Robert A. Krebs5, Elizabeth S. Meyer6, Wendy L. Paterson1, Trevor J. Prescott5, Matthew T. Rowe1, Don W. Schloesser7, and Mary C. Walsh6 Abstract - Over the past 25 years, unionid mussels in the Laurentian Great Lakes of North America have been adversely impacted by invasive dreissenid mussels, which directly (e.g., by attachment to unionid shells) and indirectly (e.g., by competing for food) cause mortality. Despite the invasion, unionids have survived in several areas in the presence of dreissenid mussels. We investigated current spatial patterns in these native mussel refuges based on surveys for unionid mussels across 48 sampling locations (141 sites) in 2011 and 2012, and documented species abundance and diversity in coastal areas of lakes St. Clair and Erie. The highest-quality assemblages of native mussels (densities, richness, and diversity) appear to be concentrated in the St. Clair delta, where abundance continues to decline, as well as in in Thompson Bay of Presque Isle in Lake Erie and in just a few coastal wetlands and drowned river-mouths in the western basin of Lake Erie. The discovery of several new refuge areas suggests that unionids have a broader distribution within the region than previously thought. Introduction The decline of native unionid mussels in the Laurentian Great Lakes of North America after the introduction and population increases of exotic dreissenid mussels (Dreissena polymorpha (Pallas) [Zebra Mussel] and Dreissena rostriformis bugensis (Andrusov) [Quagga Mussel]) has been well documented (Lucy et al. 2014, McGoldrick et al. 2009, Nalepa et al. 1996, Schloesser and Nalepa 1994, Schloesser et al. 1996, Strayer and Malcolm 2007). Native mussels in lakes Erie 1Central Michigan University, Institute for Great Lakes Research, Biology Department, 217 Brooks Hall, Mount Pleasant, MI 48859. 2University of Toledo, Department of Environmental Science, Lake Erie Center, 6200 Bayshore Road, Oregon, OH 43616. 3SUNY Buffalo State, Great Lakes Center, 1300 Elmwood Avenue, Buffalo, NY 14222. 4Kent State University, Dept. of Biological Sciences, Department of Biological Sciences, 256 Cunningham Hall, Kent, OH 44242. 5Cleveland State University, Dept. of Biological, Geological, and Environmental Sciences, 2121 Euclid Avenue, Cleveland, OH 44115. 6Pennsylvania Natural Heritage Program, Western Pennsylvania Conservancy, 800 Waterfront Drive, Pittsburgh, PA 15222. 7US Geological Survey, Great Lakes Science Center, 1451 Green Road, Ann Arbor, MI 48105. *Corresponding author - (All other authors listed alphabetically). Manuscript Editor: Robert Hershler Northeastern Naturalist 224 D.T. Zanatta, et al. 2015 Vol. 22, No. 1 and St. Clair had been in decline for decades as a result of pollution and habitat destruction (Nalepa et al. 1991), and the invasion of dreissenids in the late 1980s and early 1990s sparked fears of complete extirpation of unionids (Mackie and Schloesser 1996, Ricciardi et al. 1998). Dreissenids attach to shells of other species, thereby hampering filter feeding, respiration, and reproduction (Schloesser et al. 1996). Dreissenid mussels also impede movement of native mussels, leading to their burial in soft and unconsolidated sediments, and increasing drag and likelihood of their dislodgment by water currents (Burlakova et al. 2000, Karatayev et al. 1997, Schloesser et al. 1996). Conservation programs for these species have become increasingly crucial to prevent species loss (Haag 2012, Strayer 2006, Strayer et al. 2004) because unionid mussels are among the most endangered groups of animals in North America (Bogan 1993, Lydeard et al. 2004). In the 25 years since the invasions by dreissenid mussels in the Laurentian Great Lakes of North America, several locales were identified where native mussels continue to survive the dreissenid impacts (Bowers and de Szalay 2004, 2005, 2007; Bowers et al. 2005; Carlton 2008; Hebert et al. 1989; McGoldrick et al. 2009; Nichols and Amberg 1999; Nichols and Wilcox 1997; Schloesser and Masteller 1999; Schloesser et al. 1997; Sherman et al. 2013; Zanatta