The mercury flow through a terrestrial songbird food chain in subtropical pine forest: Elucidated by Bayesian isotope mixing model and stable mercury isotopes

Mercury accumulation and biomagnification in the barn owl (Tyto alba) food chain

Mercury (Hg) accumulation and biomagnification in the barn owl (Tyto alba) food chain were investigated using bioindicator samples from three trophic levels: (1) soil and moss (atmospheric deposition indicators), (2) small mammal fur from regurgitated pellets (herbivores and omnivores), and (3) barn owl down feathers (apex predators). Spatial analysis identified regional Hg variation in soil, fur and feathers. Statistical models explored the effects of proximity to water bodies, wetlands and nearby pollution sources. The highest total Hg (THg) concentrations were found in feathers (170 ± 160 µg kg-1, n = 246) and fur in regurgitated pellets (150 ± 200 µg kg-1, n = 150), followed by soil (63 ± 17 µg kg-1, n = 63). Bioaccumulation factors were 2.3 (soil to fur) and 2.7 (soil to feather). Biomagnification factor from fur to feathers was 1.8. Methyl Hg (MeHg), measured in a subset of samples, was 120 ± 130 µg kg-1 in fur (n = 29) and 150 ± 98 µg kg-1 in feathers (n = 42), with 75-97 % of THg in feathers as MeHg. Prey composition significantly influenced fur THg levels, with higher concentrations in diets with omnivorous prey (Apodemus flavicollis) compared to herbivorous prey (Microtus arvalis). These findings highlight the importance of diet in Hg monitoring and biomagnification studies.

Riverine songbirds capture high levels of atmospheric mercury pollution from brown food webs in forests by mercury isotopic evidence

Elevated methylmercury (MeHg) exposure poses significant risks to bird health, behavior, and reproduction. Still, the risk of MeHg exposure to forest birds, accounting for over 80 % of the world's bird species, is poorly understood. This study combines Hg isotopes and video analysis, aiming to assess MeHg exposure risks to a forest riverine songbird, the spotted forktail (Enicurus maculatus) from a remote subtropical montane forest. Noticeably, 83 % of feather MeHg concentrations of adult forktails exceeded 5000 ng g-1, a threshold level potentially impacting bird reproduction, and 50 % of feather MeHg concentrations in forktail nestlings exceeded the threshold level of 1000 ng g-1, that potentially impacts the nestling growth. Forktail nestlings ingested ∼ 99 % of their MeHg from prey within brown food webs (i.e., from forest floor, aquatic, and emergent aquatic prey). The Hg isotopes reveal that MeHg along the bird food chain is mostly derived from in situ methylation of litterfall deposited atmospheric Hg0, with limited photo-demethylation (i.e., 4-12 %) in shaded forest environments. The risk of MeHg exposure of forest songbirds correlated positively with the proportion of prey consumed from brown food webs. We recommend incorporating resident riverine songbirds in monitoring programs to better evaluate the effectiveness of the Minamata Convention, especially in remote forest ecosystems where in situ MeHg production may be underestimated.

Regulation of straw-derived DOM and clay mineral complexation on mercury accumulation in vegetables

Straw return-to-field releases substantial dissolved organic matter (DOM), which can interact with clay minerals and influence mercury (Hg) dynamics in soil-plant systems. However, its detailed mechanisms remain poorly understood. In this study, DOM-montmorillonite (DOM-M) complexes were synthesized using DOM extracted from composted rice straw (DOMrice) and rape straw (DOMrape). The objective of this study was to investigate their impacts on Hg methylation in soil and the accumulation of total Hg (THg) and methylmercury (MeHg) in vegetables. The results demonstrated that straw-derived DOM significantly increased MeHg levels in the soil and water spinach. However, humified straw-derived DOM effectively suppressed this elevation by 29.0-64.5%. Specifically, humified DOMrice resulted in lower MeHg concentrations in the soil and reduced THg and MeHg levels in water spinach compared to humified DOMrape. Natural montmorillonite reduced Hg methylation in the soil but increased the accumulation of THg and MeHg in water spinach. In contrast, the humified DOMrape-M complex significantly mitigated the MeHg accumulation in water spinach that was enhanced by montmorillonite, with a reduction percentage of 25.8-52.0%, while the humified DOMrice-M complex did not demonstrate a similar advantage. This discrepancy could be attributed to certain molecular components in DOMrape, such as higher thiol-rich protein-like fractions and oxidized S species, which could promote Hg retention within mineral layers. The reduced adsorption capacity of humified DOMrice-M for Hg2+ also emphasized the unique role of humified DOMrape-M. Overall, this study highlights the importance of humified straw-derived DOM and its interaction with soil minerals in shaping Hg dynamics within the plant-soil system.

