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Harbor mud reveals centuries of history

sampling mud in Duluth harbor NRRI scientists pull a sediment sample from the Duluth harbor that will reveal centuries of water quality history.

Euan Reavie has the kind of job a three-year old might think is just plain fun — it involves mud, lots of mud — even in the dead of winter. In mid-March Reavie and his crew hiked out on frozen water to a spot in the Duluth-Superior harbor and sunk a long tube through a hole in the ice to suck up about 300 years of muck.

Reavie is an aquatic ecologist who specializes in paleolimnology — the study of water quality history from sediments and preserved fossil records. He and fellow scientists Lisa Allinger and UMD graduate student Meagan Aliff collected sediment samples as part of the multi-organizational effort to restore areas of the St. Louis River Estuary. This site — one of seven sites that will be sampled — is one of a few that hasn’t been dramatically modified for the harbor’s intense industrial uses.

The goal of the project is to understand how water quality has changed from the 1700s, when Europeans started settling the area en masse, to today. The tube of muck holds tiny fossils of algae, called diatoms, that tell the scientists about past water quality. They can then correlate changes in the diatom populations with the quality of the water.

"Otherwise, we have no information on earlier conditions, because people obviously weren’t measuring water quality way back then," Reavie explained. "By going so far back in time we can quantify how much damage has been done and how well recent remediation is working."

The scientists will also look at the core samples for chemicals, like mercury and PCB, that have deposited onto the lake over time. The samples will also be analyzed for wild rice remains to determine how much wild rice was growing in the area before industrialization. The data will inform multiple stakeholders about remedial successes and what goals for restoration should look like.

"We need to know how far we’ve come from the days when the river was choked with organic matter and fish were dying because there wasn’t enough oxygen," said MPCA Coordinator Diane Desotelle. "And we need to know how far we still need to go."

The mud samples are taken back to the lab at NRRI and sliced off in layers that reflect sedimentary deposits over time. The mud is processed and fossils are analyzed under a microscope. What emerges from the muck is a historical timeline of changes in the estuary.

Paleolimnology is just one of NRRI’s many research efforts in the estuary to help state, local and federal agencies support the Remedial Action Plan and help them reach targeted goals that will remove the estuary’s Area of Concern designation by 2025. This project is funded by the Minnesota Pollution Control Agency and Minnesota Sea Grant.