Contemporary patterns of mercury contamination in the Portuguese Atlantic inferred from mercury concentrations in seabird tissues

Mercury concentrations in 1809 egg and plumage samples of adults and chicks of seabirds from colonies in the Portuguese Atlantic (30–40°N, 8–32°W) were determined. Species and tissues were selected to ensure varied levels of ecological (epipelagic vs mesopelagic), spatial (coastal vs pelagic) and temporal (short-to medium-term) integration of mercury contamination. There was an overall general agreement in the information provided by species and types of tissues employed. Results show a four-fold increase in mercury bioaccumulation from the epipelagic to the mesopelagic compartment. Fourteen out of 20 inter-location comparisons of mercury concentrations in tissues yield statistically significant differences. However, 11 differences did not rise above the intra-specific variability of the monitor species and were not considered to reflect genuine geographic variation in mercury contamination. The remaining three differences indicate slightly enhanced mercury contamination in the epipelagic food web adjacent to the southwest Portuguese coast, which deserves further elucidation. Seabird indications of mercury contamination in the study region were assembled with similar information in a brief meta-analysis for the North Atlantic and adjoining regions. An overall comparison indicates that contamination levels in the Portuguese Atlantic are in agreement with levels reported for populations of the same species in ‘non-pollutedʹ areas elsewhere. Such mesoscale background uniformity of mercury contamination in the North Atlantic conveys further evidence of global pollution by mercury due to atmospheric deposition at long distance from emission sources.