# Mercury levels in chicks of a long-lived seabird – parental effects and links with growth and survival

Justine Bertram, Coraline Bichet, Maria Moiron, Matteo Beccardi, Nathalie Kürten, Peter J Schupp, Sandra Bouwhuis

► To cite this version:

Justine Bertram, Coraline Bichet, Maria Moiron, Matteo Beccardi, Nathalie Kürten, et al., Mercury levels in chicks of a long-lived seabird – parental effects and links with growth and survival. *Environmental Research*, 2025, 285, pp.122283. 10.1016/j.envres.2025.122283. hal-05340069

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Submitted on 31 Oct 2025

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# Mercury levels in chicks of a long-lived seabird – parental effects and links with growth and survival

Justine Bertram<sup>a, *, b</sup>, Coraline Bichet<sup>b, c</sup>, Maria Moiron<sup>a, c, d</sup>, Matteo Beccardi<sup>a, d</sup>, Nathalie Kürten<sup>a</sup>, Peter J. Schupp<sup>d, e</sup>, Sandra Bouwhuis<sup>a, e</sup>

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## ARTICLE INFO

### Keywords:

Hg

THg

maternal effects

Parental effects

Bioaccumulation

Pollution

## ABSTRACT

Environmental mercury levels continue to rise, leading to increased exposure, especially in long-lived species at high trophic positions, such as many seabirds. During breeding, female seabirds can transfer mercury to their eggs, exposing their offspring to its harmful effects. After hatching, dietary exposure may further compound the risk, although excretion pathways exist as well. Despite extensive research, the relative contributions of maternal transfer, dietary exposure, and excretion to mercury dynamics in chicks, as well as its impacts on growth and survival, remain poorly understood. As such, across seven years, we artificially incubated 578 common tern (Sterna hirundo) eggs, and monitored the resulting chicks from hatching to fledging. We assessed total mercury levels in the eggshells, in chick blood and feathers, as well as in the blood of the parents. Chicks found dead were used to study mercury concentrations in brain, gut, kidney, liver and muscle. Mercury levels in organs were highest among chicks that died young, then lower among chicks that died by approximately day 24, suggesting growth dilution and excretion through feather growth, before being higher again among fully-grown chicks that died around fledging. Chick blood mercury levels predicted brain and feather mercury concentrations, but only showed marginal associations with those in other tissues. Maternal mercury levels were a key predictor of chick levels early in development, whereas paternal mercury levels were less consistently linked with those in chicks. Higher mercury levels in eggshells and chick blood were associated with reduced growth, and, in turn, survival. These findings demonstrate that early-life mercury exposure, shaped partly by maternal transfer, varies with chick development and survival. Future research should clarify the causalities linking mercury to chick growth, survival, recruitment and adult fitness to better understand its population-level consequences.

## 1. Introduction

Industrial emission and climate warming continue to increase environmental mercury levels around the globe (Stern et al., 2012; Obrist et al., 2018). Once in aquatic habitats, mercury is methylated by bacteria into its most toxic and bioavailable form, methylmercury (MeHg; Driscoll et al., 2013). In this form, mercury enters the food chain, where it biomagnifies (Atwell et al., 1998; Lavoie et al., 2013) and poses a significant threat to top predators feeding at a high trophic level, including many seabirds (Albert et al., 2024). In addition to their high dietary exposure to mercury, especially in piscivorous compared to

planktivorous species (Thébault et al., 2021), their relative longevity leads seabirds to accumulate mercury as they age (Bertram et al., 2024), which has the potential to further exacerbate the detrimental effects of this pollutant (Meyer et al., 1998; Scheuhammer et al., 2007; Goutte et al., 2014; Ackerman et al., 2024). These detrimental effects can manifest as reduced body condition (Ma et al., 2018; Amélineau et al., 2019; Eckbo et al., 2019; Adams et al., 2020; Grunst et al., 2023), elevated oxidative stress (Hoffman et al., 2011; Costantini et al., 2014; Eckbo et al., 2019; Soldatini et al., 2020), and endocrine disruption, potentially leading to altered behaviour (Kobiela et al., 2015; Tartu et al., 2015) and a reduced breeding probability (Tartu et al., 2013;

<footnote><sup>a</sup> Institute of Avian Research, An der Vogelwarte 21, 26386, Wilhelmshaven, Germany</footnote>

<footnote><sup>b</sup> Centre d'Etudes Biologiques de Chizé (CEBC), CNRS-La Rochelle Université, Villiers-en-Bois, France</footnote>

<footnote><sup>c</sup> Department of Evolutionary Biology, Bielefeld University, 33615, Bielefeld, Germany</footnote>

<footnote><sup>d</sup> Carl von Ossietzky Universität Oldenburg, Department for Chemistry and Biology of the Marine Environment, Terramare, 26382, Wilhelmshaven, Germany</footnote>

<footnote><sup>e</sup> Helmholtz Institute for Functional Marine Biodiversity at the University of Oldenburg, 26129, Oldenburg, Germany</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: justine.bertram@ifv-vogelwarte.de (J. Bertram).</footnote>
Goutte et al., 2015).

In addition to excreting mercury through feathers during molt (Whitney and Bristol, 2017), probably in equal measures for birds of both sexes (Bertram et al., 2022), female seabirds can deposit mercury in their eggs (Ackerman et al., 2016; Bertram et al., 2025). While this may reduce their own mercury burden (Bond and Diamond, 2009; Ackerman et al., 2020; Bertram et al., 2024), mercury deposited into the shells and contents of eggs (Peterson et al., 2017; Daniels-Abdulahad et al., 2024) will make its way into the developing chicks, which then hatch with non-negligible mercury levels (Ackerman and Eagles-Smith, 2009). After hatching, mercury levels in chicks may decrease due to excretion (via feather growth) or dilution (via body growth), but may also increase through dietary mercury uptake (Ackerman et al., 2011), with the net effect depending on the age-specific balance between these processes. Moreover, such changes can occur sequentially (Ackerman et al., 2011) or simultaneously (Lewis and Furness, 1991; Renedo et al., 2021) across tissues, as tissue-specific mercury levels are influenced by variation in exposure, metabolic activity, and excretion throughout development (Renedo et al., 2021). Assessing mercury levels and correlations across tissues and developmental stages therefore is needed to understand which processes are taking place, and when.

