By: Sébastien Descamps and Sébastien Moreau // Norwegian Polar Institute. Arnaud Tarroux, Svein-Håkon Lorentsen and Jan-Ove Bustnes // Norwegian Institute for Nature Research. Nigel G Yoccoz // UiT The Arctic University of Norway. Katrine Borgå // University of Oslo. Alice Carravieri // LIENS-CNRS. Samantha Patrick // University of Liverpool
Sentinel species are bioindicators: they reveal something about the ecosystem in which they live. They are expected to integrate broad ecosystem processes into variables that are easy to measure and interpret, thus providing an alternative way to assess ecosystem responses to environmental change.
Using a particular species to infer the state of a system is nothing new. The classic example is the “canary in the coal mine”, used to monitor air quality. For many years, contaminant levels in wildlife species have been measured to detect potential hazards both to the ecosystems where they live, and to human health. In marine ecosystems, top predators such as seabirds respond to ecosystem alterations with physiological, behavioural, and demographic changes. These responses are relatively easy to observe during the breeding season, but also outside it, thanks to miniaturised biologging devices. Together, these facts argue strongly for seabirds’ utility as indicators of change in marine ecosystems.
Links to weather and ocean processes
Antarctica’s climate is hostile (average summer temperature ‑10°C at Svarthamaren) and antarctic petrels can be hit hard by extreme weather. For instance, snowstorms can drastically reduce their breeding success. Conditions at sea may be even more important. Antarctic and snow petrels are entirely dependent on the seas adjacent to Dronning Maud Land, where they forage year-round on antarctic krill and small lanternfish. Conversely, the south polar skua can feed on inland-breeding birds (including petrels) in Antarctica in summer and spend the winter in oceans farther north. This means that petrels are directly affected by conditions in the Southern Ocean, whereas the effects on skuas are indirect, mediated through effects on its main prey, the antarctic petrel.
The movements and distribution of antarctic petrels breeding at Svarthamaren have been studied intensely in recent years. Petrels ,like all seabirds, are “central-place foragers” during the breeding season: the male and female take turns foraging at sea and incubating the egg or feeding the chick. However, recent tracking with satellite transmitters, GPS loggers, and geolocators has revealed that the foraging partner often travels >1000 kilometres from the colony, covering vast areas to search for food (about 3 000 000 square kilometres during the breeding season and even more in the non-breeding season).
The scale of antarctic petrel movements means they depend on climate and oceanographic conditions over a large portion of the Southern Ocean. Thus, changes within this vast area are expected to affect petrels from Svarthamaren in terms of foraging behaviour, diet, or life history.
There are two main modes of climate variability in the Southern Ocean: the Antarctic Oscillation (AAO) and the El Niño-Southern Oscillation (ENSO). Wind, temperature, sea-ice drift and precipitation are closely related to AAO and ENSO, and most of the variation in the reproductive success, timing of hatching, and adult survival of antarctic petrels can be accurately modelled based on these two large-scale climate signals.
The mechanisms linking petrel life history and AAO/ENSO remain speculative, but may involve changes in sea-ice habitats the petrel relies on when foraging, and changes in the availability of krill. This issue warrants further study.
Areas of ecological significance
During the Dronning Maud Land ecosystem cruise in March 2019, a highly productive system was observed over deep open water. This productivity hotspot was visible on satellite images showing sea surface colour, and was highly attractive to seabirds, including petrels. More than 1400 antarctic petrels and 800 snow petrels were observed in this area during two days of observations. These are among the highest petrel abundances and densities ever reported in this part of the Southern Ocean. This also fits with what GPS tracking shows about the distribution of antarctic petrels from Svarthamaren: they spend most of their time at sea in or near this highly productive area.
Observations like these indicate that this productivity hotspot is ecologically important and supports large numbers of top predators (seabirds and marine mammals) and prey (krill), and probably also high biodiversity. This emphasises the need to create a marine protected area in this part of the Southern Ocean.
Contaminants in skuas and petrels
Environmental contaminants such as toxic metals, persistent organic pollutants (POPs), and perfluorinated compounds are highly mobile and reach Antarctica with winds and ocean currents. They are also easily taken up by marine organisms and passed on to predators, and their tissue concentrations increase for each step in the food web. This means that seabirds can accumulate high quantities of these chemicals; it also makes the birds good bioindicators of contaminants in the food webs.
Seabirds can be vulnerable to the toxic effects of environmental contaminants. For example, high levels of POPs in south polar skuas are associated with delayed hatching and poorer chick body condition. Egg-laying antarctic petrels with high POP levels are in worse condition than those with low levels. Given that Svarthamaren lies far from pollution sources, the levels of contaminants in seabirds reflect the global dynamics of contaminant transport and transfer to remote food webs. Long-term monitoring of foreign chemicals in these species is therefore a powerful tool to trace contaminants’ presence in the environment and their potential to harm wildlife in Antarctica. It can also aid monitoring and assessment of various chemicals – regulated and unregulated – that may be harmful to antarctic ecosystems.
Antarctic petrels (and most likely snow petrels) at Svarthamaren feed mainly on antarctic krill, and forage in a vast but poorly studied region of the Southern Ocean – an area that is potentially important for krill production. Antarctic petrel movements, life history, and demography are tightly linked to ocean and climate conditions, and are relatively easy to monitor. These facts make the petrels of Svarthamaren a suitable sentinel species for the Southern Ocean. Indeed, the Ecosystem Monitoring Program (established by the Commission for the Conservation of Antarctic Marine Living Resources) already considers the antarctic petrel an indicator species, confirming its importance in detecting and alerting us to changes in critical ecosystem components in the Southern Ocean. Moreover, the seabirds of Svarthamaren can be used to monitor changes in the levels of organic pollutants and other contaminants in Dronning Maud Land.
Using top predators as sentinel species requires collecting data on various parameters: foraging behaviour, breeding numbers, phenology, diet, and productivity. Data collection must be repeated regularly over time both to detect changes and to disentangle natural variability from long-term trends. Ongoing and planned seabird research and monitoring at Svarthamaren should provide invaluable information about the state of the Southern Ocean.
- Tarroux A, Cherel Y, Fauchald P, Kato A, Love OP, Ropert-Coudert Y, Spreen G, Varpe Ø, Weimerskirch H, Yoccoz NG, Zahn S, Descamps S (2020) Foraging tactics in dynamic sea-ice habitat affect individual state in a long-ranging seabird. Functional Ecology 34: 1839-1856
- Delord K, Kato A, Tarroux A, Orgeret F, Cotté C, Ropert-Coudert Y, Cherel Y, Descamps S (2020) Antarctic petrels ‘on the ice rocks’: wintering strategy of an Antarctic predator. Royal Society Open Science 7: 191429
- Hindell et al (>40 authors) (2020) Tracking of marine predators to protect Southern Ocean ecosystems. Nature 580: 87-92, (click on “Full-text links” to access)