Study projects major loss in habitat
Changes in ocean temperatures and sea ice formation around Antarctica could imperil the region’s krill — tiny crustaceans that are at the base of the food chain. Scientists say they’ve already documented a big drop in krill populations since the 1970s. Losing more krill would reduce the amount of food available for whales, penguins, seals, squid, fish and other marine life.
A new study published online in Geophysical Research Letters says up to 80 percent of suitable krill habitat could disappear by 2100. The research examines the effects of a warmer ocean and a decline in sea ice on these small crustaceans, said Andrea Piñones, a marine scientist at Center for Advance Studies in Arid Zones (Centro de Estudios Avanzados en Zonas Aridas) in Coquimbo, Chile, and lead author of the study.
Piñones and colleagues combined climate simulations with a krill growth model to find that changes in water temperature and sea ice in Antarctic waters could shrink krill habitat by as much as 80 percent by the end of the century, potentially causing a decline in krill that could ripple throughout the entire marine food chain.
The life cycle of Antarctic krill is closely tied to many factors, including water temperature and the availability of sea ice, Piñones said. While other studies have looked into how ocean acidification, overfishing or more freshwater entering the sea from melting Antarctic glaciers could affect krill habitat, the new study examines the effects of a warmer ocean and a decline in sea ice on these small crustaceans, she said.
Understanding the effects of climate change on krill could help scientists understand what might be in store for the entire Antarctic marine food web, Piñones said.
“The idea is to give scientists a projection of how krill are going to do under these high (carbon dioxide) emissions scenarios,” she said.
Most Antarctic krill live in an area that spans from the Weddell Sea to the waters around the Antarctic Peninsula. Females lay eggs in the summer near the surface. The eggs develop and sink to depths of 700 to 1,000 meters, in the realm of the Circumpolar Deep Water current. This mass of water is about 1.8 degrees Celsius warmer than Antarctic surface waters — the ideal temperature for krill egg development. When the eggs hatch, the larvae rise up to the surface. If they don’t find food, they starve in about 10 days.
Climate models predict that, as the oceans take in more heat from the greenhouse effect, the waters of the Circumpolar Deep Water will warm 1 to 1.5 degrees Celsius by the end of the century. Meanwhile some surface waters around Antarctica are projected to warm by as much as 2 degrees Celsius. The warmer water could help krill eggs develop a little bit faster, which means they won’t sink as deep. The eggs will hatch earlier and rise to the surface sooner, but there may not be any food for them once they get to the top. And krill larvae can only survive within a certain temperature range.
“Higher than 2 degrees Celsius and they won’t develop,” Piñones said. “There are physiological limits.”
If the larvae make it past that first 10-day hurdle at the surface, they face a second challenge in the late summer and early fall when they need to fatten up make it through the winter. The last hurdle is their first winter, when juvenile krill depend on the sea ice algae for food, and crevices in the ice for shelter through the winter.
“The time when sea ice is found is very essential for the early life cycle of krill,” Piñones said. “The key here is that sea ice has been advancing later and retreating earlier in the regions where krill is declining.”
Climate models predict a 90-day delay in the formation of winter sea ice by the end of the century. Less winter sea ice means less food and shelter available for krill juveniles, which means fewer young krill will survive to adulthood and spawn the next summer, , Piñones said.
The warmer waters and lack of sea ice could lead krill habitat to shrink by as much as 80 percent by 2100, according to the new study. At that point, only localized regions along the western Weddell Sea, isolated areas of the Indian Antarctic sector and the Amundsen/Bellingshausen Sea will support successful spawning habitats for krill, according to the new study.
The loss of krill habitat could cause krill populations to decline, although the new research did not quantify the drop in krill, Piñones said. A loss of krill could have consequences that would cascade throughout the food web of marine mammals and seabirds whose rely on the crustaceans as their main source of food, according to Piñones.