Aquatic food webs and heavy metal contamination in the upper Blackfoot River, Montana

Abstract

Acid mine drainage (AMD), characterized by low pH and abundant heavy metals, is a widespread problem affecting water quality and fish habitat in Montana. Montana’s upper Blackfoot River exhibits impaired water quality from historic mining that has significantly degraded aquatic habitat and reduced fish and invertebrate abundance in impacted streams. The goal of this study is to investigate the direct and indirect effects of mine-related heavy metals contamination on aquatic ecosystems by examining changes in aquatic community composition, bioaccumulation, and toxicity risk of heavy metals along a contamination gradient in the upper Blackfoot River. Three primary research questions were addressed in this study: 1) How are macro-invertebrate communities influenced by heavy metals contamination? 2) What are the implications of changes in food web structure for exposure pathways? 3) What levels of environmental contamination produce the greatest risk to upper trophic levels? Invertebrate and fish communities impacted by heavy metals in the upper Blackfoot River were sampled in 2009 and 2010 for community composition analyses and metals concentrations. The results of this study indicate that an increase in heavy metals contamination in the upper Blackfoot River results in important changes in exposure pathways of metals entering aquatic food webs through invertebrate food sources, as well as exposure pathways of metals to fish. The greatest exposure risk to upper trophic levels from the pool of bioavailable metals in invertebrates occurred at moderately contaminated sites where moderate invertebrate abundance and moderate sediment metals levels coincided. In addition, the highest metals concentrations in fish tissue were at sites with high exposure values in invertebrates, rather than sites with the highest sediment contamination levels. The results of this study indicate that biological mechanisms influencing the movement of heavy metals in aquatic food webs are important factors for assessing toxicity risk to upper trophic levels that may not be evident when considering environmental contamination alone