Project Title: Resolving the Cause of the Recent Rise of Fish Mercury Levels in the Western Great Lakes Region

Chemicals Studied: Mercury

Geographic Areas: Western Great Lakes Region

Project Start Date: August 2009

Project Status: Completed

Synopsis: This study examines recent trends in fish-mercury (Hg) levels for inland lakes of the western Great Lakes region to determine which factor(s) are responsible for an observed upturn in fish-Hg and whether atmospheric Hg inputs to area lakes have risen or not.

An evaluation of fish-Hg records from 115 lakes in northeastern Minnesota confirms previous observations of a decline in standardized-length predator fish mercury concentrations beginning in the early 1980s and a reversal of that trend in the mid-1990s. A statistical evaluation of watershed characteristics and lake-water chemistry does not point to any variables that could explain annual percentage change (APC) in fish-Hg for individual lakes. APC fish-Hg was not correlated with factors known to influence delivery or production of methyl-Hg (MeHg) such as land cover, wetland area, lake-DOC, or -pH. An analysis of sediment-core records of Hg inputs from 39 area lakes showed generally modest changes for the period of record (1980-2010) with decreases in Hg concentration in 41% of lakes and increases in 18%. Hg accumulation rates showed the opposite trend with increases in 41% of lakes and decreases in 15%. A similar pattern was observed when the time period was divided into two intervals (1980-1995 and 1995-2010).

Changes in sediment Hg were significantly correlated with lake geometry ratio (GR), a metric for lake stratification, with shallow polymictic lakes tending to decrease in Hg concentration and increase in Hg accumulation over time; deeper dimictic lakes showed the opposite pattern. The antipathetic relationship between Hg concentration and accumulation appeared to be driven by an increasing flux of organic matter to the sediments, possibly driven by greater watershed export of dissolved organic carbon (DOC) or increasing lake productivity.

While watershed land-cover was only weakly correlated with APC in sediment Hg, other factors suggest that the increases in sediment Hg accumulation were driven by greater export of Hg from catchment soils and not changes in atmospheric Hg deposition. Although atmospheric Hg deposition may have declined over this time period, it was not evident against a backdrop of rising Hg inputs from the large pool of Hg stored in terrestrial soils. The absence of strong predictors of APC fish-Hg, including detailed sediment-core Hg records, likely stems from the infrequency of fish-Hg measurements which limits the reliability of trend data for any individual lake.

Project Contact:
Dr. Daniel R. Engstrom
Director
Science Museum of Minnesota
St. Croix Watershed Research Station
Phone: (651) 433-5953
Email: dre@smm.org