Krill vs Climate
The unsung heroes of the Antarctic food chain might be at risk.
When it comes to animals in the Antarctic, the stars of the show are often penguins and whales. Between classic novels and documentaries narrated by Morgan Freeman, it’s no wonder why these creatures hog the spotlight. But there’s one often forgotten critter that ties together the entire food chain of the Southern Ocean: Antarctic krill.
Though on average they’re no larger than your pinky finger, these tiny invertebrates are the main staple in the diets of many whales, penguins and seals.
“Krill is absolutely necessary to have this iconic ecosystem that everybody is aware of in the Southern Ocean,” says Simeon Hill, a senior scientist for the British Antarctic Survey.
But the Antarctic krill are under threat. Between rising sea temperatures and ocean acidification, climate change is krill’s biggest nemesis. By 2300, the Antarctic krill population will collapse if we do nothing about carbon dioxide emissions. Meanwhile, the krill fishery currently has very low catch limits but might soon ramp up to meet new demand.
You might know Antarctic krill (Euphausia superba) from their shell’s pink and orange hue, but their bodies are mostly translucent. The right light can give them a bluish glow, highlighting the orange speckles across their shell and the vivid green of the microscopic plants in their digestive system. All together it evokes the effect of a crustaceous stained glass window. “They’re the most spectacularly beautiful organism in the world,” says Maggie Amsler, a research associate in the biology department at the University of Alabama at Birmingham.
Krill have five sets of legs on their tails and then another five sets of feather-like legs called swimmerets. “They’re very delicate looking,” Amsler says. They beat their legs to propel themselves forward, much like they’re constantly running on an imaginary treadmill. But don’t let their delicacy fool you, Antarctic krill have to be tough to survive the harsh Antarctic environment. After hatching near the seafloor, krill larvae must swim 700 to 1000 meters to the surface, about twice the height of the Empire State Building, in order to find food within 10 to 23 days to survive.
These tiny crustaceans are one of the most abundant animal species, with total biomass estimates ranging from 379 to 500 million tonnes, compared to humans, whose total biomass is only 60 million tonnes. Krill move in swarms which can range from a few meters to kilometers wide, big enough to be seen from space.
Krill are a keystone species in the Southern Ocean, meaning they are crucial to the delicate food web of the Antarctic. Many penguins, whales and seals rely on krill as a primary source of food.
A study from December in the Proceedings of the National Academy of Sciences of the United States of America showed how two penguin species that consume Antarctic krill, the gentoo and the chinstrap, reacted very differently to shifting krill availability over time.
As krill availability declined, the gentoo adapted by eating food higher on the food chain than krill, like fish and squid, and its population has increased six-fold over the past 40 years. However, the chinstrap was unable to adapt to krill availability and its population has decreased by 30%-53%.
Penguins aren’t the only species that hang in the balance: Whales, seals, squid, ice fish, penguins, albatrosses and hundreds of other species depend on krill as a major food source.
“Krill are really important to everyone,” says Kelton McMahon, an assistant professor at the University of Rhode Island in the Graduate School of Oceanography and lead researcher on the study, “Even if you don’t eat krill or eat something that eats krill, chances are your neighbor, your competitor does, and so they are going to be influenced by it.”
Krill also play an important role in drawing carbon dioxide out of the atmosphere. During photosynthesis, phytoplankton take inorganic carbon, carbon dioxide that has dissolved from the atmosphere into the ocean, which is then passed on to krill when they’re eaten
However, krill expel a portion of that carbon through their fecal pellets which are dense and sink quite rapidly. Because of krill’s high biomass, they produce a lot of fecal matter and therefore sequester a lot of carbon. According to a 2019 review article from Nature Communication, in both shallow (170 m) and deep (1500 m) Southern Ocean sediment traps west of the Antarctic Peninsula and downstream of South Georgia, krill fecal pellets were the majority of sinking particles analyzed.
But while krill work to mitigate climate change, it is also their most pressing threat.
“It seems that krill are particularly vulnerable to the impacts of climate change because they’re cold-adapted so warming oceans has an effect on their habitat,” says Jess Melbourne-Thomas, a transdisciplinary researcher and knowledge broker for the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia’s national science research agency.
These rising temperatures have affected the distribution of krill in the Southern Ocean. Hill says the Antarctic krill population used to be spread fairly evenly across krill’s habitat range from the Antarctic Peninsula to South Georgia. However, just like retirees come December, the krill are moving south.
Angus Atkinson, a plankton ecologist and leader of the plankton ecology group at the Plymouth Marine Laboratory, says that due to the natural temperature gradient in the Southern Ocean, the ideal temperature zone for krill, roughly between 0.5 to 1 degree Celsius, has moved southward and so have the krill.
But rising temperatures don’t just affect where krill live, it affects their growth as well as a 2018 study in PLOS One shows. Emily Klein, the lead researcher on the study and a senior postdoctoral associate at the Frederick S. Pardee Center for the Study of the Longer-Range Future at Boston University, says ocean warming can stunt the growth of individual krill, meaning less biomass for the total population.
