AGU: Climate Change Could Cause Major Decline In Antarctic Krill Habitat By 2100

AGU News:
WASHINGTON, D.C. — Antarctic krill, small crustaceans key to the Antarctic marine food web, could lose most of their habitat by the year 2100, according to a new study published online in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).
Researchers 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.
Antarctic krill are centimeter-long shrimp-like crustaceans that feed primarily on phytoplankton, the small algae at the base of the marine food chain. Krill are an essential energy source for whales, penguins, seals, squid, fish and other marine life. Humans also fish for krill, which is used for aquarium food, fishing bait, pharmaceuticals and in some foods consumed by humans.

Adult krill populations have dropped by 80 to 90 percent since the 1970s. There is an ongoing scientific debate about what is causing the drop, from changes in the environment to an increase in whale populations, said Andrea Piñones, a marine scientist at Center for Advance Studies in Arid Zones (Centro de Estudios Avanzados en Zonas Aridas) – Coquimbo, Chile, and lead author of the new study published online in Geophysical Research Letters, a journal of the American Geophysical Union.

The new study seeks to understand the additional pressures krill could face in the coming decades as the climate warms. 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. They start their lives in the summertime when females lay eggs near the surface. As the eggs develop, they slowly sink to depths of 700 to 1,000 meters (2,300 to 3,300 feet) – the realm of the Circumpolar Deep Water current, a mass of deep water slightly warmer than Antarctic surface waters. At this depth, the water is a cozy 2 degrees Celsius (35.6 degrees Fahrenheit), about 1 degree Celsius (1.8 degree Fahrenheit) warmer than the water at the surface. This is the ideal temperature for krill egg development. After the eggs hatch, the krill larvae swim up to the ocean surface to feed. If they cannot find food, the larvae will starve after 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 (1.8 to 2.7 degrees Fahrenheit) by the end of the century.

Meanwhile some surface waters around Antarctica are projected to warm by as much as 2 degrees Celsius (3.6 degrees Fahrenheit). A little warming of the water means krill eggs will develop faster, they will not sink as deep below the surface, and the eggs will hatch earlier and rise to the surface sooner, according to the new study. But krill larvae can only withstand temperature changes of up to 2 degrees Celsius (3.6 degrees Fahrenheit), Piñones said.

“Higher than 2 degrees Celsius and they won’t develop,” she 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, Piñones said. 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, she 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.

“Just about everything you can think of in the Antarctic depends on Antarctic krill,” confirmed biological oceanographer Kendra Daly of the University of South Florida in Tampa, who was not involved with the new study.