Mercury concentration in Antarctic krill varies in time and space and with individual size

Environmental context: Several predators that eat Antarctic krill may be unintentionally ingesting toxic substances. Studying aspects of krill life to understand the effects of potential increases in Antarctic mercury (Hg) availability revealed that seasons, locations and individual size influence krill Hg concentration. Despite increasing human presence (potential Hg sources) in Antarctica, krill Hg content remains stable, and evidence suggests that Hg accumulates in predators by both short (krill-based) and longer food chains. 

Rationale: Mercury (Hg) is passively assimilated from the water by phytoplankton, accumulated by lower trophic levels species, and biomagnified along food chains. Any increases in its bioavailability in Antarctic waters could endanger the survival of vulnerable top predators. With Antarctic food webs reliant on krill, we must understand the temporal, spatial and biological variability in their Hg concentration to forecast ecosystem-wide impacts of rising Hg levels. 

Methodology: We sampled krill fortnightly from South Georgia, South Orkney Islands and West Antarctic Peninsula between December 2013 and September 2019 (excluding October and November months). Individuals were weighed, sexed and analysed for Hg. We assessed the importance of biological (krill size, sex and life stage) and environmental (location, time and chlorophyll-a concentration) parameters on krill Hg concentrations with generalised linear models, analyses of variance, Gaussian linear models and vector autoregressive modelling. 

Results: Temporal variation explained most of the differences in krill Hg concentrations, with location and individual size also contributing to the variability. Subsurface chlorophyll-a concentrations and the affinity of methylmercury to sulfhydryl groups of some proteins, rather than krill fatty acid content, were likely the drivers of observed annual cycles. 

Discussion: Antarctic krill Hg concentrations have remained stable since the 1990s, although our measurements were lower than most. Such a historic baseline is indispensable for continued monitoring of Antarctic ecosystems. Krill is considered a key prey species, but our findings and those of biomagnification studies suggest that there may be a gap in our understanding of trophic transfer and accumulation of Hg in some top predators. Future biomagnification studies would benefit from conducting mass balance models.