et al. 2002). We refer to such sites as unionid “refuges”. Some localities in Lake Erie and Lake St. Clair contained remnants of once-diverse assemblages (Brown et al. 1938, Goodrich and van der Schalie 1932, LaRoque and Oughton 1937), and communities may even have recovered (Crail et al. 2011) in a few areas of Lake Erie where habitat conditions have somewhat mitigated the impacts of the dreissenids (e.g., Bódis et al. 2014, Bowers and de Szalay 2007, Bryan et al. 2013, Lucy et al. 2014, McGoldrick et al. 2009, Sherman et al. 2013, Strayer and Malcom 2007). Knowledge of the extent of unionid refuges in coastal Lake Erie and Lake St. Clair is limited, as no single comprehensive and cohesive study has been made to compare mussel assemblages and habitats. Monitoring change is necessary to develop predictive models and strategies for protecting remaining native mussel populations in the Great Lakes. Each of the published papers on native mussel refuges in the Great Lakes cited above focuses on a small number of locations, and none address how frequently native mussels occur in coastal habitats and how often they are absent in what appears to be suitable habitat. Toward this end, the objectives of our study were to : 1) identify the extent of unionid assemblages in coastal areas in lakes St. Clair and Erie; 2) estimate relative abundance, density, and diversity of these assemblages, and compare modern and historical species assemblages; and 3) document shell-size distribution in refuges as potential evidence of recruitment and/or population stability. Study area The 48 localities surveyed in 2011–2012 (Table 1, Fig. 1) included embayments, coastal wetlands, drowned river-mouths, and deltaic and other shallow-water areas along shorelines of the United States in Lake St. Clair, the Detroit River, Lake Erie, and the Niagara River. The sampling localities consisted of areas where either Northeastern Naturalist Vol. 22, No. 1 D.T. Zanatta, et al. 2015 225 Table 1. Mean numbers of unionids collected per site (± S.E. for locations having multiple sites), species richness (SR), and effective number of species (eH´; Jost 2006) at 49 locations visited in 2011–2012 from coastal areas of Lake St. Clair, the Detroit River, Lake Erie, and the Niagara River. Code and locality Waterbody No. sites # specimens SR eH´ 1 Pollet Bay Lake St. Clair 2 6.0 ± 6.0 6 5 2 Goose Bay Lake St. Clair 3 40.5 ± 15 11 4 3 Fisher Bay Lake St. Clair 1 20.0 6 4 4 Big Muscamoot Bay Lake St. Clair 5 9.5 ± 3.9 11 6 5 Little Muscamoot Bay Lake St. Clair 3 47.5 ± 13 10 3 6 Pocket Bay Lake St. Clair 1 19.0 2 1 7 Bass Bay Lake St. Clair 2 19.5 ± 19.5 6 4 8 Humbug Island Detroit River 1 0.0 0 0 9 Calf Island Detroit River 1 0.0 0 0 10 Huron River (MI) mouth Detroit River 3 6.0 ± 4.0 9 7 11 Celeron Island Detroit River 1 0.0 0 0 12 La Plaisance Bay Lake Erie 1 9.0 2 1 13 North Maumee Bay Lake Erie 3 46.5 ± 18 6 4 14 North Maumee Bay - Dyked Lake Erie 3 0 0 0 15 Monroe Power Plant Discharge Lake Erie 2 42.0 ± 15 4 1 16 Potters Pond - Ottawa NWR Lake Erie 2 0 0 0 17 Brest Bay Lake Erie 1 0 0 0 18 Cedar Creek/Meinke Marina Lake Erie 1 2.0 2 2 19 Crane Creek Marsh - Ottawa NWR Lake Erie 5 56.0 ± 26.5 10 3 20 Turtle Creek Lake Erie 4 26.0 ± 9.0 4 2 21 Toussaint River - Gath Kurdy Preserve Lake Erie 4 48.0 ± 27.5 7 2 22 Portage River mouth Lake Erie 4 40.0 ± 2.7 5 1 23 Muddy Creek Bay Lake Erie 17 10.0 ± 3.1 10 3 24 Young Marsh Lake Erie 2 22.0 ± 2.0 8 5 25 Sandusky Bay Lake Erie 11 1.3 ± 1.0 5 4 26 Port Clinton Beach Lake Erie 1 1.0 1 1 27 East Harbor Lake Erie 2 23.0 ± 17 3 1 28 Cedar Point - Sandusky Bay Lake Erie 3 1.0 ± 0.6 3 3 29 Griffith Airport - Sandusky BayA Lake Erie 3 3.8 ± 2.7 2 2 30 Huron River (OH) mouth Lake Erie 3 4.4 ± 3.4 5 3 31 Old Woman Creek mouth Lake Erie 3 0.7 ± 0.4 1 1 32 Chapel Creek mouth Lake Erie 1 0.0 0 0 33 Sugar Creek mouth Lake Erie 1 0.0 0 0 34 Raccoon Creek mouth Lake Erie 1 0.0 0 0 35 Elk Creek mouth Lake Erie 2 0.0 0 0 36 Presque Isle Bay Lake Erie 15 0.9 ± 0.4 3 2 37 Thompson Bay – Presque Isle Lake Erie 3 10.0 ± 4.6 7 4 38 