Bioluminescent environment of the Southern Ocean revealed by deep-diving southern elephant seals

As a consequence of climate change, marine communities and ecosystems are shifting at all scales, but characterizing these shifts and their extent remains challenging. Notably lacking is information on mid-trophic level components such as zooplankton and micronekton. As ca. 70% of water column organisms are bioluminescent, we hypothesized that measuring changes in deep-water bioluminescence signals can provide valuable additional information for further characterizing the mid-trophic level composition of mesopelagic habitats. To test this, southern elephant seals Mirounga leonina from the Kerguelen Islands and from Peninsula Valdés in Argentina were equipped with light and motion sensors. Since these predators are large consumers of mesopelagic fauna, we expected them to target high-biomass regions and thus efficiently sample the bioluminescence fields in the Indian sector of the Southern Ocean and the southwestern sector of the Atlantic. Combining light, depth, GPS and motion data, we found that (1) bioluminescence events took place at depths ranging from 100 to 700 m, occurring deeper in Peninsula Valdés than in the Kerguelen Islands and in shallower waters during the night than during the day, suggesting organism diurnal migrations; and that (2) bioluminescence events were more intense, longer, deeper and more likely to be composed of multiple peaks when associated with a prey-capture attempt. We show that bioluminescence is a good proxy to describe marine mid-trophic levels. Combining it with other biologging sensors such as micro-sonars would yield extremely valuable data on these organisms and could be a key component in improving our understanding of mesopelagic habitats.