A retrospective study of metals and stable isotopes in seals from Swedish waters

Concentrations of Hg in Swedish marine systems are generally lower than in fresh water systems, but still often exceed environmental target levels. As top predators in many marine food webs, seals are exposed to heavy metals, like Hg and Cd, which are known to biomagnify with increasing trophic level in the food web. Therefore, studies of Hg concentrations over time are preferably conducted in relation to seal diet during the same period. By analysis of naturally occurring stable nitrogen and carbon isotopes it is possible to determine on which trophic level a certain organism is located and where it feeds. The aim of the present study was to try to establish background concentrations of Hg and some other metals in Swedish marine waters by analyses of seal bones from the 1840s and forward. Stable isotopes were analyzed in order to discover possible correlations between metal concentrations and trophic level, and changes over time in dietary intake of metals. No significant change in metal concentrations was seen over time, except for a significant decrease in Zn concentrations in Baltic Grey seal. A significant difference in δ 15N and δ13C values between species and between Baltic and Skagerrak areas was discovered. No significant correlation between δ 15N or δ13C values and metals was found, however, the seal with the highest δ 15N value (Harbor seal) also had the highest Hg concentration. Even though the δ 15N values suggested an increase over time the increase was not statistically significant. No statistically significant change over time was found in δ 13C values. It is important to keep in mind that this study was based on 44 individual seals spread over 126 years. Data on sex and age was lacking for almost half of the seals, which made statistical analysis on these parameters difficult to perform. Even though no obvious trends or differences in metal concentrations between areas and species were seen, the results from this study can serve as a base for further studies and provide valuable information for interpreting the actual contamination load in Swedish marine systems