Temporal and Spatial Distribution of Mercury in Gulls Eggs from the Iberian Peninsula

Abstract

We examined how coastal mercury contamination varied spatially and temporally across the Iberian Peninsula by measuring mercury concentrations in the eggs of the sentinel biomonitor yellow-legged gull (Larus michahellis). Samples were collected from eight colonies that ranged from the Atlantic across the south and northern areas of the Mediterranean. We also measured Hg residues in eggs of the one of the most endangered gull species in the world, the Audouin's gull (Larus audouinii) from the Ebro Delta, where colonies of yellow-legged and Audouin's gull co-occur. Fresh eggs were collected in 2009 and 2016 and samples were pooled from each colony for analysis. Mercury concentrations in yellow-legged gulls ranged between 0.4 and 2.8 mg/kg dry weight (dw); although there were no significant differences in concentrations between sampling periods, significant differences were found between colonies. Higher concentrations were associated with northern Mediterranean colonies (Columbretes and Ebro Delta), likely due to proximity to emission sources, circulatory marine currents and diet composition. Mercury concentrations in yellow-legged gull eggs were lower than those reported to result in impaired hatching. Residues in Audouin's gull eggs from the Ebro Delta were significantly higher (4.0-5.6 mg/kg dw) than those in yellow-legged gull from the same location, probably associated with dietary differences. Mercury levels in Audouin's gull were ten times above the benchmark suggested to reduce nest success by 10%. Overall, these results raise concern for adverse health impacts in this protected seabird species and further investigation in Audouin's gull eggs from the Ebro Delta is recommended.

Fig. 1

Colony locations from which eggs were sampled. These areas represent the most important gull colonies of the Iberian Peninsula and they all are declared Special Protection Area for birds

Fig. 2

Geometric mean (± 95% CI) concentration of total Hg in yellow-legged gull eggs collected from various colonies on the Iberian Peninsula. a Hg concentrations (mg/kg dw) in eggs collected in 2009 and 2016. b Data from 2009 and 2016 combined. Letters on top of bars indicates significant differences

Fig. 3

Mercury concentration in the eggs of yellow-legged gulls and Audouin’s gull from three subcolonies of the Ebro Delta collected in 2009 (clear bars) and 2016 (dark bars)

Fig. 4

Relationship between mean Hg and Se concentrations in eggs from species collecting in 2009 by sampling sites. The stippled line represents the 1:1 molar ratio between Se and total Hg and the dotted line represents the relationship between Hg and Se for yellow-legged gulls (using all data points)