Author's personal copy Characteristics of a refuge for native freshwater mussels (Bivalvia: Unionidae) in Lake St. Clair Daryl J. McGoldrick a, ⁎, Janice L. Metcalfe-Smith a , Michael T. Arts a , Donald W. Schloesser b , Teresa J. Newton c , Gerald L. Mackie d,e , Emy M. Monroe f , Johann Biberhofer a , Kennon Johnson g a Water Science and Technology Directorate, Environment Canada, P.O. Box 5050, Burlington, ON, Canada L7R 4A6 b U.S. Geological Survey, Great Lakes Science Center, Ann Arbor, MI 48105-2807, USA c U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI 54603, USA d Department of Integrative Biology, University of Guelph, Guelph, ON, Canada N1G 2W1 e Water System Analysts, 56 Avra Court, Guelph, ON, Canada N1H 7B2 f Department of Zoology, Miami University, Oxford, OH 45056, USA g Walpole Island Resource Protection, R.R. #3, Walpole Island, ON, Canada N8A 4K9 abstract article info Article history: Received 28 March 2008 Accepted 4 November 2008 Communicated by T. F. Nalepa Index words: Freshwater mussels Zebra mussels Lake St. Clair Glycogen Fatty acids Refuge The Lake St. Clair delta (∼ 100 km 2 ) provides an important refuge for native freshwater mussels (Unionidae) wherein 22 of the ∼ 35 historical species co-occur with invasive dreissenids. A total of 1875 live unionids representing 22 species were found during snorkeling surveys of 32 shallow (∼ 1 m) sites throughout the delta. Richness and density of unionids and zebra mussel infestation rates varied among sites from 3 to 13 unionid species, 0.02 to 0.12 unionids/m 2 , and b 1 to 35 zebra mussels/unionid, respectively. Zebra mussel infestation of unionids in the delta appears to be mitigated by dominant offshore currents, which limit densities of zebra mussel veligers in nearshore compared to offshore waters (13,600 vs. 28,000/m 3 , respectively). Glycogen concentrations in the tissues of a common and widespread species in the delta (Lampsilis siliquoidea) suggest that zebra mussels may be adversely affecting physiological condition of unionids in a portion of the Lake St. Clair delta. Physiological condition and community structure of unionids within the delta may also be influenced by differences in food quantity and quality resulting from the uneven distribution of water flowing from the St. Clair River. The delta likely supports the largest living unionid community in the lower Great Lakes and includes several species that have been listed as Endangered or Threatened in Canada and/or the state of Michigan, making it an important refuge for the conservation of native unionids. Crown Copyright © 2009 Published by Elsevier Inc. All rights reserved. Introduction In the Great Lakes, declines in native unionids due to the impacts of zebra mussels (Dreissena polymorpha Pallas, 1771) have been well- documented (e.g., Schloesser and Nalepa 1994, Nalepa et al., 1996, Schloesser et al., 2006). However, several “refuge” sites for unionids have been found in nearshore wetland habitats of Lake Erie where zebra mussels are present, but at low densities. A viable community of 21 species was found in Metzger Marsh along the south shore of Lake Erie in 1996 (Nichols and Wilcox, 1997), and a community of 15 species was found in nearby Crane Creek Marsh in 2001 (Bowers and de Szalay 2004). Thompson Bay, the outer harbor of Presque Isle Bay, supports a smaller community of only nine species, and annual monitoring between 1992 and 2006 suggests that the community is stable (E.C. Masteller, Pennsylvania State University. pers. comm. 2008). The locations, characteristics, and ultimate significance of these refuges are of considerable interest for unionid conservation. Zanatta et al. (2002) discovered an important refuge for unionids in the Lake St. Clair delta, where 22 of ∼ 35 historical species were found alive between 1999 and 2001. This refuge falls mainly within the territory of the Walpole Island First Nation. Eight of the species found are designated as Endangered or Threatened in Canada and/or Michigan and a ninth species, the Northern Riffleshell (Epioblasma torulosa rangiana Lea, 1838), is listed as Endangered in the US, Canada and Michigan (COSEWIC, 2007a; Michigan DNR, 2007; U.S. Fish and Wildlife Service 2007). Densities of unionids in the Lake St. Clair delta are low, ranging from 0.03 to 0.07 unionids/m 2 (Zanatta et al. 2002), compared to an average density of 2 unionids/m 2 reported for offshore waters of Lake St. Clair before the zebra mussel invasion in 1986 (Nalepa and Gauvin 1988). However, the Lake St. Clair refuge is much larger than any of the other refuge sites discovered to date in Journal of Great Lakes Research 35 (2009) 137–146 ⁎ Corresponding author. Tel.: +1 905 336 4685. E-mail addresses: Daryl.McGoldrick@ec.gc.ca (D.J. McGoldrick), janiceatstoney@sympatico.ca (J.L. Metcalfe-Smith), Michael.Arts@ec.gc.ca (M.T. Arts), dschloesser@usgs.gov (D.W. Schloesser), tnewton@usgs.gov (T.J. Newton), gerry.mackie@sympatico.ca (G.L. Mackie), monroeem@muohio.edu (E.M. Monroe), Hans.Biberhofer@ec.gc.ca (J. Biberhofer), kennon.johnson@wifn.org (K. Johnson). 0380-1330/$ – see front matter. Crown Copyright © 2009 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.jglr.2008.11.007 Contents lists available at ScienceDirect Journal of Great Lakes Research journal homepage: www.elsevier.com/locate/jglr