While exploring age-specific mercury levels in chicks, testing for remaining correlations with mercury levels of parents or eggshells can further elucidate the dynamics of mercury transfer and exposure. Maternal blood mercury levels during breeding are known to correlate with those in her eggs (Evers et al., 2003; Heinz et al., 2010a; Ackerman et al., 2020; Bertram et al., 2025), as well as those remaining in the shells of the eggs she laid even after those eggs hatched (Bertram et al., 2025). Since hatchlings likely retain similar mercury levels to those in the egg at laying (Ackerman and Eagles-Smith, 2009), a correlation between post-hatching mercury levels in chicks and maternal (or eggshell) mercury is anticipated (Heinz et al., 2010a). This correlation could also be expected for paternal mercury levels, particularly if parents mate assortatively with respect to traits linked to mercury levels (e.g. age or foraging behavior) or if male courtship feeding plays a significant role in mercury transfer to eggs (Ackerman et al., 2016; Bertram et al., 2025). As chicks grow, however, parental effects are likely to diminish (Vedder et al., 2023), unless mercury levels in the chicks' diet correlate with those in the diet of their parents. Assessing correlations between mercury levels in chicks and their parents, as well as eggshells, across chick development is key to better understand the nature of parental effects in the field of ecotoxicology.

Once the sources underlying variation in chick mercury levels have been assessed, studying potential consequences is a logical next step. So far, evidence for such consequences is mixed, with several studies reporting negative associations between chick growth and environmental (Longcore et al., 2007; Goutte et al., 2014; Amélineau et al., 2019) or dietary mercury levels (Heinz et al., 2010b; Santos et al., 2020), though others have found no such relationship (Goutner et al., 2001; Kenow et al., 2003; Pollet et al., 2017; Carravieri et al., 2021). Similarly, while fledging probability has been shown to decrease with increasing mercury exposure, both in experimental dosing studies (Albers et al., 2007; Heinz et al., 2010b) and in correlative field studies (Ackerman et al., 2008; Taylor and Bristol, 2015), other studies have reported no such effects (Bustamante et al., 2016; Carravieri et al., 2021). The absence of detectable effects in some of these studies, however, does not necessarily imply that such effects do not exist, as mercury is toxic even at low concentrations, but subtle (long-term) effects may be difficult to detect in field studies (Carravieri et al., 2021) especially when the age of chicks influences vulnerability and detectability. Given that chicks have been shown to exhibit the highest mercury levels both directly after hatching and post-fledging, negative effects may be most likely to occur at these stages (Ackerman et al., 2011, 2024). As such, comparing the effects of mercury on growth and survival across developmental stages might allow to pinpoint when detrimental effects do or do not manifest.

We, across seven years, conducted a comprehensive study on both

the sources and potential consequences of variation in mercury levels in common tern (Sterna hirundo) chicks. Common terns are long-lived migratory seabirds that are subject to relatively high mercury exposure due to their piscivorous diet (Becker and Ludwigs, 2004). They have been shown to bioaccumulate mercury as they grow older (Bertram et al., 2024) and to transfer mercury to their eggs (Bertram et al., 2025), such that studying mercury levels in common tern chicks is an important next step. Using blood, liver, muscle, kidney, gut, brain, and feather samples from chicks that hatched from artificially incubated eggs, we investigated (i) how sampling age of chicks affects the mercury concentration in these tissues, and, accounting for this, how tissue mercury levels correlate (ii) with each other and (iii) with those in parental blood sampled during incubation or remaining in the eggshells after hatching. Moreover, chicks, after having been returned to their nests of origin, were weighed three times weekly until fledging and their survival was monitored on a daily basis, such that we additionally tested whether mercury concentrations in eggshells, chick blood and parental blood are associated with (iv) chick growth and survival.

## 2. Methods

### 2.1. Study species and site

Common terns are long-lived, monogamous seabirds that primarily feed on fish up to 15 cm in length (Becker and Ludwigs, 2004). They exhibit assortative mating with respect to age (Bouwhuis et al., 2015) and blood mercury level (Bertram et al., 2025), and establish relatively stable pair bonds after starting their first breeding attempt at an average age of three (Zhang et al., 2015). Both partners share incubation of the one to three eggs in their clutch, as well as chick provisioning (Becker and Ludwigs, 2004), with males feeding more than females during the first week of the chicks' lives (Cansse et al., 2024).

This study focusses on common terns breeding in a mono-specific colony located at the German North Sea coast, at the Banter See in Wilhelmshaven (53°30'40'' N, 08°06'20'' E). The colony site consists of six artificial islands (10.7 × 4.6 m) covered with gravel and surrounded by 60 cm high walls. Since 1992, this colony is the focus of a long-term individual-based project, in which all locally fledged chicks are marked not only with rings, but also with subcutaneous transponders that allow their automatic and remote detection at the colony using an antenna system (Becker and Wendeln, 1997). Each nest and chick is monitored three times a week, although for the present study dead chicks were collected from the islands on a daily basis. The sex of chicks is determined molecularly following Becker and Wink (2003).

### 2.2. Data collection

Between 2017 and 2023, we artificially incubated 704 eggs of 271 clutches produced by 116 male and 102 female common terns of known identity, of which 578 eggs of 256 clutches produced by 108 males and 99 females hatched successfully (Table S1). Artificial incubation prevented both external environmental exposure of the eggshell after laying and potential effects of parental blood mercury levels on incubation behavior and, consequently, embryonic development. While altered incubation temperature has been shown to affect incubation duration, it does not appear to influence chick condition at hatching (Vedder et al., 2018), however, such that we do not expect parental mercury levels to be a major source of variation in embryonic growth or survival. After hatching, chicks were ringed, weighed, and returned to their original nest, where they were readily accepted by their parents (Vedder et al., 2017a). The eggshells, that could be linked to the chick because hatching took place in individual compartments, were collected from the incubator and stored in plastic cups marked with the nest number and laying order of each chick. Chicks were weighed three times weekly (Mondays, Wednesdays, and Fridays), as well as blood sampled at the age of ten and twenty days by pricking their brachial vein. Blood
samples were frozen at -18 °C. On day twenty, we also collected body feather samples from the back of the chicks' neck and stored them in plastic bags marked with the ring number. Between 2019 and 2023, the colony was checked daily for the presence of dead chicks, which were frozen as well.

