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A new study led by Trent University’s Andrew Tanentzap, and co-authored by ���ϳԹ���Դ’s John Smol and other researchers from Laurentian University and University of Leeds in England, shows the invasive spiny water flea (Bythotrephes longimanus) was present in Ontario lakes long before scientists first recorded it in 1982.

The research used lake-sediment fossils and environmental DNA (eDNA) found in mud at the bottom of lakes to build a historical record of the tiny crustacean. The methods revealed traces of the spiny water flea decades, and in some cases more than a century, earlier than traditional net surveys detected the species.

“For decades, scientists believed the spiny water flea arrived through ballast water released by trans-Atlantic ships entering the Great Lakes,” says Dr. Tanentzap. “That timeline shaped how government and conservation organizations have managed the species' spread across North America, which has come at considerable cost and caused significant economic and environmental damage to Ontario’s freshwaters.”

The results suggest the flea may have existed as a “sleeper population,” remaining rare and largely undetected until changing environmental conditions allowed its numbers to surge. That pattern helps explain why conventional monitoring – which relies on capturing organisms in nets – can miss low-abundance invaders that are small and nearly transparent.

The spiny water flea preys on native zooplankton, the tiny animals that control algae and feed young fish. Past work cited by the researchers shows invasions can cut zooplankton biomass by up to 34 per cent and reduce species diversity by as much as 54 per cent, with consequences for water quality and fisheries.

“Lake sediment profiles can act like ‘time machines’, slowly accumulating libraries of information that we can then use to undertake retrospective environmental assessments,” adds Dr. Smol. “Such historical perspectives can often provide key information for effective lake management.”

Researchers combined cutting-edge environmental DNA techniques with paleolimnology, the study of ecological history preserved in lake sediments. The approach allows researchers to reconstruct decades and even centuries of environmental change from biological traces left behind in the mud at the bottom of lakes. 

The result is a much longer ecological memory than conventional monitoring programs can provide. 
The study has been published in the .