Synchronous changes in mercury stable isotopes and compound-specific amino acid nitrogen isotopes in organisms through food chains

The relationship between stable isotope of mercury (Hg, Δ199Hg and δ202Hg) and compound-specific nitrogen isotope of amino acids (CSIA-AA, δ15NGlu and δ15NPhe) remains poorly understood. In this study, we investigated bird species and their prey in an abandoned Hg mining area, southern China to elucidate these correlations for a better understanding of Hg sources, biological transfer, accumulation and amplification through food chains. Our findings revealed distinct isotopic patterns: Δ199Hg showed a positive correlation with δ15NGlu, indicating trophic transfer processes, while a negative correlation with δ15NPhe suggested differences in Hg sources among birds. The wide ranges of δ15NPhe and Δ199Hg observed in birds appear to reflect mixtures of multiple nitrogen and Hg sources, likely due to their diverse food sources and the large variation in the proportion of MeHg in total Hg (MeHg%). The consistent slope between Δ199Hg/δ15Nphe and MeHg%/δ15Nphe, reflecting both energy and Hg sources, provides new insights into the biotransfer and accumulation of Hg in organisms. Notably, the trophic magnification factor (TMF) of MeHg observed in water birds, such as egrets, reached an exceptionally high value of 97.7 estimated from CSIA of multiple amino acids (i.e., TMFM), underscoring the significance of investigating Hg sources in birds. Our results demonstrate that the synchronous changes between CSIA-AA and odd Hg isotopes effectively identify Hg sources and transfer across multiple ecological systems.

Significant biomagnification of methylmercury in songbird nestlings through a rice-based food web: Insights from stable mercury isotopes

To elucidate the sources and transfer of mercury (Hg) in terrestrial food chains, particularly in heavily Hg-contaminated rice paddy ecosystems, we collected rice leaves, invertebrates, and Russet Sparrow nestlings from a clear food chain and analyzed the dietary compositions and potential Hg sources using stable Hg isotopes coupled with a Bayesian isotope mixing model (BIMM). Our findings indicated that MeHg exposure is dominant through the dietary route, with caterpillars, grasshoppers, and katydids being the main prey items, while the less provisioned spiders, dragonflies, and mantises contributed the most of the Hg to nestlings. We found minimal MIF but certain MDF in this terrestrial food chain and identified two distinct MeHg sources of dietary exposure and maternal transfer. We firstly found that the dietary route contributed substantially (almost tenfold) more MeHg to the nestlings than maternal transfer. These findings offer new insights into the integration of Hg from the dietary route and maternal transfers, enhancing our understanding of fluctuating Hg exposure risk during the nestling stage. Our study suggested that Hg isotopes combined with BIMM is an effective approach for tracing Hg sources in birds and for gaining in-depth insight into the trophic transfers and biomagnification of MeHg in food chains.

Unveiling the Sources and Transfer of Mercury in Forest Bird Food Chains Using Techniques of Vivo-Nest Video Recording and Stable Isotopes

Knowledge gaps in mercury (Hg) biomagnification in forest birds, especially in the most species-rich tropical and subtropical forests, limit our understanding of the ecological risks of Hg deposition to forest birds. This study aimed to quantify Hg bioaccumulation and transfer in the food chains of forest birds in a subtropical montane forest using a bird diet recorded by video and stable Hg isotope signals of biological and environmental samples. Results show that inorganic mercury (IHg) does not biomagnify along food chains, whereas methylmercury (MeHg) has trophic magnification factors of 7.4-8.1 for the basal resource-invertebrate-bird food chain. The video observations and MeHg mass balance model suggest that Niltava (Niltava sundara) nestlings ingest 78% of their MeHg from forest floor invertebrates, while Flycatcher (Eumyias thalassinus) nestlings ingest 59% from emergent aquatic invertebrates (which fly onto the canopy) and 40% from canopy invertebrates. The diet of Niltava nestlings contains 40% more MeHg than that of Flycatcher nestlings, resulting in a 60% higher MeHg concentration in their feather. Hg isotopic model shows that atmospheric Hg0 is the main Hg source in the forest bird food chains and contributes >68% in most organisms. However, three categories of canopy invertebrates receive ∼50% Hg from atmospheric Hg2+. Overall, we highlight the ecological risk of MeHg exposure for understory insectivorous birds caused by atmospheric Hg0 deposition and methylation on the forest floor.

Using live videography observation and Bayesian isotope mixing model to identify food composition and dietary contribution to inorganic mercury and methylmercury intake by songbird nestlings

Mercury (Hg) exposure is increasing in terrestrial birds; however, studies on its sources are scarce. In the present study, we elucidated the food composition of green-backed tit nestlings from three urban forest parks (CPL, AHL, and LCG) using live videography observation (LVO). Furthermore, the daily dietary intakes of inorganic Hg (IHg) (MDIIHg) and methylmercury (MeHg) (MDIMeHg) were determined using the Bayesian isotope mixing model (BIMM) to uncover the nestlings' specific dietary Hg contribution. Both LVO and BIMM indicated that Lepidoptera (primarily caterpillar) constituted the primary food source for the nestlings in the three forests, accounting for approximately 60% of their diet in all three forest parks. The estimated MDI of Hg revealed that lepidopterans and spiders primarily contributed to IHg exposure, with a co-contribution ratio of 71.8%-97.7%. Unexpectedly, dietary MeHg was mostly derived from spiders; the highest contribution ratio of 93.6% was recorded at CPL, followed by another peak ratio of 92.9% at LCG. However, the dietary exposure was primarily IHg, accounting for 69.8% (AHL), 62.0% (LCG), and 61.3% (CPL) of the nestlings. Our study findings highlight the importance of dietary IHg transfer in evaluating the effects of Hg in nestlings. LVO, coupled with BIMM, is an effective tool for determining the food compositions of songbird nestlings and estimating the contribution of specific diets.