Water-level fluctuations substantially alter the fauna, flora, and microbial community of nearshore aquatic ecosystems. Water-level management has the potential to strongly influence a wide variety of ecosystem processes.

Many northern temperate lake food webs experience substantial methylmercury contamination, which is partially mediated by the action of sulfate reducing bacteria occurring in sediments that are periodically inundated. For lakes with methylmercury problems, water-level management could be designed to reduce methylmercury contamination.

USGS and NPS scientists compiled a long-term dataset (1997-2015) of mercury content in young-of-year Yellow Perch (Perca flavescens) from six lakes in or near Voyageurs National Park and examined whether mercury content was associated with water-level fluctuation. Many water-level metrics co-vary and appear to have strong associations with Yellow Perch mercury. However, these associations appear to vary by lake, and lake-specific models are needed to identify relationships between water-level fluctuation and Yellow Perch mercury content.

Lake-specific models showed some variation among lakes, but also supported strong associations between water-level fluctuations and annual variation in Yellow Perch mercury content. The study lakes underwent a change in water-level management in 2000, when winter water-level minimums were increased by about 1 meter in five of the six study lakes, reducing annual water-level fluctuation on those lakes.

In four of these five lakes, the change in water-level management likely reduced Yellow Perch mercury content, relative to the previous water-level management regime.

Project Publications

• Mercury and water level management in lakes of northern Minnesota