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Songbird reproductive success can decline from consuming mercury-contaminated aquatic insects, but assessments of hydrologic conditions influencing songbird mercury exposure are lacking. We monitored breast feather total mercury (THg) concentrations and reproductive success in the U.S. federally listed endangered Cape Sable Seaside Sparrow (CSSS: Ammospiza maritima mirabilis) over three breeding seasons in the Florida Everglades. We used model comparison to explore the influence of annual hydrologic variation on adult CSSS THg concentrations, and tested mercury effects on individual reproductive success (individuals’ mate status, apparent nest success, and total productivity) that were scaled to estimates on population productivity using a demographic model. We identified four hydrologic models that explained annual variation in adult THg concentrations, with the top model showing a negative association between THg concentrations and drought length of the previous breeding season and a positive association between THg concentrations and dry-season water recession rate (model adjusted R2 = 0.82). Adult male mating probability declined by 63% across the range of THg concentrations observed. We found no mercury effect on CSSS nest success or total productivity. However, demographic modeling suggested the reduced mating could produce a 60% decrease in population productivity compared to a scenario with no THg impact. Our results suggest that CSSS mercury exposure is influenced by local hydrologic conditions that can increase early breeding failure (lack of breeding initiation) and potentially limit population productivity. This study is the first to describe CSSS mercury exposure and its potential reproductive costs at the individual and population levels. 

Methylmercury (MeHg) and, to a lesser extent, inorganic mercury (IHg) contamination of rice is a global public health concern, but little is known about how soil and grain Hg concentrations respond to elevated CO2 (ECO2), or how ECO2 alters movement of Hg through the soil-plant-grain system. To advance knowledge of how Hg contamination of rice will change in the future, this study explored the effect of elevated CO2 (ECO2, c. 800 ppm) on soil, iron plaque, root, stem/leaf, and grain concentrations of MeHg and IHg. We observed evidence that ECO2 increased accumulation of MeHg, but not IHg, in rice grain. For IHg, ECO2 did not alter its uptake from the soil, translocation through the plant, or concentration in rice grain. However, ECO2 did reduce uptake of IHg from the air into leaf tissues, likely as a result of the reduced stomatal conductivity and thus more limited direct uptake from the air. Methylmercury concentrations in the grain of plants grown at ECO2 were significantly higher than those of plants grown at ambient CO2. Moreover, MeHg concentrations were also elevated in stem/leaf (82 %) and root tissue (37 %) for ECO2 plants, although the root-tissue results were not statistically significant. In contrast, soil MeHg concentrations were virtually indistinguishable between treatments, indicating that higher rice grain MeHg concentrations were not likely due to higher microbial IHg methylation in soil. Plant uptake of MeHg into stem/leaves and grain from the soil was significantly greater in the ECO2 treatment; however, translocation patterns of MeHg within the plant itself did not differ between treatments. Notably, these patterns existed despite consistently lower transpiration in the ECO2 treatment, and thus less mass flow of solute towards and through the plant. Our results indicate that as CO2 concentrations rise, the human health risks related to MeHg in grain will likely increase.