Mercury Isotopes Reveal an Ontogenetic Shift in Habitat Use by Walleye in Lower Green Bay of Lake Michigan

Abstract

In general, fish residing in rivers differ from fish residing in lakes in their mercury (Hg) isotope ratios. Specifically, fish residing in lakes typically show enriched values for the isotope ratios of δ202Hg (mass-dependent fractionation of isotope 202Hg) and Δ199Hg (mass-independent fractionation of isotope 199Hg) compared with fish residing in rivers, because photochemical effects acting on Hg isotope ratios are stronger in lakes than in rivers. Whole-fish determinations of Hg isotope ratios in age-0 and adult (ages 4–11) walleye (Sander vitreus) caught in the Fox River, the main tributary to lower Green Bay of Lake Michigan, were dissimilar. Age-0 fish exhibited a river signature for δ202Hg and Δ199Hg, with means equal to 0.00 and 0.26‰, respectively. Significantly elevated levels of δ202Hg and Δ199Hg were observed in adult fish, indicating that adult fish primarily resided in the bay. Our results implied that the Fox River serves as a nursery area for juvenile walleye in the Fox River–lower Green Bay ecosystem. Moreover, corrections for photochemical fractionation of δ202Hg revealed that age-0 and adult walleye shared the same source of Hg in this ecosystem. In addition, Hg isotope ratios did not significantly differ between the sexes of adult walleye, suggesting that these ratios did not fractionate during the Hg elimination process.

Enhanced Susceptibility of Methylmercury Bioaccumulation into Seston of the Laurentian Great Lakes

“…In the Great Lakes, the total mercury (HgT) and MeHg concentrations in water are among the lowest in the literature (100–400 pg L –1 and 1–10 pg L –1 , respectively). However, HgT concentrations in commonly consumed predator fish are 20–40 million-fold higher − than concentrations in water and are often near or above the U.S. Environmental Protection Agency (U.S. EPA)-Food and Drug Administration threshold for twice weekly consumption (equivalent to two 4 ounce meals week –1 for a pregnant 78 kg female at less than a 230 ng g –1 wet weight or 770 ng g –1 dry weight Hg concentration in consumed fish) . Additionally, Lakes Superior, Huron, and Michigan all receive primarily background-level atmospherically deposited Hg, yet they have elevated fish HgT concentrations when compared to the more anthropogenically impacted Lakes Erie and Ontario. , A comparison of Hg levels in fish relative to water indicates that one or more of the foundational processes (MeHg formation, bioaccumulation, and biomagnification) is highly efficient in the Laurentian Great Lakes, especially within Lakes Superior, Huron, and Michigan, and therefore, these lakes are more sensitive to Hg inputs than other aquatic ecosystems.…”

It’s complicated: Heterogeneous patterns of genetic structure in five fish species from a fragmented river suggest multiple processes can drive differentiation

“…These systems are generally characterized by relatively warmer temperatures, higher productivity and turbidity, and more macrophyte cover compared to the larger lake to which they are connected (Höök et al, 2007 ; Larson et al, 2013 ). Consequently, these habitats are important nursery and refuge areas for juvenile fish (Altenritter et al, 2013 ; Brazner & Beals, 1997 ; Madenjian et al, 2018 ). In our study, evidence for an ontogenetic habitat shift of young GL fish into the lower Boardman River was strongest for smallmouth bass.…”

Methylmercury Stable Isotopes: New Insights on Assessing Aquatic Food Web Bioaccumulation in Legacy Impacted Regions

“…In this study, we quantified HgT and MeHg concentrations and stable isotope values of each species to examine the bioaccumulation of Hg across a biological taxon with complex Hg-source portfolios in the lower Fox River. The Fox River was chosen as a study location because it has extensive legacy Hg contamination but relatively low fish Hg concentrations. − This disconnect makes it an ideal system for the application of MeHg and HgT isotopes because the relation between legacy sediment sources and receptor Hg burdens is unclear and warrants further examination. We hypothesized that different isotope values would be displayed across lower trophic levels (e.g., seston, benthic invertebrates, and prey fish) as a function of Hg speciation and that legacy contamination may mask other sources of Hg to different food web compartments.…”

“…Fish species were composited based on the location and total length (TL). Resident fish were targeted for this study 31 including juvenile or young-of-the-year prey fish (gizzard shad TL <70 mm, emerald shiner TL <25 mm, and yellow perch TL <50 mm). All biological samples were kept on ice in the field and stored at −20 °C prior to lyophilization and homogenization.…”