Disentangling the effects of habitat biogeochemistry, food web structure, and diet composition on mercury bioaccumulation in a wetland bird.

Methylmercury (MeHg) is a globally pervasive contaminant with known toxicity to humans and wildlife. Several sources of variation can lead to spatial differences in MeHg bioaccumulation within a species including: biogeochemical processes that influence MeHg production and availability within an organism's home range; trophic positions of consumers and MeHg biomagnification efficiency in food webs; and individual prey preferences that influence diet composition. To better understand spatial variation in MeHg bioaccumulation within a species, we evaluated the effects of habitat biogeochemistry, food web structure, and diet composition in the wetland-obligate California black rail (Laterallus jamaicensis coturniculus) at three wetlands along the Petaluma River in northern San Francisco Bay, California, USA. The concentration of MeHg in sediments differed significantly among wetlands. We identified three sediment and porewater measurements that contributed significantly to a discriminant function explaining differences in habitat biogeochemistry among wetlands: the porewater concentration of ferrous iron, the percent organic matter, and the sediment MeHg concentration. Food web structure and biomagnification efficiency were similar among wetlands, with trophic magnification factors for MeHg ranging from 1.84 to 2.59. In addition, regurgitation samples indicated that black rails were dietary generalists with similar diets among wetlands (percent similarity indices > 70%). Given the similarities in diet composition, food web structure, and MeHg biomagnification efficiency among wetlands, we concluded that variation in habitat biogeochemistry and associated sediment MeHg production was the primary driver of differences in MeHg concentrations among black rails from different wetlands.

Potential effects of mercury on threatened California black rails.

San Francisco Bay (SFB) estuary sediments contain high levels of mercury (Hg), and tidal marsh resident species may be vulnerable to Hg contamination. We examined Hg concentrations in California black rails, a threatened waterbird species that inhabits SFB tidal salt marshes. We captured 127 black rails during the prebreeding and postbreeding seasons and examined the influence of site, sex, and year on Hg, methylmercury (MeHg), and also selenium (Se) concentrations in feathers and blood. Feather Hg concentrations averaged 6.94 microg/g dry weight (dw) and MeHg and Se concentrations in blood averaged 0.38 and 0.42 microg/g wet weight (ww). We used Akaike's information criterion model selection process to evaluate the importance of year, site, sex, and age on patterns of MeHg concentrations; sex and year were the most important of these factors. Feather Hg concentrations (dw) were higher in males (8.22 microg/g) than females (6.63 microg/g) and higher in adult birds (7.36 microg/g) than in hatch-year birds (4.61 microg/g). A substantial portion of SFB black rail populations may be at risk of reproductive effects due to MeHg contamination, as 32-78% of feathers and <10% of blood samples exceeded no observed adverse effect levels. Sea level rise and other anthropogenic threats to endemic tidal marsh species such as black rails may be exacerbated by the presence of MeHg. Further study of population demographics and toxicological effects would further elucidate the effects of MeHg contamination on black rail populations in SFB.