Population biology of a failed invasion: Paleolimnology of Daphnia exilis in upstate New York

Viable diapausing eggs of Daphnia exilis, a species within the Daphnia simi lis species complex, occur in sediments of Onondaga Lake, New York. The dis covery of this species, which otherwise occurs exclusively in temporary sal ine ponds in southwestern North America, represents a range extension of 1, 000 km. Pb-210-dating of the sediments containing diapausing eggs indicates that D. exilis was present in Onondaga Lake between the mid-1920s and the early 1980S. The species' introduction, successful colonization, and subseq uent disappearance from the water column correspond temporally with distinc t events in the history of industrial activity along the shores of the lake and with the paleoecological record of this activity deposited in Onondaga Lake sediments. Only the most recently deposited diapausing eggs (late 197 0s to early 1980s) hatch during laboratory incubation; older eggs may not b e viable because of toxic concentrations of mercury pollution in older sedi ments. The, D. exilis eggs that have hatched have had strikingly low geneti c (allozyme) variation in comparison with the variation documented for popu lations in the southwestern United States by Hebert and Finston (1993). Exp loration of various invasion scenarios through simulated introduction of ge notypes from the southwestern United States suggests that a single genotype established the D. exilis population in Onondaga Lake. These observations document the ecological and microevolutionary patterns associated with an i nvasion by an exotic crustacean that currently persists only in the sedimen t egg bank.