Document Changes in Reservoir Management on Mercury Accumulation in Fish Within Voyageurs National Park

Principal Investigators:
John Sorensen, University of Minnesota Duluth, 218-726-8469
Michael Sydor, University of Minnesota Duluth, 218-726-7205

Project Personnel:
Larry Kallemeyn, USGS-BRD, International Falls Biological Station, 218-283-9821

Project Duration:
October 2001 – January 2005

Funding:
U.S. Department of the Interior, Administered through the Midwest Region NPS and the GPCESU.

Summary:
Significant water resource issues for Voyageurs National Park (VNP) include water level management for Rainy Lake and the Namakan Reservoir (comprising Namakan, Kabetogama, Sand Point, Crane, and Little Vermilion lakes), and mercury contamination of the food web. Water levels in these six large lakes have been controlled by a hydroelectric dam at the outlet of Rainy Lake, and by regulatory dams on Namakan’s two outlets since the early 1900s. While all these lakes existed as natural water bodies, the present day reservoirs are larger, and regulated to satisfy a variety of water users. Since these are international waters, shared by Canada and the United States, they are regulated by the International Joint Commission (IJC).

The IJC uses “rule curves”, which are bands of permitted high- and low-water levels throughout the year, to regulate this system of lakes. Since 1970 the “rule curves” have allowed larger-than-natural fluctuations in lake levels on the Namakan Reservoir to maintain less-than-natural- fluctuations on Rainy Lake. However, on January 6, 2000, new "rule curve" guidelines were established to provide a more natural hydrologic regime.

Since 1987, the University of Minnesota-Duluth (UMD), in cooperation with VNP and the USGS-International Falls Biological Station, has been determining mercury contamination across a host of biotic and physical parameters. As part of that effort, annual variations in mercury accumulation in young-of-year yellow perch have been assessed at one location on Sand Point Lake since 1991. Based on that data, mercury concentrations in fish appeared to be positively correlated with water level fluctuations for that sampling site.

Because preliminary data suggest a positive correlation between water level fluctuations and mercury concentrations in fish, an important implication is that a reduction in water level fluctuations with the new “rule curve” could have a dramatic beneficial impact on mercury in game fish. The goals of this project are to examine the apparent water level - fish mercury relationship on a broader scope to test whether the preliminary findings are broadly applicable. This will be accomplished by studying all six lakes affected by the Rainy Lake and Namakan Lake dams as well as several lakes outside the Park boundaries. Annual changes in other environmental variables (temperature, lake pH etc.) will also be studied as possible covariates in explaining annual fish mercury variations.

This study is designed to include three sampling years, the last of which ends in the Fall 0f 2003. Sample analyses are currently underway for the first two years of fish collection.