Source: Melbourne‐Thomas, J., Corney, S. P., Trebilco, R., Meiners, K. M., Stevens, R. P., Kawaguchi, S., … & Constable, A. J. (2016). Under ice habitats for Antarctic krill larvae: Could less mean more under climate warming?. Geophysical Research Letters.
Antarctic krill are small, semi-transparent crustaceans similar to shrimp that are about 2 inches long and live in the waters around Antarctica. But despite their small size, Antarctic krill are actually probably the most abundant animal on earth in terms of biomass, making up a total mass of 500 million tons. The name krill derives from a Norwegian word meaning ‘whale food’, and they are the main food source for many species of whales, seals, penguins and other birds, making them a very important part of the ecosystem.
Winters in Antarctica are cold and extreme, so krill have adapted their growth and life cycle around these tough winters. Food is very scarce in the open ocean during the winter, and adult krill can survive long winters without any food, but young krill larvae cannot. To survive the winter, the krill larvae retreat under sea ice, where they can feed on algae that grows on the underside of the ice to survive the winter. The sea ice is just like a winter resort for the larvae, because as well as providing food, the rough surface of the ice also provides shelter, preventing the krill from being swept away by currents and hiding them from their predators.
Sea ice is so important for the survival of krill populations, that researchers observed that in some places, in years when the ice cover expands further north, the size of krill populations in that region increased noticeably. But, because the harsh conditions make it so challenging to measure krill populations, especially under the sea ice, it is still a mystery if krill depend on the total extent of sea ice, and how their populations might be responding to sea ice changes.
Sea ice surrounding Antarctica is changing in response to changes in the surrounding atmosphere and ocean that are driven by climate change. As temperatures increase, ice is getting thinner, and the area of ice is changing, and expected to shrink in the future and warmer temperatures melt the ice. A team of scientists from Hobart, Australia, worked together to gain some insight into how Antarctic krill populations might respond to these sea ice changes.
The krill housing market
First, before the researchers could predict how krill populations will respond to changes in sea ice, they needed to identify all of the sea ice habitats around Antarctica that krill are likely to be found. They used a model that represents current sea ice accurately to locate potential habitats for krill in the winter, assuming two things determine whether the ice is suitable for krill: 1) food availability and, 2) the complexity of the ice. Algae need enough sunlight and nutrients to grow, so the researchers identified regions of ice where algae were likely to be growing by looking at sunlight availability and ice thickness, because studies have shown that algae are much more likely to thrive under ice that is 0.5-1 m thick. To assess ice complexity (the lumps and bumps that provide shelter for larvae) the scientists looked at the “ridging rate” of the ice. Ridging happens when sea ice is put under pressure, causing the ice to form ridges and making the surface underneath the ice very uneven. Using these criteria, they found that there is currently very little habitat for krill larvae in July and August, but by September, there is a large habitat area that wraps around Antarctica.
Future housing growth
Once they had identified the under ice habitats where krill are likely living now, the researchers restarted the model, but changed the model setup to simulate a warmer climate, like we expect in the year 2100 as a result of climate change. The results were an unexpected win for future krill larvae. In the warm scenario, the extent of sea ice in the winter decreased dramatically because warmer temperature melt much of the ice. But surprisingly, even though the total ice area decreased in the warm scenario, the area of ice that was suitable habitat for larvae actually increased significantly. The authors pose three possible reasons for the increase in habitat area in spite of a decrease in ice cover: 1) either there was thinner ice, 2) more light available, or 3) the complexity of the under ice surface increased in some places, or a combination of these factors. The researchers found that the ice did become significantly thinner, making more areas where algae are likely to be found on the underside of the ice. Additionally, a higher ridging rate was observed in many places, making more rough surfaces suitable for krill larvae to hide out.
The result of this study can only tell us how potential winter habitats for Antarctic krill larvae may change in response to sea ice changes, but larvae may be affected by other things, like currents and predators, which will determine if they occupy these habitats. Much more work is needed to figure out all the ways in which this important species will respond to future change, but this is an important first step, and suggests that the future Antarctic krill might be more resilient to climate change than we previously thought.