Short-term impacts of forest fire on fish and lake water mercury concentrations from northern Minnesota lakes

Background/Question/Methods

Mercury in aquatic systems is of concern due to potential negative impacts on human and wildlife heath stemming from rapid bio-magnification in aquatic food webs.  The southern Boreal ecosystem is sensitive to mercury contamination, with elevated mercury concentrations identified in remote lakes.  Fire has been identified as having an important role in mercury cycling, re-distributing previously stored mercury and possibly impacting aquatic systems.  Prescribed fire is one tool being used in the Superior National Forest to reduce fuel loads.  Understanding the impact of fire, both prescribed and wildfire, on mercury cycling is important for this area, as the region is important for recreation, including fishing.  The overall objective of this project is to determine the impact of fire on fish and lake water mercury concentrations in northern Minnesota lakes.  Eight lakes were monitored in the Boundary Waters Canoe Area Wilderness from 2004 to 2012.  Lakes were grouped into pairs consisting of one burn lake and one un-burned control lake.  Lake monitoring included annual collection of yellow perch (Perca flavescens), as well as total and methyl mercury in lake water. 

Results/Conclusions

Of the four lake pairs, three were impacted by fire between 2004 and 2012.  All three were subjected to fall prescribed burns, and one of the three was severely burned by wildfire in 2007, approximately 2.5 years following the prescribed burn.  Initial analysis of fish mercury concentrations measured differences among lake pairs and burn status.  The impact of fire on fish mercury concentration differed between the two burn lakes included in the preliminary analysis.  Analysis of lake water total mercury concentration did not measure differences among lake pairs, but differences among burned and un-burned lakes were measured.  Similar to fish mercury changes, the impact of the fire on lake water total mercury concentration differed between the two burned lakes.  Initial results indicate that fire does impact mercury concentrations in lake water and fish, but further analysis of the impact of fire severity and portion of each watershed burned, time since fire, and the impact of repeated fire is necessary.