Mercury Exposure in Double-crested Cormorant Chicks from Lake ErieErie

Visscher, Eva G.

Mercury Exposure in Double-crested Cormorant Chicks from Lake ErieErie

Files

Visscher_Eva_MSc_2026_thesis.pdf (1.19 MB)

Date

2026-01-23

Authors

Visscher, Eva G.

Abstract

Mercury (Hg) concentrations in the Great Lakes have declined since the early 1970s, yet legacy contamination from industrial point sources continues to impact some ecosystems. Double-crested Cormorants (Nannopterum auritum) accumulate Hg through their piscivorous diet and may be adversely affected by this exposure to Hg. This study compared Hg burdens between colonies in Lake Erie’s western and eastern basins that are more, and less Hg contaminated, respectively. Chicks at Middle Sister Island (west) had red blood cell and feather Hg three times higher than at Mohawk Island (east). Maternal transfer could not explain this difference as egg Hg was similar between sites, implicating post-hatch diet as the Hg source. Stable isotope data indicated a higher trophic position in the west, consistent with higher Hg exposure. These results highlight geographically uneven dietary exposure to Hg and pinpoint west basin cormorants as a particularly Hg-impacted population, notably at early life stages.

Summary for Lay Audience

Mercury is a naturally occurring element that can become a pollutant in certain circumstances. In water, it can change into methylmercury, a form that builds up in animals and becomes more concentrated at each step of the food chain. My study asked a simple question: why do cormorant chicks in one part of Lake Erie carry more mercury than those in another? I compared Double-crested Cormorant chicks from two long-standing colonies: Middle Sister Island in the lake’s west basin and Mohawk Island in the east. To see when and how chicks were exposed, I measured mercury in three tissues that record different time windows: eggs (pre-hatch, what mothers pass to embryos), red blood cells (post-hatch, recent meals), and feathers (post-hatch, exposure over the time they form). Egg mercury was similar at both colonies, suggesting embryos began life with a similar amount of mercury in their bodies. After hatching, however, west basin chicks had about three times more mercury in blood and feathers than east basin chicks. I used natural chemical markers in chick tissues that reflect diet and position in the food web. These markers showed that west basin chicks were feeding higher in the food web, meaning the energy reaching them likely passed through more steps in the food chain. Because methylmercury increases at each step, every extra link gives it more chances to build up before it reaches the chicks. These results point to diet after hatching, rather than maternal transfer, as the main cause of the difference. In other words, mercury risk is not uniform within a single lake: local hotspots of dietary exposure persist even as overall mercury has declined. Because cormorant chicks respond quickly to changes in what and where they feed, they are practical sentinels for finding places and pathways where mercury remains elevated. Targeting those hotspots can guide monitoring, habitat protection, and fish-consumption advice for people.