| Summary: | Female northern elephant seals, Mirounga angustirostris, fast entirely during nursing while their pups may quadruple in weight over the 25-day suckling period. Stable carbon and nitrogen isotope ratios (d13C and d15N used as diet markers) and total mercury concentrations (Hg) were measured in tissues (blood cells, serum, full blood, milk and blubber) from 20 mother-pup pairs on day 4 and 21 of lactation. A systematic difference was observed between isotopic values in blood cells and serum of the mothers, linked to diet and distinct biochemical composition between blood components. d13C differed between inner and outer blubber layers and were similar between inner blubber and milk. High Hg concentrations (up to 350 ng.g-1 ww at the beginning of lactation) were found in the blood of mother elephant seals linked to their high d15N values in blood cells. Hg transfer through placenta and milk was observed between mothers and pups. A significant increase of blood Hg concentration in mothers (+285 ng.g-1ww) and a decrease in pups (-93 ng.g-1ww) were observed between days 4 and 21 of lactation. Both processes were explained by a remobilization of proteins and lipids during fasting and milk production in mothers and by a dilution of Hg in pups due to their high mass increase during this period. To conclude, physiological processes and body condition should be considered carefully when interpreting stable isotope ratios and Hg concentrations in the framework of biomonitoring.
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