Parents of known identity were blood sampled using the method described by Becker et al. (2006). Briefly, we inserted a Mexican kissing bug (*Dipetalogaster maxima*) in an artificial hollow egg, which was then placed in the nest of a target bird when that bird was observed to be incubating by a mobile antenna placed around the nest. The bug was allowed 20-30 min to feed on the bird's blood through small holes along the egg. After extracting up to 0.5 ml of the bird's blood from the bug's abdomen with a syringe, it was stored at -18 °C. Mercury concentrations measured in blood samples obtained using this method strongly correlated with those measured in blood samples collected by pricking the brachial vein of the same bird (r = 0.93, 95 % CI [0.83-0.98]), validating the use of bugs for blood sampling (see Bertram et al., 2024 for details).

## 2.3. Mercury analysis

Before mercury analysis, eggshell samples were dried in an incubator for 24 h at 50 °C and ground to a fine powder. Feather samples were washed three times in alternately demineralized water and acetone and cleaned in the respective liquid for 15 min in an ultrasonic bath (Sonorex Super RK510H, Bandelin, Germany). Dead chicks were defrosted, and dissected to collect the liver, pectoral muscle, kidney, gut and brain. Contents of gut samples were removed and the guts thoroughly washed to avoid contamination by feces. While for eggshells and feathers we determined the dry weight (dw) mercury concentration, organ and (both adult and chick) blood samples were not dried prior to mercury analyses, such that we determined their wet weight (ww) mercury concentration.

Although MeHg is the toxicologically relevant form of mercury most directly associated with bioaccumulation and ecological risk, we were only able to measure total mercury (THg). However, THg is a reliable proxy for MeHg levels (Ackerman et al., 2013; Renedo et al., 2017, 2021; Rutkowska et al., 2019) and additionally is most widely used for analysis, therefore allowing us to compare our results to those obtained in previous studies. Concentrations of THg were measured using a Direct Mercury Analyzer (DMA-80, MLS Mikrowellen-Labor-Systeme GmbH, Germany). For this, 30 mg of either eggshell powder or organ sample, 20 µl whole blood, or 1-2 feathers, respectively, were inserted into a nickel sample boat and weighed. In the DMA-80, the respective sample was dried and thermally decomposed in an oxygen stream at 750 °C, combusting all compounds except mercury, which was captured via gold amalgamation. The trapped mercury was then released by heating to 850 °C and quantified using atomic absorption spectrometry at 253.7 nm. Analytical blanks were included in each measurement batch to ensure the device remained contamination-free. Accuracy was verified using certified reference materials (DORM-3 or DORM-4; fish protein from the National Research Council of Canada, with certified mercury concentrations of 382 or 412 µg·kg⁻¹ dry weight), yielding mean recoveries (±SD) of 104 ± 4 % and 106 ± 6 %, respectively. The laboratory's QA/QC procedures also included regular participation in the QUASIMEME international performance study program, consistently achieving satisfactory results. To further validate that our measurement method was reliable, we conducted double blind measurement of all wet samples. Doing so, we obtained very strong correlations between two measurements for the same blood samples (r = 0.88, 95 % CI [0.81-0.92]; Bertram et al., 2024), as well as chick organ samples (r = 0.98, 95 % CI [0.98-0.99]; Fig. S1) with a relative difference (±SE) of 8.7 (±0.5) %. Eggshell samples also were measured twice, and these measurements were strongly correlated too (r = 0.98, 95 % CI [0.96-0.97]; Bertram et al., 2025), while feather samples were measured three times, with these three measurements displaying a high repeatability (R = 0.92; 95 % CI [0.91-0.92]).

## 2.4. Statistical analyses

All analyses were performed, and figures created, using R (version 4.3.3, R Core Team, 2023). Models were implemented using the R packages "MCMCglmm" (Hadfield, 2010), "lme4" (Bates et al., 2015) and "mgcv" (Wood, 2023). P-values for linear mixed models were obtained using "lmerTest" (Kuznetsova et al., 2017), with the significance level set at p < 0.05. Data were visualized using "ggplot2" (Wickham, 2016) and "interactions" (Long, 2022). For all MCMCglmm models, we used inverse gamma priors and specified iterations and thinning intervals to achieve a minimum effective sample size of 1000. Mercury data were log-transformed and assumed to follow a Gaussian error distribution.

All models included the combination of brood size and hatching order ('brood size hatching order') as a six-level categorical variable. As random effects, we included 'year' to capture potential inter-annual variation in THg concentrations, nest identity to take into account the non-independence of chicks from the same clutch, and either 'maternal' or 'paternal identity' to account for repeated sampling of 60 fathers and 55 mothers across 2-6 years.

### 2.4.1. Age effects

To assess the age-specificity of a chick's organ THg concentration, using the subset of 199 chicks found dead in the colony, we used a Bayesian multivariate model (MCMCglmm; Hadfield, 2010) with the 'THg concentration' of the liver, pectoral muscle, kidney, gut, and brain as dependent variables, and both a linear and quadratic term for the chicks' age at death ('death age') as covariates. Given the statistical significance of the quadratic age term (see Results), we then estimated the age at which the minimum THg was observed, as well as the slope of the pre- and post-minimum age effects for each organ. To do so, we first repeated our analysis using five univariate linear mixed models (LMM, one for each organ), and used the model estimates and standard errors to determine the age (±SE) at which THg was at its minimum, also calculating the corresponding THg concentrations. Then, to estimate the slope of the pre- and post-minimum age effects, we reran the univariate models for each organ, but replaced the quadratic age term with two new fixed effects: (a) a binary 'pre-peak' variable (0 for post-peak, 1 for pre-peak), and (b) the interaction between the linear 'death age' and the 'pre-peak' variable. The main effect of age in this model quantifies the post-peak age effect, while the interaction between 'death age' and the 'pre-peak' quantifies the pre-peak effect as a deviation from the post-peak effect. The sum of both terms therefore provides the total pre-peak effect (Bouwhuis et al., 2009; Dingemanse et al., 2020).