Warmer oceans aren’t the only manifestation of climate change that could affect krill. Due to human activity, there is a higher concentration of carbon dioxide in the atmosphere. While our oceans absorb about 30% of the carbon dioxide released into the atmosphere, the added carbon dioxide goes through a series of chemical reactions with seawater making the water slightly acidic.
According to a 2013 study in Nature, if nothing is done to curb carbon dioxide emissions and their effects on ocean acidification, krill hatching success will decrease by at least 20% by 2100. By 2300, the entire population could collapse because the krill habitat would no longer be suitable for hatching.
But not all studies point to the end times for krill. One 2018 study in Communications Biology showed that adult krill were able to survive for one year in the laboratory exposed to near-future rates of ocean acidification while growing, storing fat, maturing and maintaining respiration rates.
However, So Kawaguchi, a principal research scientist at the Australian Antarctic Division, says many of the studies that point to the resilience of krill to ocean acidification use adult krill, who have the ability to regulate their body fluid’s pH to counteract the acidification. Younger krill lack that mechanism, and even though adult krill can self-regulate, it requires energy that would be devoted to reproduction and growth.
Melbourne-Thomas also led a study from 2016 published in Geophysical Research Letters which suggested that thinner sea ice could mean that more light can reach the underside. This could mean more algae for krill larvae to eat during winter and early spring when food is scarce.
However, Melbourne-Thomas maintains that most research points to the effects of climate change having a negative impact on krill, and Hill agrees. “This is good that…not everything might be as bad as it could be, but unfortunately, most of the information we have still suggests that the cumulative effect…[of climate change is] likely to reduce krill populations and distribution,” he says.
And though climate change is by far the biggest threat to krill, a thriving krill fishing industry also has the means to impact the Southern Ocean. But while krill fishing certainly doesn’t mitigate the effects of climate change, for now, it doesn’t drastically amplify them either.
Currently, krill is harvested for aquaculture feed, like that used in fish farms, and for use as a dietary supplement in the form of krill oil.
“People don’t realize that there is a krill fishery, because they’re these little shrimp that people don’t really think of,” Klein says.
Krill oil, like fish oil, is rich in omega-3 fatty acids and its popularity is growing. According to a 2016 report from Persistence Market Research, the global krill market was valued at $267.7 million in 2014 and is projected to be worth $703 million in 2022.
George Watters, the US scientific committee representative for The Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) and the director of the Antarctic Ecosystem Research Division at the National Oceanic and Atmospheric Administration’s (NOAA) Southwest Fisheries Science Center, says “we’re going to need new protein sources” to feed a growing human population.
Higher demand for krill could lead to increases in krill fishing, but for now, the CCAMLR, an international body that manages fisheries in the Southern Ocean, has put conservative catch limits on krill fishery in the Southern Ocean with a total annual catch at around 0.3% of the unexploited biomass of krill in the southwest Atlantic of the Southern Ocean.
Between climate change, predators and fishery, there are a lot of moving parts affecting krill in the Southern Ocean, and they often overlap. Klein says that the areas in the Southern Ocean where climate change could have the greatest impact on krill are the same areas with a concentration of krill fishing and where krill predators need to forage.
“There is a range of krill quality,” Klein says, “It turns out that the krill that’s best for krill oil is also the krill that these predators like to eat when they’re raising [seal] pups and raising [penguin] chicks.”
Klein’s 2018 study on ocean warming’s effect on krill biomass also predicted how current krill fishing could affect the consequences of ocean warming. CCAMLR has set a depletion threshold of 75% of the unimpacted biomass of krill.
“Seventy-five percent is going to be our threshold for when things are going to get riskier, or we’re going to be more and more concerned,” Klein says.
Excluding the effects of climate change, with current harvest rates of krill there was almost no risk that krill biomass would dip below that 75% depletion threshold. Including the effects of climate change, the study suggests that continuing to fish at current harvest rates would in some cases further the depletion of krill biomass, but discontinuing krill fishing would do little to mitigate the effects of climate change.
However, there’s a lot of work being done to study and conserve krill. CCAMLR relies on a team of scientists from across the world to keep a watchful eye on the health of krill and the greater ecosystem.
Watters has wanted to be a fishery biologist since he was a kid while fishing with his father. “I always used to wonder, ‘How do they know how many I get to keep?’”
Now that he’s all grown up, his research now focuses on how krill fishing and the availability of krill affects the populations of krill predators like penguins and whales. “My role as the [US scientific committee] representative is to take everything I learned from that process and try to distill it down into some useful advice for policymakers,” he says.
Klein says that sometimes it’s difficult to get progress after publication of papers, but the particular study she led in 2018 on ocean warming was rewarding because the results were directly incorporated into policy. “It was pretty exciting to be part of work that was immediately relevant and…useful to decision-makers,” she says.
In the meantime, it’s important to keep in mind what is at stake: Without krill, all those documentaries, novels and animated hits might not have their stars. “We like to know that there are penguins and seals happily living in Antarctica,” says Melbourne-Thomas. So the next time you sit down to watch Happy Feet or listen to whale sounds as you fall asleep, know that it’s all brought to you by a small, pink crustacean.