Freshwater mussel community response to warm water discharge in western Lake Erie
Introduction
North America hosts the most diverse group of unionids (Bivalvia: Unionidae) in the world with over 300 documented species (Bogan, 1993). Historically, the western basin of Lake Erie had the largest populations of unionids within the Laurentian Great Lakes, probably due to relatively warm temperatures, shallow depths, and high flushing rates (Nalepa et al., 1991). However, unionid abundances in western Lake Erie have been declining since the 1960s (Nalepa et al., 1991) due to pollution and habitat alteration (Stevens and Neilson, 1989, Nalepa et al., 1991, Morang et al., 2011). When Eurasian dreissenid mussels (Bivalvia: Dreissenidae) invaded Lake Erie in 1986 (Schloesser et al., 1996), they exacerbated unionid declines by fouling their shells and possibly competing for food and oxygen (Schloesser and Nalepa, 1994, Parker et al., 1998). Ricciardi et al. (1998) reported that all unionid species were extirpated within 4–8 years in many areas that developed high dreissenid populations.
Although habitat degradation and dreissenid mussels have reduced unionid distributions and abundances in western Lake Erie, some habitats still support abundant populations and diverse communities (Crail et al., 2011). Habitat characteristics that allow coexistence of unionids with dreissenids are unclear (Bowers and de Szalay, 2003), but understanding these characteristics may be important for the conservation and management of unionids. Recent studies suggest several reasons why certain habitats have low dreissenid fouling rates. Strong currents reduce the likelihood that dreissenid pediveligers will settle out of the water column and attach to unionids (Bowers and de Szalay, 2003). Deep layers of unconsolidated sediments allow unionids to burrow, thus escaping colonization and possibly even “shedding” attached dreissenids (Nichols and Wilcox, 1997). Also, dreissenid predators may remove attached dreissenids from the shells of unionids (Bowers et al., 2005). The reason why dreissenid colonization rates are relatively low in some habitats is almost certainly multi-factorial, so that habitats offering a combination of key features may be most likely to allow coexistence of exotic and native mussels.
A seiche in October, 2011, along the southern shore of the western basin of Lake Erie revealed a diverse community of unionids living within the discharge plume of First Energy's Bayshore Power Plant (BPP) at Oregon, Ohio. The plant takes water from the Maumee River as it enters the western basin (Ager, 2009) and discharges it into a small bay partially separated from the larger basin by an island of sediments dredged from the Toledo shipping channel (Fig. 1). Herein, we present field data from unionid sampling we completed in the vicinity of the power plant's thermal plume. We also present data from three additional studies done within and adjacent to this thermal plume. Using these data, we elucidated the likely factors contributing to unionid success in this habitat. The goal of this study was to identify the key habitat characteristics and positive feedbacks among bivalves that allow native unionids to coexist with two exotic taxa (Dreissena spp. and Corbicula fluminea). Our findings may be important for unionid conservation in western Lake Erie.
Section snippets
Study area
Our study area was located in the shallow (< 2 m depth) waters within 200 m of the southern shoreline of Lake Erie, within the Maumee Bay (Fig. 1). This included sites (1) within the power plant's thermal plume (ca. 0.4 km east of the BPP), (2) at Bayshore Park (2.0 km east of BPP), and (3) at the University of Toledo's Lake Erie Center (4.0 km east of BPP). The BPP receives cooling water from an intake canal connected to the east side of the mouth of the Maumee River. Heated water is discharged
Unionids
The Shannon diversity index (H′) was calculated for each of the 0.5 ha sample plots in 2011. However, the single plot sampled at each site does not permit statistical comparisons between sites. In contrast, the 95% confidence intervals about the means for density and H′ of the four 0.01 ha plots collected at Bayshore Park in 2009 were compared to the density and diversity of the community found in the 0.01 ha plot within the thermal plume of the power plant in 2011.
Only Leptodea fragilis was
Unionids
We found 2657 unionids representing 13 species during the 2011 seiche. There were 715 unionids in the 0.01 ha plot representing 11 species, and 1942 unionids representing 11 species in the three 0.5 ha plots (Table 1). This includes 3 species listed in Ohio as threatened or as species of concern (Ohio Department of Natural Resources). For the 0.5 ha plots (2011), the thermal plume had 598 individuals, Bayshore Park had 531, and Lake Erie Center had 97. The density of unionids in the 0.01 ha plot
Discussion
Our results indicate a larger, more diverse community of unionids living within the thermal plume of the Bayshore Power Plant than at other more exposed locations along the southwest shore of Lake Erie. Clearly unionids have not been extirpated by dreissenids, despite concerns by Ricciardi et al. (1998). The low rate of dreissenid infestation within the discharge embayment is probably one of the reasons why unionids are thriving in this habitat. Positive flow from the power plant may reduce
Conclusion
The thermal plume at the Bayshore Power Plant provided favorable habitat for unionids due to a combination of interacting factors. Dreissenid density was low, sediment and potential food availability (sediment organic matter) was high, and temperature and flow (plant discharge = 8–33 m3 s− 1) were elevated within the embayment. All of these factors may individually and synergistically contribute to the greater sizes, density, and diversity of the unionid community in the thermal plume. Despite
Acknowledgments
We thank J. Gottgens for reviewing this paper and providing laboratory support. We also thank T. Fisher and The University of Toledo's Lake Erie Research Center for providing additional lab support. We also thank two anonymous reviewers for suggestions that improved this work. Financial support was provided by the U.S. Fish and Wildlife Service grant #30181AG152, the National Science Foundation GK-12 program grant #DGE-0742395, and the Department of Education TCT program grant #P381B080006. We
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