Krill are a primary building block in the ocean’s food chain. They are a major source of nutrition to large and small creatures alike and are a great tool to estimate the health of a particular marine ecosystem. 

Marine beings are more sensitive to changes in temperature and this is particularly true for krill, which relies heavily on seasonal changes in its reproduction. Fluctuations in Antarctic krill populations every five to seven years is often a result of climate change, according to a new research paper published by researchers from the Oregon State University.

Climate change largely affects the biological cycles of seasonally dependant creatures throughout the world, including krill. Female krill feel the effects of warming oceans as it changes weather patterns and creates a shortage of food during the reproduction season according to Kirsten Steinke, a doctoral student working with biological oceanographer Kim Bernard at Oregon State.

“This ecologically important species serves as the base of the food web in the Antarctic peninsula, supporting everything from whales to penguins to seabirds,” said Steinke, the study’s lead author in a media release. “Understanding the connection between the environment and population health is critical for predicting future demographic patterns and responses to climate change in the krill population.”

The study published in the journal Marine Ecology Progress Series studies the impact of warming oceans on Antarctic krill, also known as Euphausia superba – a type of zooplankton that can live five to seven years and grow to a length of a little more than two inches.

A prominent breeding ground for krill, the Atlantic peninsula is a large fishing zone for krill. Krill fishing is a highly lucrative business and the krill are mainly used to produce fish food for the aquarium trade. They are a dense source of protein and also used in fish-based health supplements and are fished in high quantities. The Antarctic marine ecosystem is at risk of overfishing krill and being such a huge breeding area, this affects the krill population worldwide. An estimated 313,000 tons of krill was harvested in the region in 2018.

“This region is critically important because it is both a popular fishing spot and one of the biggest spots for krill spawning and it is also warming more quickly than other parts of Antarctica,” Steinke said. “There has been a notable poleward contraction of the population and a decrease in population size in recent years.”

How is climate change affecting krill?

Why is there such a large drop or fluctuation in krill density in the region? If overfishing is a major reason, the numbers should decline steadily instead of a periodic fluctuation. This points to a more insidious cause that is harder to predict and control. 

“You tend to see two years of high krill recruitment, meaning a high proportion of juvenile krill in the population, and then a crash, and then the population starts to rebound again,” said oceanographer Kim Bernard, who is a part of the team researching krill in the Antarctic Penninsula. “Understanding what is driving that cycle is critical.”

Using historic data, the team found a direct link between the reproductive health of female krill and the seasonal increases in population. The health and maturity of the female krill population should be in peak condition during spawning season and this allows for more offspring the following year.

And these mature female krill were affected by changes in the water temperature around the Antarctic waters. Seasonal variations were the main driving factor for the fluctuation in the number of healthy female krill during the spawning season.

The climate in the western Antarctic Peninsula is determined by two factors – the Southern Annual Mode (SAM) and the Multivariate El Niño Southern Oscillation Index (MEI). They aid female krill by bringing in nutrient-rich waters periodically, allowing them to stay healthy.

SAM and MEI are natural climate patterns but they are changing as the planet warms. The SAM in particular has been trending positive, meaning it has been warmer and more intense. That positive phase is projected to continue under climate change, the study said.

“The SAM was found to be really important to driving the health of the female krill,” she said. “As the SAM continues to trend positive, it will continue to get warmer, and that suggests a negative effect on the overall condition of female krill during their spawning season.”

Future Impact

“It is really critical to start including climate change impacts as part of the plan,” Bernard said. “Antarctic krill are a super unique and fascinating species. So many predators feed on them. If you have a collapse of the krill population, you would be putting all of those populations at risk.”

“If there are a lot of mature female krill, the chicks can bulk up and survive the winter,” Bernard said. “But the Adélie penguin population has plummeted at the northern parts of the Antarctic Peninsula in recent years, in part because of changes in the krill population.”

This shows a need for critical management of krill fishing and understanding how nature is adapting to rapid changes in climate. Krill are crucial to the marine ecosystem as a whole. By managing fishing during the downtrend in reproductive patterns can really help manage fishing activity. If there is a predictable fluctuation in healthy females, controlling fishing in the area can help keep numbers healthy for the next season where more healthy females are present.

The effects of global warming are unavoidable but our methods and way of life can adapt to help give the natural ecosystem more time to heal. The study of krill goes to show all the ways the natural weather systems are changing, causing changes in the smallest organisms. The negative impact to small, ecosystem builders in any food chain could cause a major slide in the population of larger creatures. As seen with the penguin population, larger creatures are often dependant on creatures lower in the food chain. By monitoring climate change, we can try and help nature heal.