### 2.4.2. Correlations among THg concentrations of chick tissues

We first assessed the correlation between organ THg concentrations by calculating Pearson correlation coefficients. To investigate the correlation between THg concentrations in chick blood and organs, we, for the subset of 93 chicks that died at an age older than 10 days, ran a second multivariate model, including the five organ-specific 'THg concentrations' as dependent variables (as above), and the THg concentration in chick blood ('chick blood THg day 10') as an explanatory variable. For the 59 chicks that died between day 10 and day 19, blood THg at day 10 was used, while for the 34 chicks that died at day 20 or later, we used blood THg at day 20 as an explanatory variable. We also included the 'interval between blood sampling and death' (in days) as a covariate to account for changes in organ THg with chick age. To investigate how THg concentrations in chick blood and feathers were correlated, we ran three LMMs. In the first model, we included 'chick blood THg day 20' as the dependent variable and 'chick blood THg day 10' as a covariate. Next, we ran a second LMM with 'feather THg' concentration as the dependent variable and 'chick blood THg day 20' as a covariate, while in the third one we additionally included 'chick blood THg day 10' as a covariate.
### 2.4.3. Parental and eggshell effects

To analyze how parental blood THg concentrations correlate with THg concentrations in chick organ samples, we ran two more multivariate models, one for each parental sex, with the linear and quadratic effects of the chicks' age at death as covariates, as above, but this time in interaction with maternal or paternal 'blood THg concentrations'. Since the interactions were not significant for fathers, we repeated the model without them to test the main paternal effect.

To also assess whether THg levels in chick organs correlate with those remaining in their eggshells after they had hatched, we employed two multivariate models. The first model included 'THg concentrations' in each organ as dependent variables, with the linear and quadratic effects of chicks' death age as covariates interacting with 'eggshell THg'. As the interaction effects were not significant, we excluded them in the second model.

To assess how THg concentrations in the remaining chick tissues correlate with parental THg concentrations, we ran six LMMs, three for each parental sex, with either 'chick blood THg day 10', 'chick blood THg day 20', or 'feather THg' as a dependent variable and 'parental blood THg' as a covariate. Additionally, to evaluate whether eggshell THg concentrations affect chick mercury levels, we ran three more LMMs using 'chick blood THg day 10', 'chick blood THg day 20', or 'feather THg' as a dependent variable and 'eggshell THg' as a covariate.

### 2.4.4. Consequences for chick growth and survival

Between 2017 and 2023, we collected 4128 mass measurements of 492 chicks. To investigate potential effects of mercury on chick growth, we used four generalized additive mixed models (GAMMs) with a normal error distribution, each incorporating the THg concentrations of a different tissue as the primary explanatory variable: (i) eggshell THg, (ii) chick blood THg at day 10, (iii) maternal blood THg, or (iv) paternal blood THg. Each model included a smooth term for the variable 'day' of measurement to capture nonlinear growth patterns over time, as well as a smooth term for the respective 'THg concentration' to account for its possible nonlinear influence on chick mass. Additionally, an interaction between 'day' and 'THg concentration' was included to explore whether the relationship between chick growth and THg levels changed across the growth period. We included 'chick identity' as a random effect. Models were fitted using maximum likelihood (ML) to enable effective model comparison.

To assess whether chick THg levels predict chick survival, we fitted two generalized linear mixed models (GLMMs) with a binomial error structure. The dependent variable was 'survival' (coded as '1' for chicks that survived and '0' for those that died). In the first model, we focused on survival until day 10, a critical early developmental stage identified in previous studies (Vedder et al., 2017b), and included 'eggshell THg concentration' as a covariate to capture early-life chick THg levels. In the second model, we included 'chick blood THg day 10' as a covariate to extend the analysis to cover survival between day 10 and fledging. To determine whether potential effects on chick survival stem from direct mercury toxicity or are mediated through impacts on chick development, we reran both models including either the chick mass at hatching ('chick hatch mass') or at day 10 ('chick mass day 10') as covariates, respectively.

## 3. Results

We successfully measured total mercury (THg) concentrations in 575 of 578 eggshells, as well as in at least one wet tissue (i.e. blood or organs) of 485, and feathers of 269, of the chicks resulting from these eggs. THg concentrations varied considerably across tissues, with the highest mean THg concentration measured in feathers, followed by liver, kidney, muscle, gut, brain, blood, and eggshells (Table 1). The hatching order of the chicks within their brood did not explain a significant amount of variation in their THg concentrations in any tissue, but inter-annual fluctuations were strong (Fig. S2, Table S2).

<caption>**Table 1**</caption>

Mean total mercury (THg) concentrations ± standard errors (SE) in µg·kg⁻¹, as well as minima and maxima and sample size, for common tern eggshells (in dw), chick organs (in ww), chick and parental blood samples (in ww) and chick feathers (in dw). Sample sizes vary among organs, because not all organs could be collected from every chick, particularly when chicks were very small or not found very soon after dying.

<table><thead><tr><th></th><th></th><th>mean THg</th><th>SE</th><th>min THg</th><th>max THg</th><th>n</th></tr></thead><tbody><tr><td>eggshells</td><td></td><td>19.1</td><td>2.4</td><td>1.8</td><td>911.9</td><td>575</td></tr><tr><td rowspan="5">chick organs</td><td>liver</td><td>653.7</td><td>33.1</td><td>139.4</td><td>2464.5</td><td>200</td></tr><tr><td>muscle</td><td>394.6</td><td>25.1</td><td>43.8</td><td>2080.6</td><td>198</td></tr><tr><td>kidney</td><td>455.7</td><td>24.4</td><td>110.5</td><td>1888.9</td><td>175</td></tr><tr><td>gut</td><td>290.2</td><td>17.3</td><td>54.9</td><td>1359.4</td><td>188</td></tr><tr><td>brain</td><td>210.8</td><td>11.2</td><td>43.7</td><td>850.1</td><td>199</td></tr><tr><td rowspan="2">chick blood</td><td>day 10</td><td>74.4</td><td>3.4</td><td>11.5</td><td>378.6</td><td>328</td></tr><tr><td>day 20</td><td>64.3</td><td>2.8</td><td>15.3</td><td>180.7</td><td>265</td></tr><tr><td>chick feathers</td><td></td><td>2255.8</td><td>30.7</td><td>1290.8</td><td>3988.9</td><td>262</td></tr><tr><td rowspan="2">parental blood</td><td>maternal</td><td>782.9</td><td>11.9</td><td>288.1</td><td>1490.7</td><td>193</td></tr><tr><td>paternal</td><td>1122.4</td><td>18.6</td><td>262.7</td><td>3895.8</td><td>215</td></tr></tbody></table>

### 3.1. Age effects

Variation in THg concentrations of organ samples was explained by the quadratic term for the age at which the chicks had died (Fig. 1, Table S3 (a)). Organ THg concentrations first decreased until c. day 24, after which it increased again. The liver reached its minimum earliest (c. day 21), the muscle latest (c. day 25), and the different models testing for the rates of pre-peak decreases and post-peak increases in THg concentrations also showed some slight differences among organs, with the liver exhibiting the slowest de- and fastest increase and muscle the fastest de- and slowest increase (Table S4).

