Pleistocene refuge areas and postglacial dispersal of copepods of the northeastern United States

The distribution of common freshwater copepods of the northeastern United States has a marked association with major meltwater drainage systems and coastal marine submergence zones formed during the retreat of the Wisconsinan ice sheet. Postglacial dispersal routes inferred from the deglaciation chronology indicate that zooplankton could have come from several major Pleistocene refuges in the Mississippi Valley, Appalachian Plateaus, and mid-Atlantic Coastal Plain, and from northern locations on the Atlantic Coastal Plain. Contemporary distribution patterns suggest that some species of copepods dispersed primarily in surface waters during advances and retreats of continental glaciers. These include species of the calanoid Skistodiaptomus and the cyclopoid Diacyclops thomasi that occur in coastal lowlands and in lower elevation lakes in mountainous areas that could be flooded by retreating ice-front lakes. There is little evidence, except for inadvertent introductions through human activities, that these species have expanded beyond their original postglacial dispersal boundaries. In contrast, other calanoid and cyclopoid species were widely distributed and are dominant in high-elevation lakes and lowland lakes behind mountain barriers unreachable by species dependent on water transport. These species commonly occur in permanent and temporary ponds and may produce dormant stages that are readily dispersed.