Factors Affecting Mercury Concentrations in Iowa Fishes


Principle Investigator:

Michael J. Weber

Clay L. Pierce

Student Investigator:

Nathan T. Mills (M.S. graduate)


Darcy Cashatt, and other Iowa DNR staff


June 1, 2014 to July 31, 2016

Funding Source(s):

Iowa Department of Natural Resources


Goals and Objectives:

  • Evaluate temporal changes in mercury concentrations in largemouth bass to guide mercury sampling protocols.
  • Develop models to predict the concentration of mercury in a range of fishes as a function of fish total length across natural lakes, impoundments, reservoirs, and rivers to guide consumption advisories.


Conclusions and Recommendations:

Largemouth bass mercury concentrations varied across months in Red Haw Lake, with the highest concentrations observed during July, and the lowest concentrations observed during October. In contrast, largemouth bass mercury concentrations were similar across months in Twelve Mile Lake. Fish mercury concentrations in Iowa lakes are generally low, with mercury concentrations <0.30 mg/kg for ~90% of fishes collected and mercury concentrations below detectable levels (<0.05 mg/kg) for ~40% of fishes.

Detected mercury concentrations in lakes were highest in muskellunge, northern pike, walleye and largemouth bass, lowest in black and white crappie and bluegill, and positively related to fish length and age. Lake mean depth, pH, watershed:lake area ratio, and percent of watershed as forested land, grasslands and open water were positively related to fish mercury concentrations, whereas lake area and percent of watershed as agriculture and developed land were negatively related to mercury concentrations. Finally, detected mercury concentrations were on average 28% higher in shallow natural lakes compared to other lake types. Combined, these factors explained 74% of the variation in detectable fish mercury concentrations in Iowa lakes.

Fish mercury concentrations in rivers were also generally low and similar among species. Fish mercury concentrations were positively related to length, age, trophic position and δ13C signatures. Human Threat Index and percent of watershed as open water were negatively related to fish mercury concentrations, whereas percent of watershed as forested land was positively related to fish mercury concentrations. Additionally, phosphorous, nitrogen-ammonia, and sulfate were weakly negatively related to mercury concentrations, whereas water hardness was weakly positively related to fish mercury concentrations. Fishes collected from the Paleozoic Plateau ecoregion had the highest mercury concentrations compared to those collected from other ecoregions across Iowa. Together, these factors explained 70% of the variation in fish mercury concentrations in Iowa rivers.

01/01/2014 to 12/31/2016