Gull Point - Presque Isle Lake Erie 2 0.0 0 0 39 Duck Pond - Presque Isle Lake Erie 2 11.0 ± 10 4 2 40 Twelve Mile Creek mouth Lake Erie 1 0.0 0 0 41 Sixteen Mile Creek mouth Lake Erie 1 0.0 0 0 42 Twenty Mile Creek mouth Lake Erie 1 0.0 0 0 43 Silver Creek mouth/ Walnut Creek mouth Lake Erie 1 0.0 0 0 44 Cattaraugus Creek mouth Lake Erie 1 0.0 0 0 45 Big Sister Creek mouth/ Bennett Beach Lake Erie 1 0.0 0 0 46 Eighteen Mile Creek backwater Lake Erie 2 0.0 0 0 47 Strawberry Island Niagara River 3 28.0 ± 14 3 1 48 Spicer Creek mouth/ Grand Isle Niagara River 1 45.0 4 2 49 Bayshore – South Maumee BayB Lake Erie 4 1201.5 ± 400.5 13 4 Mean values (locations 1–48) 13.1 3.33 1.75 AFrom D. Kapusinski (unpubl. data). BFrom Bryan et al. (2013) and not included in the analyses (of 48 sites) used herein. Northeastern Naturalist 226 D.T. Zanatta, et al. 2015 Vol. 22, No. 1 live unionids had been observed in recent years (~10 y), shells had been collected recently (Crail et al. 2011, McGoldrick et al. 2009, Zanatta et al. 2002), or where unionids were thought to possibly occur based on habitat features including water depth (<1 m, protected embayments, drowned river mouths), substrate type (soft sand or mud), and vegetation (e.g., chara, bullrushes). At each locality, one or more sites were surveyed depending on areal extent (totaling 141 sites across the 48 locations). For example, we sampled 17 sites at Muddy Creek Bay (location 24, ~7.4 km2) and 11 sites at inner Sandusky Bay (location 26, ~73 km2), but only sampled single sites at the mouths of small tributaries (<0.1 km2). Methods Each survey site was sampled for unionids within a rectangular-shaped area of 0.5 ha, typically 50 m by 100 m but dimensions were dependent on the layout of available habitat. We marked the rectangle corners and recorded the coordinates with a hand-held GPS unit. Each site was searched by tactile methods (due to poor visibility caused by highly turbid water) for 2 person-hours (e.g., 4 searchers for 30 minutes each). Search method varied due to substrate type, water clarity, and water depth; wading, raking, and snorkeling were used at a majority (90%) of sites. Relatively deep sites (>1.5 m) were sampled by SCUBA. We do not believe that varying methods biased the results among sites. We identified all Figure 1. Map showing collection localities (codes from Table 1) in Lake St. Clair, the Detroit River, Lake Erie, and the Niagara River sampled in 2011–2012. The location of the study area in northeastern North America is shown in the insert. Northeastern Naturalist Vol. 22, No. 1 D.T. Zanatta, et al. 2015 227 collected material to species, measured standard shell length of all specimens, and determined gender for species that have sexually dimorphic shells. Dreissenid data for these sites was collected as part of another study and is presented there (see Burlakova et al. 2014). We determined the mean numbers of unionids per site (± standard error [SE]) for each location by averaging data among sites sampled within each location, the species richness (SR) by enumeration of the number of species present at a location (cumulative across sites), and the diversity by a Shannon diversity index (H´), which we then converted to effective number of species (calculated as eH´, Jost 2006). We also calculated the mean shell lengths of the 13 most common or widespread species at each location. The presence of small-size classes (<40 mm length, except <30 mm length for small species and <20 mm length for Taxolasma parvum [Lilliput]) in samples was interpreted as evidence of recent recruitment (Haag 2012). Results A total of 1923 live specimens (22 species) was collected from 30 of the locations sampled between June and August in 2011 and 2012 (Tables 1, 2; Fig. 1). No mussels were found at the remaining 18 locations. The most abundant species were Quadrula quadrula (Pimpleback; n = 780), Pyganodon grandis (Giant Floater; n = 311), Lampsilis siliquoidea (Fatmucket Clam; n = 262), Amblema plicata (Threeridge; n = 199), and Leptodea fragilis (Fragile Papershell; n = 92) (Table 2). Collectively these 5 species accounted for 85.5% of the total number of specimens. The most widespread species (proportion of sites found) were: P. grandis (38.9%), Q. quadrula (27.3%), A. plicata (21.6%), L. fragilis (17.3%), and L. siliquoidea (14.4%). The assemblages in Lake Erie, the Detroit River, and the Niagara River differed considerably from that in Lake St. Clair (Table 2). In Lake Erie, Q. quadrula, P. grandis, and A. plicata dominated numerically, toogether accounting for 84% of the total assemblage. The most abundant species in Lake St. Clair, in contrast, were L. siliquoidea, Potamilus alatus (Pink Heelsplitter), Ligumia nasuta (Eastern Pondmussel), and Villosa iris (Rainbow Mussel), which together accounted for 80% of the assemblage. The mean abundance of unionids in the St. Clair delta (23.8 ± 6.2 inidviduals/ site) was more than twice that of the other sampled regions grouped together (10.9 ± 2.7 individuals/site) (Table 2). However, 5 of the 7 highest-density sites were in western Lake Erie: at Crane Creek Marsh (location 19), Toussaint Creek (21), North Maumee Bay (13), Monroe power-plant discharge (15), and the Portage River mouth (22). Mean unionid abundance in the western basin of Lake Erie was similar to that of the St. Clair delta, although variation among locations was 40% higher than in the latter (Table 3). Evidence of recent recruitment was found for 9 of the 13 most common species, but at only 11 of the 48 locations surveyed (Table 4): 3 in Lake St. Clair, 5 in the western basin of Lake Erie, 2 in Sandusky Bay, and just 1 farther east, at Presque Isle, PA. Northeastern Naturalist 228 D.T. Zanatta, et al. 2015 Vol. 22, No. 1 Discussion Our findings provide further evidence that Great Lakes nearshore beach/littoral habitats still contain native mussel populations (Crail et al. 2011, Schloesser et al. 1997) despite the presence of dreissenids and a long history of declining water Table 2. Comparison between unionid assemblages in Lake St. Clair and Lake Erie and the Detroit and Niagara rivers in 2011–2012. Rel. abund. = relative abundance; % of sites = proportion of all sites in which species was found. Waterbody Lake Erie and Detroit and Lake St. Clair Niagara rivers # of Rel. # of Rel. % of Species mussels abund. mussels abund. sites Subfamily Anodontinae Anodontoides ferussacianus Lea (Cylindrical Papershell) 1 0.2% 0 0.0% 0.7% Lasmigona complanata Barnes (White Heelsplitter) 0 0.0% 23 1.5% 8.6% Lasmigona costata Rafinesque (Flutedshell) 5 1.2% 0 0.0% 2.9% Pyganodon grandis (Say) (Giant Floater) 13 3.1% 298 19.8% 38.9% Strophitus undulatus Say (Squawfoot) 12 2.9% 0 0.0% 2.9% Utterbackia imbecillis (Say) (Paper Pondshell) 0 0.0% 9 0.6% 2.2% Subfamily Ambleminae Tribe Amblemini/Pleurobemini/Quadrulini Amblema plicata Say (Threeridge) 12 2.9% 187 12.4% 21.6% Elliptio dilatata Rafinesque (Spike) 1 0.2% 0 0.0% 0.7% Fusconaia flava Rafinesque (Wabash Pigtoe) 21 5.1% 22 1.5% 8.6% Pleurobema sintoxia Rafinesque (Round Pigtoe) 2 0.5% 1 0.1% 1.4% Quadrula pustulosa Lea (Pimpleback) 0 0.0% 5 0.3% 2.9% Quadrula quadrula Rafinesque (Mapleleaf) 0 0.0% 780 51.7% 27.3% Tribe Lampsilini Lampsilis cardium Rafinesque (Plain Pocketbook) 14 3.4% 1 0.1% 5.0% Lampsilis siliquoidea Barnes (Fatmucket Clam) 251 60.5% 11 0.7% 14.4% Leptodea fragilis Rafinesque (Fragile Papershell) 2 0.5% 90 6.0% 17.3% Ligumia nasuta Say (Eastern Pondmussel) 24 5.8% 28 1.9% 10.8% Ligumia recta Lamarck (Black Sandshell) 1 0.2% 0 0.0% 0.7% Obliquaria reflexa Rafinesque (Three-horn Wartyback) 0 0.0% 10 0.7% 4.3% Potamilus alatus (Say) (Pink Heelsplitter) 35 8.4% 22 1.5% 12.2% Toxolasma parvum (Barnes) (Lilliput) 0 0.0% 15 1.0% 7.9% Truncilla donaciformis Lea (Fawnsfoot)* 0 0.0% 0* 0.0% 0.7% Truncilla truncata Rafinesque (Deertoe) 0 0.0% 6 0.4% 3.6% Villosa iris (Lea) (Rainbow Mussel) 21 5.1% 0 0.0% 5.0% Total unionids 415 1508 Total collection sites 17 sites 125 sites Abundance (unionids/site ± S.E.) 23.8 ± 6.2 10.9 ± 2.7 Species richness (SR) 15 16 Shannon diversity (H´) 1.55 1.53 Effective number of species 5 5 *Many were collected during a fall seiche at Bayshore (Maumee Bay) in October 2011 (Bryan et al. 2013). Northeastern Naturalist Vol. 22, No. 1 D.T. Zanatta, et al. 2015 229 quality (Nalepa et al. 1991). The present study encompassed a much wider geographic area than previous investigations (e.g., Lake St. Clair delta [McGoldrick et al. 2009, Zanatta et al. 2002], Crane Creek [Bowers and de Szalay 2004], and Presque Isle Bay of Lake Erie [Masteller et al. 1993]), and all surveys were obtained within a 2-year time frame, which allowed for comparison of abundance, richness, and diversity among locations not possible with previous data . Five of the seven sites surveyed in the St. Clair delta had among the highest species richness and diversity in the entire region surveyed. Interestingly, the composition of the unionid assemblages in Lake St. Clair has changed little since 1986, prior to the Zebra Mussel invasion (Nalepa and Gauvin 1988). However, assemblages on the Canadian side of the delta (Pocket Bay and Bass Bay), which Table 4. Shell lengths, locations (codes from Table 1) with evidence of recent recruitment (length < 40 mm*,†), and proportion of locations with evidence of recent recruitment for common and/or widely distributed species in coastal areas of Lake St. Clair, the Detroit River, Lake Erie, and the Niagara River visited in 2011–2012. Range of Locations with Proportion of locations shell lengths in mm evidence of with evidence of recent Species (# of individuals) recent recruitment recruitment (# of locations) Amblema plicata 15–126 (n = 199) 13, 19, 21, 23 25% (n = 16) Fusconaia flava* 26–81 (n = 43) 2, 3, 4 27% (n = 11) Lampsilis siliquoidea 34–151 (n = 262) 2 13% (n = 8) Lasmigona complanata 24–165 (n = 23) 19 20% (n = 5) Leptodea fragilis 11–158 (n = 92) 13, 19, 25 20% (n = 15) Ligumia nasuta 49–110 (n = 52) - - (n = 10) Obliquaria reflexa* 17–63 (n = 10) 15, 19, 25 60% (n = 5) Potamilus alatus 58–138 (n = 57) - - (n = 16) Pyganodon grandis 11–155 (n = 311) 13, 25, 37 12% (n = 25) Quadrula quadrula 11–117 (n = 780) 19, 21, 22, 23, 25, 30 50% (n = 12) Strophitus undulatus* 33–43 (n = 12) - - (n = 3) Toxolasma parvum† 14–41 (n = 15) 19, 25, 30 43% (n = 7) Villosa iris* 36–67 (n = 21) - - (n = 5) *Recent recruits for small species considered when length < 30 mm. †Recent recruits for T. parvum considered when length < 20 mm. Table 3. Mean unionid abundance by location visited in 2011–2012 across 6 bathymetrically separate regions of the Lake Erie watershed, and the coefficient of variation (V) among locations in each region. Number of Mean number of Regions locations unionids/site SE V Lake St. Clair 7 23.8 6.2 34.4 Detroit River 4 3.0 1.5 100.0 Lake Erie western basin 13 22.5 6.0 48.2 Lake Erie Sandusky Bay 5 9.3 4.4 59.2 Lake Erie central basin 10 2.7 1.4 80.7 Lake Erie eastern basin 9 8.1 5.6 102.6 Niagara River 48 12.1 2.5 69.4 Northeastern Naturalist 230 D.T. Zanatta, et al. 2015 Vol. 22, No. 1 contained several rare species and have been considered among the highestquality refuges in the region (McGoldrick et al. 2009, Zanatta et al. 2002), may be in decline. In Lake Erie, the highest native mussel abundance we found exists near the warm-water discharge of the Bayshore power plant in Maumee Bay (location 49; Fig. 1), where unionids were collected during a seiche (standing wave) event (Bryan et al. 2013) that caused the water levels at that site to drop considerably at that time. However, data collected during a seiche event are not directly comparable to methods used in the present study because sampling is much more effective where large areas of the lake bottom are exposed. The sampling revealed 13 species and >1200 unionids in a 0.5-ha site, which is two orders of magnitude higher than that of any location surveyed in the present study. Other Lake Erie locations in which we found a large number of species were the mouth of the Huron River, MI; several of the large drowned river-mouths of the western basin (Toussaint Creek, Turtle Creek, and the Portage River), and two coastal wetlands (North