Northern Redistribution of Freshwater Pearly Mussels (Bivalvia: Unionoidea) during Wisconsin Deglaciation in the Sothern Glacial Lake Agassiz Region: A Review

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Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138 and Department of Biology Northeastern University, Boston, Massachusetts 02115

ABSTRACT.-The chronology of the reinvasion of freshwater pearly mussels (Bivalvia: Unionoidea) into the southern Glacial Lake Agassiz region is reconstructed from the published record of the latest Pleistocene (Wisconsin) deglaciation of the region and known spatial and temporal distributions. The earliest invading unionoidean fauna entered the region above the driftless area of southeastern Minnesota and western Wisconsin between 14,000 and 11,000 years before present; the remaining species colonized the region postglacially. The formation of St. Anthony Falls is correlated with the drainage of Lake Agassiz from the Herman to the Campbell Beach level as an estimate of its date of formation, and a new chronology and mechanism of mussel dispersal into the Red River of the North basin is proposed.


Malacologists, working in the southern Glacial Lake Agassiz region (e.g., Wilson and Danglade, 1914; Dawley, 1947; Cvancara and Cvancara and Cvancara et al., see below; Clarke, 1973, 1981) and elsewhere (e.g., Simpson, 1896; Ortmann, 1924; Johnson, 1970, 1980), recognized the effect of the latest Pleistocene (Wisconsin) deglaciation on the distribution of freshwater pearly mussels (Bivalvia: Unionoidea) in the Canadian Interior and Great Lakes basins. The meltwater connections produced by wasting glaciers allowed the migration of mussels upstream from southern refugia (Johnson, 1980). Once water levels fell after glaciers receded, the basins took on their present configurations, limiting the dispersal of aquatic organisms such as fish-dispersed unionoideans across the divides.

The dispersal chronology of mussels into the region's now disjunct watersheds can be determined by analyzing their past and present distributions in the light of the known sequence of interbasin connections. Dawley (1944, 1947) recognized these aquatic connections, but limited geological information prohibited her from proposing a mechanism or chronology of dispersal. Cvancara (1966, 1967, 1970, 1975, 1976, 1983; Cvancara et al., 1966; 1972; 1976, 1979; Ashworth and Cvancara, 1983) contributed a wealth of information on the Recent and fossil Unionoidea of the southern Lake Agassiz basin but speculated little on the origin of the region's mussel community. Underhill (1989) has previously proposed a dispersal chronology for the reinvasion of the Minnesota's ichthyofauna; this is discussed below.

Present hydrology.-The southern Glacial Lake Agassiz region is composed of the headwaters of three major continental drainage basins (the Great Lakes, Hudson Bay and Mississippi River basins), and was formerly occupied by the early stages of the Glacial Lake Agassiz watershed, It is here divided into five drainage systems: the Lake Superior, Lake of the Woods, Red River, and Upper and Lower Mississippi River systems (Fig. 1).

The Lake Superior System, that portion of the Great Lakes basin within the region, includes Lake Superior and its tributaries in Ontario, Minnesota and Wisconsin. The Hudson Bay basin in the region includes only those tributaries of the Nelson River above Lake Winnipeg. For the purposes of this paper, the Nelson River watershed is subdivided into two drainage systems: the Red River System (the Red River of the North and all of its tributaries) and the Lake of the Woods System (the Winnipeg River with its tributaries).

The remainder of the study area drains to the Gulf of Mexico. This basin is subdivided into the Upper and Lower Mississippi River systems (following Underhill, 1989; also Graf, 1997) based on their divergent histories and disparate mussel communities. The former is that portion of the Mississippi River and all of its tributaries above St. Anthony Falls at Minneapolis, Hennepin County, Minnesota. …