Mercury Exposure And Mercury Isotope Fractionation In Birds At Lancang Jingmai Airport,Yunnan Province

Mercury（Hg）is a heavy metal pollutant that is widely distributed in nature and poses a significant risk of exposure to wildlife and human health.Given their sensitivity to environmental changes,birds are commonly regarded as valuable indicators of pollution.With a widespread global distribution and ease of research and observation,birds are often employed for monitoring environmental pollutants.As high trophic organisms,birds are particularly susceptible to mercury pollution,which can result in substantial bioaccumulation.Yunnan Province,situated in the southwest of China and bordering Myanmar,Vietnam,and Laos,is home to a diverse range of species and boasts having the most abundant bird resources in China.This makes it an optimal location to investigate the exposure of birds to environmental pollutants.The study focused on utilizing bird samples obtained through accidental deaths during bird driving at Jingmai Airport in Lancang County,Yunnan Province.Multiple tissues and organs were dissected and collected for analysis,alongside samples of invertebrates,surrounding soil,and dominant plants in the bird habitat.The analysis included measurements of total mercury（THg）,methylmercury（Me Hg）,carbon isotope(δ¹³C),nitrogen isotope(δ¹⁵N),and mercury isotopes(δ²⁰²Hg,Δ¹⁹⁹Hg,Δ²⁰¹Hg)to investigate mercury exposure,food chain transmission,potential food sources,and isotope fractionation characteristics across different avian tissues and organs.Key findings of the study include:（1）Birds exposed to mercury at airports exhibited a wide range of mercury concentrations,ranging from 16 ng/g to 11956 ng/g,and 85%of the birds had THg concentrations below 1000 ng/g.The levels of mercury varied significantly amongst ecological groups as well.Average mercury concentrations for waders were 2926±2454 ng/g,raptors were 850±861ng/g,songbirds were 394±339 ng/g,climbing birds were 209±246 ng/g,and land birds were 76±53 ng/g.The mercury content in various tissues varied significantly,with feathers having the highest mercury content at1374±2213ng/g,nails at 1626±2146 ng/g,the beak at 1120±1288ng/g,the liver at 1126±1700ng/g,and the heart at 665±960ng/g,the gizzard at358±476 ng/g,the kidney at 973±391 ng/g,the pectoral muscle at 431±621ng/g,the leg muscle at 480±653 ng/g and the gastric contents at 215±317ng/g.（2）The mercury content of invertebrates and other samples was as follows:cotton locusts had the lowest mercury content（14±3ng/g）,fish had the highest mercury content（328±66ng/g）,dragonflies had a very high mercury content（200±145ng/g）because of their larval environment,and plants had the lowest mercury content（16±14ng/g）.With a mean value of769±545ng/g,the airport’s soil had an extremely high concentration of mercury.（3）Birds,invertebrates and other samples of carbon nitrogen isotope composition characteristics showed that birds’carbon isotopic range was-32‰to-14.07‰,invertebrates’and fish’s carbon isotope in the range of-32.06‰～-11.83‰,plants’range was-29.4‰～-11.70‰.Nitrogen isotopes ranged from 2.43‰to 13.44‰in birds,with the highest trophic grade in schrenck’s bittern.The nitrogen isotope values of invertebrates and fish ranged from-9.26‰to 9.19‰,among which the nitrogen isotope value of the larvae was negative,which was related to their feeding on moss.The nitrogen isotope value of fish was the highest,and its trophic level was also the highest.There was no significant difference in carbon isotopes among different tissues of birds,but there was a difference in nitrogen isotopes,which was related to nutrient transfer during metabolism.（4）The Bayesian stable isotope mixing model’s findings revealed that invertebrates were favoured by raptors and songbirds at the airport,mountain turtle doves preferred C3 plants,and wading birds mostly consumed fish and shrimp from nearby rivers.With TA values of 114 and131,respectively,the primary and premium consumers of the airport have a high niche space with a balanced nutritional structure and a diverse range of species.（5）The isotopic fractionation ofδ²⁰²Hg in birds revealed that dietary metabolism might influence the outcomes ofδ²⁰²Hg fractionation.MDF for waders（represented by the greater painted snipe）was-0.1±0.26‰,birds of raptors（represented by the collared scops owl）was-1.19±0.4‰,and songbirds（represented by the common magpie）was-1.18±0.22‰.Rufous woodpecker represented climbing birds with a value of-1.01±0.46‰and Oriental Turtle Dove represented land birds with a value of-1.22±0.49‰.The fractionation ofδ²⁰²Hg was also considerable,suggesting that food consumption and trophic level may be directly associated to the mercury levels in various tissues.（6）Diet and metabolism can cause MDF and MIF in birds.Combined with Bayesian mixed model analysis,the food sources of waders and other land birds were significantly different,resulting in different MDF and MIF.The slope ofΔ¹⁹⁹Hg/Δ²⁰¹Hg was 1.24,indicating that the non-mass fractionation of Hg isotopes in airport birds might be influenced by the combination of photoreduction of inorganic Hg and photodegradation of methylmercury before food entry into the food chain.Light isotopes produced after liver demethylation metabolism were enriched in other parts of the body,especially feathers,nails,and beak,leading to lowerδ²⁰²Hg in these three parts.