Maumee Bay and Young Marsh). The latter two locations have habitats similar to Crane Creek (Bowers and de Szalay 2007, Bowers et al. 2005). Similar to the results of Schloesser et al. (2006), we found few unionids around islands at the outlet of the Detroit River. Only one location outside of western Lake Erie (Thompson Bay, outer Presque Isle Bay) still possessed a remnant mussel assemblage that apparently does not contain dreissenids (Burlakova et al. 2014), but populations in that region were much reduced compared to those reported by Masteller et al. (1993). Many of trhe sampling localities in central and eastern Lake Erie had few or no unionids (e.g., locations 34–46; Fig. 1). While relatively large densities of unionids were found at 2 locations in the upper Niagara River, neither of these had relatively high species richness or diversity (instead they were numerically dominated by L. siliquoidea and P. grandis), and none showed evidence of recent recruitment. Unexpectedly , and despite intensive sampling, the large, shallow (generally <2 m depth) region of Sandusky Bay (in western Lake Erie) contained small populations. Densities there were low and often below detectable limits, which equated to a large number of coastal sites where no mussels were found (albeit with evidence of recent recruitment for several species). Most individuals in Sandusky Bay were found in the mouth of the Sandusky River where it flows into Muddy Creek Bay (location 23), and few specimens occurred in the bay itself. Therefore , as abundances declined in Lake Erie, species assemblages have shifted compared to those of the 1950s and 1980s (Nalepa et al. 1991). Lake assemblages were previously dominated numerically by L. siliquoidea, P. grandis, and L. nasuta (Nalepa et al. 1991), as is still the case in Lake St. Clair. The coastal assemblage present in western Lake Erie has become dominated by Q. quadrula, P. grandis, and A. plicata, although unionid richness (SR) and diversity metrics (H´ and effective number of species) have rebounded to levels similar to that found in Lake Erie in the 1950s (Nalepa et al. 1991). A recent geographic expansion is suggested by the present results because Q. quadrula was rarely collected in earlier surveys (e.g., Clark 1944, Nalepa and Gauvin 1988). Northeastern Naturalist Vol. 22, No. 1 D.T. Zanatta, et al. 2015 231 The shift in species composition in Lake Erie is potentially a result of differential tolerance to mortality caused by dreissenid mussels. Studies from the 1990s, not long after Zebra Mussels invaded Lake Erie (ca. mid-1980s), suggest that some unionid species were more likely to survive dreissenid fouling than others (reviewed in Schloesser et al. 1996). Explanations for unionid survival included the ability to burrow quickly, gender (males appeared to be less impacted than brooding females), shell robustness (thick/heavy shelled species seemed to be less affected), and the length of brooding period (species with short brooding time were found to be less influenced). These characteristics, which are shared by many of the taxa that live in drowned river-mouths and shallow embayments (e.g., A. plicata, species of Quadrula and Fusconaia) correspond to an equilibrium life-history strategy (Haag 2012). The robust shells and short-term brooding period of these surviving species may well give them some resilience to dreissenid fouling. Two other species predominantly found in Lake Erie, L. fragilis and P. grandis, possess an opportunistic strategy of high fecundity, early age of reproduction, and short life spans (Haag 2012). Although these 2 species were not especially abundant in the stream mouths, fresh empty shells of both were frequently found on beaches (Crail et al. 2011). Host-fish abundance may have contributed to their persistence as well; P. grandis is a host generalist, and Leptodea fragilis appears to be dependent on Aplodinotus grunniens (Rafinesque 1819) (Freshwater Drum; Watters et al. 2009), a large benthic molluscivore fish that has become very abundant in western Lake Erie