Indigenous Unionid Clam Refugia from Zebra Mussels in Michigan Inland Lakes
Hollandsworth, Donna, Lowe, Rex, Badra, Peter, The American Midland Naturalist
ABSTRACT.-Zebra mussel presence and lake water calcium concentrations were compared to determine if a calcium concentration threshold exists which might preclude colonization of zebra mussels in Michigan inland lakes. The shallow littoral zones of nine northern Michigan inland lakes with calcium concentrations near 28.3 mg/L and considered to have had potential zebra mussel introductions through recreational boating were studied. A survey of native unionids was carried out in the lakes for the purpose of establishing a species inventory list for each lake. Zebra mussel densities were determined for lakes where zebra mussels were detected. Water samples were collected and calcium concentrations determined for each study lake. In five softwater lakes (lakes with calcium concentrations <28.3 mg/L), zebra mussels were not found. Zebra mussels had colonized three of four lakes where calcium concentrations were >28.3 mg/L. Five unionid species were found in seven of the nine lakes surveyed. Unionids were present in four of five lakes where calcium concentrations <28.3 mg/L and where zebra mussels were not detected suggesting a possible unionid refuge from zebra mussel induced mortality and/or extirpation in softwater lakes.
Zebra mussel (Dreissena polymorpha Pallas) invasion into the Great Lakes region has resulted in high mortality or extirpation of native unionids from all or parts of Lake Erie, Lake St. Clair (Schloesser et al, 1998) and the Detroit River (Schloesser et al, 1998). Extirpation of native unionids has occurred primarily in open water, but small remnant populations occur in the Lake St. Clair delta (Zanatta et al, 2002) and three areas of Lake Erie: Presque Isle Bay, Erie, PA (Schloesser and Masteller, 1999), Metzger Marsh near Toledo, OH (Nichols and Amberg, 1999) and near the mouth of die Raisin River, Monroe, MI (Schloesser et al, 1997) . In contrast, litde is known about impacts of zebra mussels on native mussels in Michigan inland lakes although zebra mussels occur in at least 260 Michigan lakes to date (Michigan Sea Grant, 2008; United States Geological Survey Nonindigenous Aquatic Species Website, 2008).
Studies of zebra mussel habitat requirements have found that pH, temperature, salinity, substrate, nutrients and calcium may limit zebra mussel colonization success (Strayer, 1991; Ramcharan et al, 1992; Stanczykowska and Lewandowski, 1993; Medina and Rasmussen, 1994; and Hincks and Mackie, 1997). The establishment of zebra mussels and dieir impact on unionids in European lakes has been well documented. In European lakes, calcium concentrations >28.3 mg/L were required for survival of zebra mussel veligers and to support zebra mussel colonization (Ramcharan et al, 1992). Results of a risk assessment for zebra mussel invasion into 3000 North American streams and rivers showed that most successful zebra mussel colonizations were in regions where calcium concentrations were >28.3 mg/L or where surface water drained high calcium areas into regions where concentrations were <28.3 mg/L (Whittier et al., 2008). However, calcium requirements for successful zebra mussel reproduction and colonization have been reported as low as 20 mg/L in laboratory study (Cohen and Weinstein, 2001).
In contrast, calcium requirements of unionids vary among species and some inhabit softwater lakes (i.e., lakes with calcium concentrations ≤28.3 mg/L) (Boycott, 1936; Mackie and Flippance, 1983). In a study of six low-alkalinity lakes in Ontario, Pyganodon (=Anodonta) grandis and Elliptio complanata -were found in Blue Chalk Lake and Harp Lake with calcium concentrations of 2.99 mg/L and 3.15 mg/L respectively (Rooke and Mackie, 1984). A large population of P. grandis has also been reported in Shell Lake, a small arctic lake, with a calcium concentration of 10 mg/L (Green, 1980) . Elliptio complanata is known to occur in calcium-poor waters (Hinch et al, 1988) and is found in Mirror Lake, New Hampshire with calcium concentrations of 2-3 mg/L (Strayer et al, 1981). …