A U-Turn for Mercury Concentrations over 20 Years: How Do Environmental Conditions Affect Exposure in Arctic Seabirds?

Mercury (Hg) is highly toxic in its methylated form (MeHg), and global change is likely to modify its bioavailability in the environment. However, it is unclear how top predators will be impacted. We studied blood Hg concentrations of chick-rearing black-legged kittiwakes Rissa tridactyla (2000-2019) in Svalbard (Norway). From 2000 to 2019, Hg concentrations followed a U-shaped trend. The trophic level, inferred from nitrogen stable isotopes, and chlorophyll a (Chl a) concentrations better predicted Hg concentrations, with positive and U-shaped associations, respectively. As strong indicators of primary productivity, Chl a concentrations can influence production of upper trophic levels and, thus, fish community assemblage. In the early 2000s, the high Hg concentrations were likely related to a higher proportion of Arctic prey in kittiwake's diet. The gradual input of Atlantic prey in kittiwake diet could have resulted in a decrease in Hg concentrations until 2013. Then, a new shift in the prey community, added to the shrinking sea ice-associated release of MeHg in the ocean, could explain the increasing trend of Hg observed since 2014. The present monitoring provides critical insights about the exposure of a toxic contaminant in Arctic wildlife, and the reported increase since 2014 raises concern for Arctic seabirds.

Landfast ice: a major driver of reproductive success in a polar seabird.

In a fast-changing world, polar ecosystems are threatened by climate variability. Understanding the roles of fine-scale processes, and linear and nonlinear effects of climate factors on the demography of polar species is crucial for anticipating the future state of these fragile ecosystems. While the effects of sea ice on polar marine top predators are increasingly being studied, little is known about the impacts of landfast ice (LFI) on this species community. Based on a unique 39-year time series of satellite imagery and in situ meteorological conditions and on the world's longest dataset of emperor penguin (Aptenodytes forsteri) breeding parameters, we studied the effects of fine-scale variability of LFI and weather conditions on this species' reproductive success. We found that longer distances to the LFI edge (i.e. foraging areas) negatively affected the overall breeding success but also the fledging success. Climate window analyses suggested that chick mortality was particularly sensitive to LFI variability between August and November. Snowfall in May also affected hatching success. Given the sensitivity of LFI to storms and changes in wind direction, important future repercussions on the breeding habitat of emperor penguins are to be expected in the context of climate change.

Increased sea ice concentration worsens fledging condition and juvenile survival in a pagophilic seabird, the snow petrel.

Polar sea ice is changing rapidly, threatening many taxa in the Arctic and the Antarctic. Little is known about the effects of sea ice on early life-history traits of sea ice specialist species, although juvenile stages are a critical component of population dynamics and recruitment. We examined how annual variation in sea ice concentration (SIC) affects juvenile survival and body condition at fledging in the snow petrel Pagodroma nivea using long-term datasets encompassing 22 years for body condition and 37 years for juvenile survival. We show that SIC and southern annular mode (SAM), the principal mode of variability of the atmospheric circulation in the Southern Hemisphere, have strong nonlinear effects on juvenile survival and body condition. Below ca 20-30% SIC, body condition remained stable, but decreased almost linearly for higher SIC. Juvenile survival was negatively related to SIC and to SAM during the chick rearing period. We suggest that the base of the sea ice food web would be directly affected by sea ice conditions, thus acting locally on the abundance and structure of prey communities.