Hg in stream invertebrates
Hg is a heavy metal whose toxic, organic form methylmercury, is produced in aquatic ecosystems such as streams. Through the process of biomagnification, Hg concentrations increase exponentially from low trophic levels (bottom of the food web) to high trophic levels (top predators). Therefore, organisms at the base of the food web (herbivores) tend to have lower Hg concentrations than those at the top of the food web (predators).
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In streams, macroinvertebrates are generally organized into four functional feeding groups: shredders, collectors, grazers/scrapers, and predators. While predators prey on animal tissues and are thus at the highest trophic position, the other three groups consume particulate organic matter and vegetation occupying lower trophic positions.
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_Biomagnification_Graphic_ed.jpg)
NPS, Public domain, via Wikimedia Commons
Purpose
The purpose of this project is to identify major taxa in each of the four functional feeding groups from Strong's Creek (Ogden, UT) and determine Hg concentrations in each group. Our goal is to collect at least one representative taxa from each functional feeding group to understand how Hg flows through this food web. This project began in June 2024 as a collaboration between myself and Dr. Chris Hoagstrom, the aquatic ecologist in the Zoology Department. Together, we assembled a group of eager undergraduate students to lead the field collection and conduct Hg analysis in the lab.
How do we collect aquatic macroinvertebrates?
By flipping lots of rocks! Macroinvertebrates often cling to rocks in fast moving water and we collect and sort what we find attached. We also use kick nets to find macroinvertebrates associated with piles of vegetation and sediment. We then bring the insects back to the lab for further taxonomic identification and Hg analysis.
Why is it important to understand Hg in stream invertebrates?
In addition to being part of the aquatic food web, aquatic macroinvertebrates also play crucial roles in adjacent terrestrial food webs. Many groups of aquatic macroinvertebrates (such as mayflies) undergo metamorphosis into flying adults and become important prey for terrestrial predators such as spiders and songbirds. Any Hg that the invertebrates accumulated in the stream during their larval stage is then transferred out of the stream and into terrestrial predators. Our goal is to determine how much Hg is being accumulated by aquatic macroinvertebrates during their larval stage to understand the risk of accumulation of Hg to nearby terrestrial predators.
Chumchal et al. 2022, https://doi.org/10.1002/etc.5267