A Meta-Analysis of Mercury Biomagnification in Freshwater

Overview

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Key numbers

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When vertebrates or primary producers were included, a site’s trophic magnification factor was 18.6 ± 6.2 to 54.1 ± 7.7% higher
and wildlife population and community-level impacts.5−9 mean of 8.3 ± 7.5 (trophic magnification factors, arithmetic
Assessing these risks is often the first step to identify mean ± standard deviation).11 Yet, there is limited under-
biomagnification within and among sites limits the utility of emergent adult samples (Table S1). We also compiled a
biomagnification. Where data were available, we also assessed 206), or summary arithmetic means and standard errors where
how including vertebrate and primary producer samples necessary (n = 159). As needed, data were extracted from
functional groups to assess the influence of taxonomy on Hg percent moisture estimate of 76% when necessary.86 We
webs: biomagnification factors (BMFs) and trophic magnifi- from THg concentration using a conversion factor of 90%,87
Data Acquisition and Preparation. In January 2023, we based on medians and geometric means were 110.0 ± 2.5%
compiled relevant published studies on Hg biomagnification in and 109.2 ± 2.4% respectively of the BMFs based on
Table 1. Trophic Assignment Approachesa
concentration and continuous trophic level, which was flagged data. Effect size estimates were all within 10% of each
estimated from δ15N ratios adjusted for each site’s baseline other, all standard error ranges and 95% credible interval
following Post.90 We selected long-lived gastropods and ranges overlapped, and conclusions were not affected by the
endpoint are listed in Tables S2 and S3. publication as a random effect to minimize method-driven
benefits and drawbacks (Table 1). The “Functional” approach Wishart prior distributions.103 Models were assessed for
0.7% for MeHg and 12.6 ± 1.0% for THg respectively
(8%), mean Hg recovery of quality control samples >10% from underestimated for heterogeneous datasets using frequentist
certified values (11%), isotopic data collected after lipid methods.100,104 All endpoints were log10-transformed for
extraction or acid rinse (1%), mixing of samples from likely normality prior to modeling, and we report back-transformed
different food webs (6%), and data reporting issues like wet estimates and credible intervals. We also present geometric
weight concentrations (3%) or missing summary statistics means for log-normally distributed data and arithmetic means
(4%). We also flagged potential outliers from datasets with a for normally distributed data. All statistical analyses were
communities based on geometric summary data (solid points) and intercept estimates and 95% credible intervals from Bayesian random effects
understand how organism taxonomy affects Hg movement and invertebrates were 36.6 ± 5.0% higher than TMFs for
TMFs for freshwater communities11 and lower than TMFs were still 18.6 ± 6.2% higher than the site’s invertebrate-only
calculated with a wider range of taxa within sites. Taxonomic TMF (Figure 2a), suggesting lower MeHg biomagnification
0.3 (“Traditional” BMF; geometric mean ± standard error),
with 95% credible intervals from 1.6 to 5.3 (Figure 1 and Table
community-level TMF mean of 8.3 ± 0.7 from Lavoie et al.11
means of our invertebrate dataset, which ranged from 2.8 ± 0.2
a 41.6 ± 0.9% probability that it is below 3. Invertebrate THg
0.5,11 with geometric means from our dataset ranging from 1.3
± 0.3 to 2.6 ± 0.5 and 95% credible intervals from 1.1 to 4.1
1.0% probability that it is below 2 (posterior probability magnification factors (TMFs) for invertebrates only (TMF2 in
range (n = 13), (b) invertebrates and all vertebrates (n = 65), and (c)
freshwater food webs,11 potentially because overrepresented all taxa including primary producers (n = 25). TMFs were estimated
biomagnification estimates.26 arithmetic means and standard errors for percent differences. Each bar
Figure 3. Scatterplot and linear regressions (purple) of log mean prey methylmercury (MeHg) concentration and (a) MeHg biomagnification
vertebrates were 54.1 ± 7.7% higher than the site’s thiol-containing amino acids like cysteine and methio-
primary producers were still 27.9 ± 4.8% and 54.2 ± 7.1% appropriate biomagnification estimates in risk assessment and
(Table S3). Community-level TMFs (including vertebrates, ∼40% for each doubling of prey MeHg concentration

Methods (brief)

biomagnification within and among sites limits the utility of emergent adult samples (Table S1). We also compiled a
be more ethically, logistically, and fiscally feasible to sample focused only on data from freshwater ecosystems because of
how including vertebrate and primary producer samples necessary (n = 159). As needed, data were extracted from
biomagnification. adjusted reported sample sizes to weight composite samples
individuals in each composite sample as the sample size where
odology, we would like to define the two commonly used composite sample size ranges were reported. Where necessary
Functional BMF Functional feeding group based on taxonomy Largest site sample size Least accurate trophic-level estimate, assigns
for invertebrates92 and vertebrates93,94 samples to broad categories based solely on
Traditional BMF & δ15N data with a standard trophic More accurate continuous trophic Assumes constant Δ15N across all samples,
Integrated BMF Subset for samples where “Functional” and Most accurate Smallest site sample size
data required for each endpoint, and sample sizes for each sample size as a measure of data credibility and included
was the most qualitative of the four, simply assigning effective sample sizes over 1,000 and convergence of sampling
“Integrated” approach that removed samples with “Specialized” across all methods − within sites, frequentist and Bayesian
(8%), mean Hg recovery of quality control samples >10% from underestimated for heterogeneous datasets using frequentist
certified values (11%), isotopic data collected after lipid methods.100,104 All endpoints were log10-transformed for
extraction or acid rinse (1%), mixing of samples from likely normality prior to modeling, and we report back-transformed
This discrepancy may be due to smaller sample sizes, more across ranges of prey MeHg concentration (the BMF
brates in digestion, sulfhydryl protein content, and other predator concentrations using a constant BMF are under-

Implications

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Update history

The five most recent substantive edits to this page, classified major (evidence or structure moved), correction (a published value or statement was wrong and has been fixed), or minor (narrative rewritten without changing the underlying evidence). Each description is derived from what the edit did to this page; the linked commit is the authoritative record, routine regeneration passes are excluded, and the full version history lives in git. When DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.

Commit	Date	Change	Description
b01ec52c	2026-08-04	major	2 sections added
d49e450f	2026-08-03	major	5 sections added; narrative text revised