Synopsis: Recent reports of rising fish-mercury levels in lakes of the western Great Lakes region raise concerns that atmospheric mercury deposition may be increasing. Deposition-monitoring data are inconclusive on this matter, and other ecosystem factors, possibly related to climate change, may have increased biotic exposure to methyl-mercury, the bioaccumulative form present in fish. This project would help determine which factor(s) are most responsible for the upturn in fish-Hg and whether atmospheric Hg inputs to area lakes have actually risen or not. There are two parts to the study: (1) the reconstruction of recent Hg accumulation changes at high resolution in dated sediment cores from 40 regional lakes, and (2) an in-depth comparison of fish trends with lake and watershed characteristics to determine why some lakes show increasing levels of fish-Hg while others do not. This analysis should help address public concerns regarding the efficacy of emission controls in reducing Hg deposition and ultimately Hg levels in fish.

Chemicals Studied: Mercury

Geographic Areas: Minnesota Lakes

Project Duration: 2 years

Potential Results and Implications: Public perception is currently focused on the idea that atmospheric emissions and deposition must have increased in recent years. This is serious from a public-policy perspective, as it could lead to the impression that efforts to control mercury emissions are either ineffectual or possibly overwhelmed by rising emission from elsewhere (e.g. Asia). A clearer understanding of what actually caused fish-Hg levels to rise would go far to address such concerns. While direct monitoring data show no regional increase in wet-deposition of Hg over the last decade, longer-term evidence from lakesediment records should resolve the question far more conclusively. And such interpretations
would also be enhanced by the determination of ecosystem factors that might explain differences in fish trends among the individual lakes – especially those potentially related to climate change.

Project Contact:
Dr. Daniel R. Engstrom
Director
St. Croix Watershed Research Station
16910 152nd St. North, Marine on St. Croix, Minnesota 55047
Phone: (651) 433-5953
Email: dre@smm.org