<figure><img src="image_4.png" /></figure>

**Fig. 1.** Total mercury (THg) concentration (in log, µg·kg⁻¹) in common tern chick organs in relation to the age at which the chick died. Points represent the year-normalized data for the liver (light-blue), muscle (red), kidney (dark-blue), gut (green), and brain (yellow) sample of individual chicks. Solid lines depict the relationships between age at death and THg concentrations for each tissue, while the shaded areas represent the 95 % confidence intervals around the predicted relationships. The vertical dashed line represents the mean inflection point of all organs combined (24 days). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
## 3.2. Correlation among THg concentrations of chick tissues

THg concentrations in organ samples (Fig. S3) were highly correlated (ranging between r = 0.87–0.95). While higher blood THg concentrations were associated with higher THg concentrations in the brain, independent of the interval between sampling events, only marginal associations were detected with those in the liver, muscle, kidney, or gut (Fig. 2, Table S3 (b)). Blood THg concentrations measured at day 10 and day 20 post-hatching showed a weak positive correlation (β = 0.144, p = 0.040; Fig. 3a, Table S5 (a)). Chicks with higher THg concentrations in their blood at both day 10 and day 20 also had higher THg concentrations in their back feathers at day 20 (Table S5 (b)), although the significant correlation between feather and blood THg at day 20 disappeared when both blood THg values (at day 10 and day 20) were included simultaneously in the model (Fig. 3b–Table S5 (c)). This indicates that the relationship between feather and blood THg at day 20 may be largely attributable to the underlying correlation between blood THg concentrations at day 10 and day 20.

## 3.3. Parental and eggshell effects

THg concentrations in the liver, muscle, kidney, and gut, but not brain, showed significant positive correlations with maternal blood THg concentrations, but these correlations were reduced among chicks that died late during development (Fig. 4, Table S3 (c)). In contrast, paternal blood THg concentrations did not significantly correlate with THg concentrations in chick organs at any chick age (Fig. 4, Table S3 (d) and (e)). THg concentration in eggshells, however, significantly correlated with those in chick organs independent of chick age (Fig. 4, Table S3 (f) and (g)).

THg concentrations in maternal blood (β = 0.199, p = 0.052), but not paternal blood (p = 0.147) or eggshell (p = 0.773), showed a trend to be

associated with blood THg concentrations in chicks 10 days post-hatching (Fig. 5, Table S6 (a)), while no such trend was observed at day 20 (p<sub>mat</sub> = 0.427; p<sub>pat</sub> = 0.847; p<sub>egg</sub> = 0.114; Fig. 5, Table S6 (b)). Blood THg concentrations of both parents as well as the eggshells were, however, strongly correlated with THg concentrations in the back feathers of the corresponding chicks (β<sub>mat</sub> = 0.178, p<sub>mat</sub> < 0.001; β<sub>pat</sub> = 0.118, p<sub>pat</sub> = 0.004; β<sub>egg</sub> = 0.040, p<sub>egg</sub> < 0.001; Fig. 5, Table S6 (c)).

## 3.4. Consequences for chick growth and survival

Our generalized additive mixed models revealed that chick mass gain was time-dependently influenced by THg concentrations in the shells of the eggs from which they hatched, with chicks that hatch from eggs with higher shell THg growing slower in the first days after hatching, but catching up to reach a similar body mass at fledging (Fig. 6, Table 2 (a)). Additionally, we found chicks with higher THg concentrations in their blood at day 10 to exhibit slower growth throughout the pre-fledging period (Fig. 6, Table 2 (b)), although they too caught up at very late ages. While maternal THg concentrations were not associated with the mass gain of their chicks (Fig. 6, Table 2 (c)), high paternal blood THg concentrations were associated with heavier chicks late in the chicks' development (Fig. 6, Table 2 (d)).

Chicks that did not survive until day 10 had higher THg concentrations in their eggshells compared to chicks that did (β = −0.355, p = 0.028; Fig. 7a, Table S7), and chicks that did not fledge had higher THg concentrations in their blood at day 10 (β = −1.276, p < 0.001; Fig. 7b–Table S7). Because these effects disappeared when adding hatch mass (β = −0.211, p = 0.200; Table S7) or mass at day 10 (β = −0.504, p = 0.275; Table S7) to the models, respectively, the negative associations between eggshell and chick blood THg and survival are unlikely to be causal, rather to be underpinned by reduced growth being associated with reduced survival.

# 4. Discussion

Mercury contamination in seabird chicks can be shaped by a combination of maternal transfer, dietary uptake and developmental processes and may impair chick health. In this study, we measured mercury levels across multiple tissues and developmental stages in common tern chicks, and linked those to mercury levels in blood of their parents over a period of seven years. Mercury levels in chicks varied substantially within and among tissue types, exhibited significant annual fluctuations (Fig. S2, Table S2), and were strongly influenced by the chicks' age at sampling. The mercury levels in eggshells and in the blood of both parents were also correlated with those of the chicks, although these correlations varied among tissue types and parental effects diminished as chicks grew older. Additionally, we found evidence that mercury concentrations were associated with mass development and survival.

## 4.1. Age effects

Variation in mercury concentrations across chick organs was associated with the chicks' age at death. Chicks that died shortly after hatching exhibited the highest mercury concentrations, while concentrations were lower in chicks that died until approximately day 24 (the average inflection point across tissues), before being higher again in those chicks that died at older ages. This finding aligns with that of Ackerman et al. (2011), who reported similar age-specificity of organ THg concentrations in Förster's tern (*Sterna forsteri*) chicks, although the inflection point occurred a little earlier in this species, at day 20. This quadratic pattern is likely driven by multiple factors. While elevated mercury levels in young chicks could be attributable to maternal transfer of mercury to eggs (Ackerman and Eagles-Smith, 2009; Bertram et al., 2025), the subsequent decline, if underpinned by a within-individual process, may result from a combination of (i) excretion through feather development and (ii) dilution through rapid growth (Ackerman

