methylmercury burdens of

Overview

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Key numbers

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Athabasca River was the source of 62–94% of the bioaccumulated mercury in otter, fish, and tern
food web structure using carbon and nitrogen stable isotopes45. With publicly available water chemistry data for
lake) in the study area (Table 1; Fig. 2). Fish from the Athabasca River and River Otter from the PAD had similarly
low values of δ202Hg and Δ199Hg, with means ranging from − 0.85 to -0.78‰ and 0.57 to 0.68‰, respectively.
The highest average values of δ202Hg (mean ± SE = 1.00 ± 0.10‰) and Δ199Hg (4.05 ± 0.14‰) were observed in
Fig. S2, Table S1). Statistical results for pair-wise comparisons of mean Hg isotope values of biota are provided
in Supplemental Table S1.
The large range of Δ199Hg (0.05–4.99‰) among biota suggested an environmental gradient in photochemical
was 1.28 ± 0.01 (r2 = 0.99, p < 0.001, n = 344) (Supplemental Fig. S3), which is consistent with laboratory
observations of isotopic fractionation from photodemethylation of aqueous MeHg46–48. The mean (± SE) of
(Pearson r = 0.65, p < 0.001, n = 334). After correction of δ202Hg values for MDF during MeHg photodegradation
(see Methods), we found River Otter, lake fish, and river fish did not differ significantly in δ202Hgcorr (means =
had the highest mean value (-0.27‰) (Supplemental Fig. S2, Table S1). A weak positive correlation was found
between the Δ199Hg and Δ200Hg of biota (Pearson r = 0.35, p < 0.0001, n = 344), which suggested there was a
Table 1. Mercury concentrations and mercury stable isotope values of biota from the Athabasca River,
Peace-Athabasca Delta, and Lake Athabasca. Values are means ± 1 standard deviation with the minimum
Delta, and Lake Athabasca. Data points are means (± standard error) and sample sizes are provided in Table 1.
the main source of bioaccumulated mercury in River Otter (mean (95% credible interval) = 94% (85–99%)) and
Common Tern eggs (78% (73–82%)) collected from the PAD. In western Lake Athabasca, river contributions
were also the main source of mercury for Walleye (85% (71–97%)), Caspian Tern eggs (84% (80–87%)), preyfish
with consistently dominant contributions of mercury from the Athabasca River (Supplemental Table S2).
concentration during the period of 2009 to 2022, with higher mean concentrations in Caspian Tern eggs
the sample size was small (n = 4 years). Similarly, THg concentrations of Caspian and Common Tern eggs from
year (Supplemental Fig. S5) as well as the water level of Lake Athabasca (Supplemental Table S3). Together, the
δ13C, egg δ15N, and Athabasca River flow (model r2 = 0.40, p < 0.001, n = 265; Supplemental Table S4). Other
(Supplemental Fig. S5, S6), Δ199Hg ranged ~ 1.5-2‰ among eggs in any given year, which indicates variation in
was important, as indicated by carbon and nitrogen stable isotope values of eggs. Eggs with higher δ15N had more
with the modelled contribution of mercury originating from the Athabasca River. Data are means of 10 eggs
and 95% credible intervals for modelled river contributions.
concentrations in unfiltered river water ranged from 0.08 to 19.35 ng/L of THg (global mean = 3.33 ng/L,
n = 548, from 2008 to 2022) and 0.02–0.40 ng/L of MeHg (global mean = 0.09 ng/L, n = 391, from 2013 to 2022).
coincident with the period of high discharge (Supplemental Fig. S7). Mean annual concentrations of THg (2.07–
(range = 39–210 kg/year) and 3.0 ± 1.3 kg/year of MeHg (range = 1.0–5.4 kg/year). On average, more than 80%
the Athabasca River from May to August in the previous year (Pearson r = 0.77, p = 0.027, n = 8). No correlation
sediment TOC and concentrations of both THg (Pearson r = 0.79, p 0.004, n = 11 sites) and MeHg (Pearson r = 0.96,
r = -0.62, p = 0.041, n = 11 sites). The later trend reflected the rapid settling out of higher THg concentration
Higher plankton MeHg concentrations (mean ± SD = 24 ± 2 ng/g) were observed at sites < 10 km from the
Athabasca Delta, and plankton concentrations were approximately 50% lower at sites farther away (11 ± 2 ng/g)
published literature for sites across North America (Supplemental Table S5, Datafile). The Hg isotope signatures
MeHg (> 85%, Supplemental Table S6) while abiotic matrices contained primarily or entirely inorganic mercury.
rain. The mean δ202Hgcorr of river fish (corrected for MDF during photochemical processes) was − 1.0 ± 0.1‰
body have > 85% MeHg – Supplemental Table S6) and it has been experimentally demonstrated that MeHg

Methods (brief)

eggs collected from the delta or lake. A time series from 2009 to 2022 showed mercury loads from
bitumen seeps, industry samples) precluded an estimation of contributions from oil sands operations
colonial waterbird eggs. Further, eggs collected after high flow years had less MIF of Δ199Hg, thereby potentially
and maximum in parentheses. Latin names for sampled species are: a Notropis spp. bSander vitreus, cLontra
Delta, and Lake Athabasca. Data points are means (± standard error) and sample sizes are provided in Table 1.
Common Tern eggs (78% (73–82%)) collected from the PAD. In western Lake Athabasca, river contributions
the sample size was small (n = 4 years). Similarly, THg concentrations of Caspian and Common Tern eggs from
per year (sample sizes varied slightly in some years) and error bars are standard errors for THg concentration
A transect extending ~ 60 km from the Athabasca River mouth into western Lake Athabasca was sampled
from the outflow of the Athabasca River. Data points are means ± standard error of triplicate samples.
delta. Food web uptake of MeHg was examined using water column samples of plankton (biomass > 200 μm).
process samples were measured in the study area while data for leaf litter, rain, and soil were taken from the
of terrestrial samples were clustered together, namely sediment, soil, leaf litter and bitumen seeps, with negative
and industry process samples had similar isotope values to terrestrial matrices from across North America (i.e.,
Fig. 5. Isotope biplots (δ202Hg, Δ199Hg, Δ200Hg) of mercury in air, sediment and oil sands industry samples
Natural bitumen seeps sampled within the Athabasca River Basin and oil sands industry process samples had
of bitumen and industry samples were consistent with previous isotopic data generated a decade ago by Blum
signatures for terrestrial matrices and oil sands-related samples precluded an estimation of contributions from

Implications

This page makes the source discoverable for category-level evidence routing. Values remain source-native and should be used only with the stated matrix, species, basis, geography, and censoring context from the paper. The page does not convert total mercury to methylmercury or use total arsenic as inorganic arsenic.

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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