since dreissenid mussels invaded (ODW 2013). Our evidence for recent recruitment correlated well with abundance, richness, and diversity. Although our sampling methods were not especially efficient at detecting juvenile unionids, 9 of the 13 most-common and/or widespread species— including Q. quadrula, P. grandis, and A. plicata in western Lake Erie—showed evidence of recruitment at several locations. In contrast, a concern exists that some of the more common species in the St. Clair delta (e.g., L. siliquoidea and L. nasuta) and Presque Isle (e.g., A. plicata, F. flava, and L. nasuta) exhibited few signs of recent reproduction. These results lead to a final important question: what varies between sites with and without unionids? This study described the current distribution of native mussel assemblages in Lake St. Clair and Lake Erie, and research continues to characterize and model habitat variables of refuges that enable unionids to persist at those locations. Sherman et al. (2013) found that while both North Maumee Bay in Lake Erie and the delta of Lake St. Clair contain unionids, they differ in habitat features (drowned river-mouth vs. deltaic), substrate (softer in Lake Erie), and water chemistry (higher pH and turbidity in Lake Erie). The St. Clair deltaic environment may suppress dreissenid impacts because its flow remains river-like with a retention/replacement time of only 7 days compared to 2.6 years for Lake Erie (GLIN 2014). A persistent flow may allow periodic life-history strategists, including L. siliquoidea and L. nasuta, to sustain populations; we note in this context that the unionid assemblage of the St. Clair Northeastern Naturalist 232 D.T. Zanatta, et al. 2015 Vol. 22, No. 1 delta is similar to those of small streams in the Mississippi watershed (Haag 2012:table 8.1) with the addition of 4 species presumed to use Freshwater Drum as a host fish (L. fragilis, P. alatus, Truncilla truncata, and T. donaciformis; Watters et al. 2009) and therefore not typically present in small streams. North Maumee Bay had the highest water-level fluctuations (>1 m water-level changes during seiches) of any of the locations from across the Great Lakes, a characteristic that may largely exclude dreissenid mussels. The native mussel compositions we found in 2011–2012 are difficult to directly compare to species compositions found in previous studies. Most offshore sampling suggested a limited unionid presence, and therefore coastal refuges may represent the vast majority of what remains in Lake Erie. Within coastal refuges of western Lake Erie, species composition has changed in comparison to historical assemblages. Lake St. Clair populations showed signs of decline, although the aggregate community has not changed substantially. Our main concern is that the remaining species assemblages will succumb to mortality caused by dreissenid mussels. Only time will tell whether shifts that have been observed in Lake Erie will also occur in Lake St. Clair in the future. Acknowledgments Funding for this project was provided by the US Fish and Wildlife Service, Great Lakes Fish and Wildlife Restoration Act (project No. 30191-A-G152). Scientific collection permits were provided by wildlife agencies of Michigan, Ohio, Pennsylvania, New York, and Ontario. Special thanks to the dive crew, J. Bateman, G. Longton, D. Okon, and M. Shackelford, from DTE Energy. Additional assistance in the field was provided by: M. Scott, E. Nederhoed, J. Bergner, J. Twichell, E. Bertram, L. Adams, L. Kolich, and E. Marlow from Central Michigan University; B. Morgan, M. Hickin, B. Brdek, J. Martin, and K. Shreve from Kent State University; M. Begley from Cleveland State University; N. Bryan, C. Florence, S. Schnapp, J. Sieracki, and L. Volmar from the University of Toledo; R. Miller, A. Halmi, and C. Folb from Western Pennsylvania Conservancy; Dr. E. Masteller from the Tom Ridge Environmental Center; V. Karatayev from Cornell University; and I. Porto-Hannes from SUNY University at Buffalo. Dr. T. 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