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Year-normalized total mercury (THg) concentrations (in log, µg·kg⁻¹) in common tern chick organs in relation to the THg concentration in blood sampled closest to death (at either day 10 or day 20). Points represent the partial residual of year-normalized THg concentrations in liver (light-blue), muscle (red), kidney (dark-blue), gut (green), and brain (yellow) samples, solid lines the model predictions of the respective organs and shaded areas the 95 % confidence intervals around the predicted relationships. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 3. Correlations between common tern chick total mercury (THg) concentrations (in log, µg·kg⁻¹) (a) in blood 10 and 20 days after hatching, and (b) in feathers and blood at day 10. Points represent the partial residual of year-normalized THg concentrations in samples, solid lines the model predictions and shaded areas the 95 % confidence intervals around the predicted relationships.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Correlations between common tern chick organ total mercury (THg) concentrations (in log, µg·kg⁻¹) and maternal blood (left), paternal blood (middle), and eggshell (right) THg concentrations. Points represent the partial residual of year-normalized THg concentrations in five different organ samples, lines the fitted relationships for chicks sampled at a dead at the lower (3 days), middle (11 days) and upper (21 days) age tercile median (from light to dark red, respectively).</figcaption></figure>

et al., 2011) during a period when chicks predominantly consume small fish (mostly herring; Cansse et al., 2024) that presumably have only very low mercury levels (e.g. Storelli et al., 2007; Kammann et al., 2023). Once chick development nears completion, which is typically around day 26–28 in the common terns of our colony (Becker and Ludwigs, 2004), these opportunities for mercury reduction diminish. At the same time, dietary changes to larger fish, comparable to those consumed by adults (Stienen et al., 2000; Morris, 2013) and potentially higher in mercury, could drive the observed late-stage increase.

Although mercury concentrations in all organs displayed a similar pattern of age-specificity, the estimated inflection points and rates of decrease and subsequent increase differed slightly between the traits. Mercury concentrations in muscles decreased most strongly, likely due to dilution during their rapid growth (Lewis and Furness, 1991), and the potential redistribution of mercury stored in muscles during feather development (Renedo et al., 2021). This decline continued until an inflection point at approximately 25 days, the latest among all organs analyzed, after which muscle mercury concentrations showed the

slowest rate of increase. In contrast, mercury concentrations in liver and kidney samples decreased less strongly, with inflection points at day 21 and day 22, respectively, the earliest among all organs. This likely is connected to the role these organs play for mercury detoxification through demethylation: they convert MeHg into, and store it as, inorganic mercury (Kim et al., 1996; Eagles-Smith et al., 2009; Renedo et al., 2021).

## 4.2. *Correlations among THg concentrations of chick tissues*

Inter-organ correlations in mercury levels were high, but, contrary to a previous study on common loon chicks (Gavia immer; Kenow et al., 2007), mercury levels in blood and organs of common tern chicks were only marginally correlated, with only the relationship between those in blood and the brain being statistically significant. One plausible explanation for this discrepancy is that with our mercury measurement method we were only able to quantify total mercury concentrations rather than distinguishing between mercury species. Since mercury in
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Correlations of maternal (grey) and paternal blood (black) as well as eggshell (brown) total mercury (THg) concentrations (in log, µg·kg⁻¹) do not exist with THg concentrations in blood of common tern chicks at day 10 (left) or 20 (middle), but do with those in chick back feathers (right). Points represent the partial residual of year-normalized THg concentrations in samples, lines the model predictions and shaded areas the 95 % confidence intervals around the predicted relationships. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Relationships between total mercury (THg) concentrations (in log, µg·kg⁻¹) in eggshells (upper left), chick blood at day 10 (upper right), maternal (bottom left) and paternal blood (bottom right) and the mass gain of common tern chicks throughout their pre-fledging phase. For visual purposes the graphs represent the lower (blue), middle (red) and upper (orange) tercile median of the respective THg concentrations, but all results from analyses presented in the text are obtained using THg concentrations as continuous variables. Chicks that died before day 40 contributed data up to their last recorded measurement. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<caption>**Table 2**</caption>

Results from four generalized additive models, testing whether total mercury (THg) concentration measured in (a) eggshells, (b) chick blood at day 10, (c) maternal or (d) paternal blood are correlated with the individual mass gain of 492 common tern chicks. Parametric coefficients are reported as mean estimates with associated standard errors (SE), and edf (effective degrees of freedom) represents the complexity of a smooth term. Terms in bold are significant (p-value <0.05).

