Rewriting the history of the invasive spiny water flea in North America.

Introduced species cause significant environmental impacts, making timely detection key for mitigation. The spiny water flea, Bythotrephes longimanus, is considered one of the most ecologically and economically destructive species in North American freshwaters. For decades, management focused on limiting its spread from the Laurentian Great Lakes, where it is believed to have been released via ballast water from trans-Atlantic ships entering the St. Lawrence Seaway in the 1980s. Combining microfossil analysis from 210Pb-dated sediments, radiocarbon dating of chitinous remains, and sediment DNA, we show Bythotrephes was present at low densities in inland lakes in Ontario, Canada, at least 100 years earlier than initially reported. We then consider how its recent proliferation coincided with widespread environmental change and shifts in other processes influencing its establishment and population growth. These results challenge widely accepted assumptions about the history of Bythotrephes in North America and highlight the importance of temporal perspectives for managing species introductions and their impacts.