Mercury accumulation in the food web of the Scotia Sea, Southern Ocean

Abstract

The aim of the work presented in this thesis was to understand processes of bioaccumulation and biomagnification of mercury (Hg) in a Southern Ocean (SO) food web, and to evaluate the effect of organism size and habitat in Hg accumulation during the last decade. To do this, tissues of various species occupying different trophic levels were analysed in samples collected over various sampling years (2006/07 and 2016/17) from areas with distinctive environmental characteristics.

Different accumulation patterns were found: Antarctic krill juveniles had higher Hg concentrations than adults; in squid, Hg concentration increased with individual size in one species (Alluroteuthis antarcticus), decreased in another (Galiteuthis glacialis), and in another still, there was no obvious relationship (Bathyteuthis abyssicola); for myctophid fish there was a consistent increase of Hg concentration with fish size, with the exception of Electrona antarctica females.

Proportions of organic Hg also varied between trophic groups, from relatively low (15-37%) in krill to virtual 100% in all myctophid tissues. Regarding Hg tissue allocation, squid muscle was the tissue that had highest Hg, followed by digestive gland and gills. Myctophids’ Hg concentrations were higher in the liver and heart than in muscle or gills.

Geographic differences in Hg concentration in krill were found, with individuals from the South Orkney having Hg levels 5 to 7 times higher than South Georgia: this geographic variation was not found in myctophids.

As expected, when evaluating Hg along the food web, POM spell out had the lowest Hg levels, followed in increasing concentration by zooplankton, squid, myctophid, notothenid fish and seabirds. Predators exhibited variability in Hg levels which corresponding to the trophic level of their diets, with lower G15N levels corresponding to lower Hg concentrations. The mid food web groups (squid and myctophids) showed a decreasing trend in Hg level over the last decade, but that difference was not reflected in top predators, for which Hg levels were higher in 2016/17 than in 2007/08. This difference between years may be due to a decrease in the abundance of krill that year, which would have necessitated a change by krill predators to myctophids, a higher Hg body burden prey. This thesis revealed details of Hg contamination in SO biota, emphasizing the role of atmospheric transportation in global mercury contamination. Present-day regional warming may lead to increasing Hg availability in the SO as glacial melt is releasing contaminants previously trapped following atmospheric precipitation. Climate change, pollution and growing fishing pressure are together placing increased pressure on SO marine ecosystems and the living resources they contain.