<table><thead><tr><th rowspan="2">parametric coefficients</th><th colspan="3">(a) eggshell THg</th><th colspan="3">(b) chick blood THg day 10</th><th colspan="3">(c) maternal blood THg</th><th colspan="3">(d) paternal blood THg</th></tr><tr><th>estimate</th><th>SE</th><th>p-value</th><th>estimate</th><th>SE</th><th>p-value</th><th>estimate</th><th>SE</th><th>p-value</th><th>estimate</th><th>SE</th><th>p-value</th></tr></thead><tbody><tr><td>intercept</td><td>76.069</td><td>3.111</td><td>&lt;0.001</td><td>83.352</td><td>3.654</td><td>&lt;0.001</td><td>76.598</td><td>3.028</td><td>&lt;0.001</td><td>76.013</td><td>2.842</td><td>&lt;0.001</td></tr><tr><td>brood size_hatching order (reference 1_1)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>2_1</td><td>-1.509</td><td>1.948</td><td>0.439</td><td>-1.661</td><td>2.113</td><td>0.432</td><td>-2.086</td><td>1.948</td><td>0.284</td><td>-0.499</td><td>1.818</td><td>0.784</td></tr><tr><td>2_2</td><td>-3.025</td><td>1.991</td><td>0.129</td><td>-4.408</td><td>2.231</td><td><strong>0.048</strong></td><td>-3.607</td><td>1.991</td><td>0.07</td><td>-2.671</td><td>1.869</td><td>0.153</td></tr><tr><td>3_1</td><td>-1.648</td><td>1.926</td><td>0.392</td><td>-2.196</td><td>2.100</td><td>0.296</td><td>2.106</td><td>1.933</td><td>0.276</td><td>0.405</td><td>1.770</td><td>0.819</td></tr><tr><td>3_2</td><td>-2.234</td><td>1.956</td><td>0.253</td><td>-4.021</td><td>2.163</td><td>0.063</td><td>2.482</td><td>1.964</td><td>0.206</td><td>-1.164</td><td>1.804</td><td>0.519</td></tr><tr><td>3_3</td><td>-8.148</td><td>2.021</td><td>&lt;0.001</td><td>-8.304</td><td>2.330</td><td>&lt;0.001</td><td>-8.615</td><td>2.032</td><td>&lt;0.001</td><td>-6.964</td><td>1.869</td><td>&lt;0.001</td></tr><tr><td>smooth terms</td><td>edf</td><td></td><td>p-value</td><td>edf</td><td></td><td>p-value</td><td>edf</td><td></td><td>p-value</td><td>edf</td><td></td><td>p-value</td></tr><tr><td>day</td><td>8.104</td><td></td><td>&lt;0.001</td><td>8.018</td><td></td><td>&lt;0.001</td><td>8.131</td><td></td><td>&lt;0.001</td><td>8.385</td><td></td><td>&lt;0.001</td></tr><tr><td>THg</td><td>1.001</td><td></td><td>0.204</td><td>4.487</td><td></td><td>&lt;0.001</td><td>1.000</td><td></td><td>0.517</td><td>2.054</td><td></td><td>0.167</td></tr><tr><td>day*THg</td><td>7.251</td><td></td><td>&lt;0.001</td><td>12.689</td><td></td><td>&lt;0.001</td><td>3.276</td><td></td><td>0.197</td><td>6.676</td><td></td><td>&lt;0.001</td></tr><tr><td>random effects</td><td>estimate</td><td>SE</td><td>p-value</td><td>estimate</td><td>SE</td><td>p-value</td><td>estimate</td><td>SE</td><td>p-value</td><td>estimate</td><td>SE</td><td>p-value</td></tr><tr><td>nest identity</td><td>7.825</td><td>0.215</td><td>0.426</td><td>44.088</td><td>8.228</td><td><strong>0.008</strong></td><td>7.594</td><td>0.216</td><td>0.494</td><td>36.422</td><td>3.302</td><td>0.064</td></tr><tr><td>year</td><td>5.793</td><td>1661.364</td><td>&lt;0.001</td><td>5.644</td><td>1455.952</td><td>&lt;0.001</td><td>5.777</td><td>1500.309</td><td>&lt;0.001</td><td>5.780</td><td>1667.817</td><td>&lt;0.001</td></tr><tr><td>parental identity</td><td>0.067</td><td>0.001</td><td>0.417</td><td>3.807</td><td>0.558</td><td>0.252</td><td>0.009</td><td>0.000</td><td>0.540</td><td>0.069</td><td>0.001</td><td>0.345</td></tr><tr><td>chick identity</td><td>278.803</td><td>7.962</td><td>&lt;0.001</td><td>164.150</td><td>11.517</td><td>&lt;0.001</td><td>274.5</td><td>8.063</td><td>&lt;0.001</td><td>284.885</td><td>11.429</td><td>&lt;0.001</td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Total mercury (THg) concentrations (in log, µg·kg⁻¹) (a) in eggshells of chicks that did and did not survive until day 10 and (b) in blood at day 10 of chicks that did and did not fledge. The width of the violin plots represents the raw data distribution, while boxplots show the median (horizontal black line within the boxes) and the first and third quartiles. Whiskers represent the minimum (first quartile – 1.5 × inter-quartile range) and maximum (third quartile + 1.5 × interquartile range) concentrations and outliers are represented by black dots. We year-normalized the data for visualization purposes only.</figcaption></figure>

blood comprises almost exclusively MeHg (Renedo et al., 2021), correlations between blood and other tissues would be expected only if the MeHg fraction in those tissues remains stable over time (Kenow et al., 2007). However, due to detoxification and storage functions of the liver, kidney and muscle (Renedo et al., 2021), large parts of the mercury we measured in those organs might occur in its demethylated form. Consequently, mercury levels in these organs may potentially be less affected by, and/or lag behind by MeHg circulating in the blood (Kenow et al., 2007), due to inter-organ variation in mercury turnover rates (Renedo et al., 2021). The significant correlation between mercury in blood and brain is then also logical, given that mercury in the brain is largely found to be bound as MeHg to a cysteine complex (Clarkson and Magos, 2006), in which it can readily cross the blood-brain-barrier (Aschner and Aschner, 1990; Yin et al., 2008). This, however, raises concerns regarding the toxicological impact of mercury, for example on its capacity to induce oxidative stress (Hoffman et al., 2011; Costantini

et al., 2014; Eckbo et al., 2019; Soldatini et al., 2020) or interfer with endocrine function (Tartu et al., 2013, 2015; Goutte et al., 2015; Kobiela et al., 2015), as it has been demonstrated in multiple avian species (reviewed in Bottini and MacDougall-Shackleton, 2023). Importantly, since MeHg is the biologically active and toxicologically most relevant form, the use of THg data limits our ability to infer the extent of bio-accumulation and potential toxicity in tissues where demethylation is likely to occur. Future studies should therefore further investigate potential correlations between mercury species in these tissues to refine our understanding of tissue-specific mercury dynamics and their consequences.

Blood mercury levels at day 10 and day 20 after hatching were only weakly correlated. To investigate how this may relate to among-individual variation in growth between these two time points, we ran an additional model testing how the proportional mass gain between day 10 and day 20 related to the proportional change in blood THg
concentration during this same time (Table S8). Consistent with the findings of Ackerman et al. (2011), this model showed that greater growth was associated with smaller increases in mercury concentrations in the blood (Fig. S4). Although this may at first seem counterintuitive, because more growth would result from more food intake and therefore more mercury intake, this suggests growth dilution to compensate for such an effect. Moreover, when including chick growth between day 10 and day 20 as a covariate in the model assessing the correlation between mercury levels at these time points, this correlation increased by approximately 5%–19% and became statistically significant (p = 0.011).

Mercury levels in blood at both day 10 and day 20 were strongly associated with mercury levels in the back feathers of chicks, with the effect of blood mercury levels at day 10 being notably stronger. This stronger association between blood mercury at day 10 and feather mercury makes sense, given that chicks are in their main period of growing feathers and thus depositing mercury into those around day 10. At day 20, body feather development is largely completed (Nisbet and Drury, 1972) and the connection between back feathers and the blood stream is largely atrophied (Appelquist et al., 1984).

