Toward Resurrection Ecology: Daphnia mendotae and D. retrocurva in the Coastal Region of Lake Superior, among the First Successful Outside Invaders?

Abstract

In Lake Superior, almost all embayment and coastal species depend on overwintering stages. Diapausing eggs from sediments provide a long-term record of species presence in addition to furnishing individuals for genetic characterization and experimental studies (resurrection ecology). Here we begin to clarify the historical development of present day species distributions in Lake Superior by examining species composition in time and space around the Keweenaw Peninsula. Keweenaw Bay sediments document the relatively recent increased abundance (perhaps arrival) of Daphnia mendotae and D. retrocurva in coastal assemblages, two species previously assumed to be characteristic of late summer assemblages. Ephippial eggs are confirmed to species by hatching experiments and genetic characterization. The timing of D. mendotae and D. retrocurva appearance coincides roughly with initial ship traffic through the St. Marys River, suggesting colonization from the lower Great Lakes at that time. An alternative hypothesis is that run-off from forest clearance altered coastal waters and encouraged Daphnia development. Eutrophication and increased abundance of planktivorous fishes, particularly perch and smelt, may also have contributed to Daphnia species succession in the Keweenaw Waterway, whereas recent interactions with Bythotrephes in Keweenaw Bay may be pushing assemblages towards dominance of D. mendotae and Holopedium. Contact between D. mendotae and resident D. dentifera in the Keweenaw Waterway led to hybridization. In the waterway, hybrids were more common in the past, yet hybridization continues in small ponds and embayments near regions of species contact.