### 4.3. Parental and eggshell effects

In addition to the correlations among mercury concentrations across tissues within chicks, we also found significant correlations between mercury levels of chicks and their parents, with these correlations being strongly influenced by the tissue type and the chicks' age at sampling. When chicks died young, mercury concentrations in their liver, muscle, kidney, and gut were significantly correlated with maternal blood mercury levels, which is most likely attributable to maternal mercury transfer to eggs (Ackerman and Eagles-Smith, 2009). This maternal effect was weaker in chicks that died later, which mirrors recent findings of maternal effects on offspring body mass declining during chick development as well (Vedder et al., 2023), and makes sense once parents start feeding their offspring with prey items that would be much smaller than the ones they would have eaten themselves (Cansse et al., 2024). The absence of a correlation between maternal mercury concentrations and those in the chick brain also makes sense in the light of our previously discussed finding, that mercury levels in the chicks' brain correlate with those in chick blood. This does, however, mean that this correlation comes from the part of the chick blood mercury level that is not correlated with that of its mother, suggesting it may be more reflective of recent mercury uptake through the chick's diet.

While maternal mercury levels significantly correlated with those in most of the chick organs, no such correlation was found for paternal mercury levels. At first sight, this seems intriguing, given that our previous work showed that mercury levels in both parents correlate with each other and with those in the contents of their eggs (Bertram et al., 2025) and because fathers take the largest share of provisioning the chicks in the first week after hatching (Cansse et al., 2024). However, we also found that mercury levels in the shells of hatched eggs correlated only with those in the blood of the mother, not the father (Bertram et al., 2025). This suggests that although mercury in the content of the eggs is derived from recently ingested food, as expected for an income breeder, mercury found in the eggshells might primarily originate from maternal stores rather than from that recently taken up from the diet. If, in turn, mercury found in the chicks' organs originates mostly from that they extract from the shells during embryonic development, the pattern starts making sense. Our finding that remaining eggshell mercury levels correlated both with maternal mercury levels (Bertram et al., 2025) and that in the chicks' organs further supports this idea.

The positive correlation between mercury levels in chick feathers and those in the blood of their parents, as well as their eggshells suggests that chicks might excrete mercury received from both the egg content and the shell into their feathers. However, further work is needed to understand the specific physiological mechanisms and temporal

dynamics of these patterns. Using a single-dosing-of-mothers experimental approach and comparing how mercury levels in eggshells and egg contents decrease as these mothers produce multiple eggs, for example, could help clarify when and how mercury is incorporated into different egg components. Moreover, male and female common terns of the study population are known to forage in different habitats, at least during incubation (Militão et al., 2023), and to exhibit distinct provisioning behavior during chick rearing (Cansse et al., 2024), which is likely to affect the mercury exposure of chicks. At present, however, we can only address overall parental effects on chick mercury levels, but are not yet able to link chick mercury concentrations to the specific parts of their diets obtained from their individual parents. Future work should be focused on this.

### 4.4. Consequences for chick growth and survival

Our analyses of the effect of mercury on growth showed that higher mercury levels in the eggshells were associated with a slower growth in younger chicks, although this effect diminished in older chicks. Paternal blood mercury levels additionally correlated with the mass of older chicks, with fathers exhibiting higher mercury levels tending to have heavier chicks, potentially due to their ability to forage on larger, more contaminated fish, which not only elevates their own mercury levels but also contributes to increased chick mass through provisioning. Maternal blood mercury levels, however, were not associated with chick mass growth at any life stage, potentially due to their reduced involvement in feeding (Cansse et al., 2024). Blood mercury levels in chicks measured at day 10, however, were negatively associated with mass gain throughout development, with chicks exhibiting slower growth when mercury levels were higher. This may either result from deficient parental provisioning leading to a reduced growth preventing the dilution of mercury through tissue expansion, or from causal negative effects of mercury on chick development (Longcore et al., 2007; Heinz et al., 2010b; Amélineau et al., 2019; Santos et al., 2020). In terms of survival, we found mercury levels to predict chick survival in two phases during the rearing period: higher levels of mercury remaining in the eggshell were associated with a reduced likelihood to survive to day 10, and higher levels of chick blood mercury at day 10 were associated with a reduced likelihood to fledge. Given that these effects disappeared when adding hatch mass or day 10 mass to the models, respectively, we suggest that the causal agent underpinning variation in chick survival is growth, rather than mercury pollution. Finding out whether mercury in chicks is causally related to growth, or at which mercury levels, therefore will be important.

## 5. Conclusion

Overall, our results indicate that common tern chicks in the studied population hatch with substantial levels of maternally transferred mercury. Maternal effects diminish as chicks grow, most likely due to the chicks' ability to reduce maternally transferred mercury levels through growth dilution and excretion via feather development. Despite this ability to reduce their mercury burden, which varies across tissue types, we found evidence of negative correlations between mercury in chick blood and growth, although causal mechanisms underlying this relationship require further investigation. After chick growth and feather development are completed, the chicks' mercury levels increase rapidly, potentially compromising survival after fledging, although we were unable to quantify this in our study. An experimental study on zebra finches reported that mercury exposure during the chick phase alone can affect reproductive success in later life (Paris et al., 2018), emphasizing the long-term negative effects of mercury, even in the absence of further exposure - while the birds in our study experience continued exposure throughout their lives. As such, future research should focus on the long-term effects of early-life mercury exposure in wild birds, assessing the process of recruitment, as well as annual reproductive performance and survival of birds with respect to their 'mercury history', ideally also
separating effects of methylmercury from those of other forms of mercury.

### CRediT authorship contribution statement

**Justine Bertram:** Writing – original draft, Visualization, Investigation, Formal analysis, Conceptualization. **Coraline Bichet:** Writing – review & editing, Investigation. **Maria Moiron:** Writing – review & editing, Formal analysis. **Matteo Beccardi:** Writing – review & editing, Investigation, Formal analysis. **Nathalie Kürten:** Writing – review & editing, Investigation. **Peter J. Schupp:** Writing – review & editing, Resources. **Sandra Bouwhuis:** Writing – review & editing, Supervision, Project administration, Conceptualization.

### Ethical approval

The study was performed under licenses of the city of Wilhelmshaven (63-04/03) and the Lower Saxony State Office for Consumer Protection and Food Safety (33.14-42502-04-14/1392, 33.19-42502-04-19/3068 and 33.12-42502-04-24-00555), Germany.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

We would like to thank all field assistants who helped with data collection, as well as U. Pijanowska and P. Schwarz for laboratory support. Special thanks goes to P.H. Becker for setting up the long-term common tern study at the Banter See and G. Wagenknecht for being a backbone of the overall project throughout the sampling period. We would also like to thank four anonymous reviewers for their constructive comments that helped us improve this manuscript.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.envres.2025.122283.

### Data availability

Data will be made available on request.

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