PORTLAND, Ore. — Penguins preserve records of Antarctic environmental change. The birds’ feathers and eggshells contain the chemical fingerprints of variations in diet, food web structure and even climate, researchers reported February 12 at the American Geophysical Union’s 2018 Ocean Sciences Meeting.
The Antarctic environment has changed dramatically in recent decades. Overfishing has led to a decline in krill, small swimming crustaceans that are a key food source for birds, whales, fish and penguins in the Southern Ocean. Climate change is altering wind directions, creating open water regions in the sea ice that become hot spots for life.
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These changes have cascading effects on food webs and on the cycling of nutrients. “Penguins are excellent bioarchives of this change,” says Kelton McMahon, an oceanic ecogeochemist at the University of Rhode Island in Kingston.
Penguins are at the heart of the Antarctic food web, and their tissues are known to capture details about what they’ve eaten. Different food sources contain different proportions of carbon and nitrogen isotopes, forms of the elements with different numbers of neutrons. For example, food sources such as krill and fish have different amounts of nitrogen-15 relative to nitrogen-14. The tissues of penguins, such as feathers and eggshells, preserve these proportions.
Previous studies had already noted a large shift in isotopic values in penguin tissues in the last 80 years, but those studies couldn’t distinguish between shifts in the penguins’ diet versus climate-related shifts in the isotopic values of the microscopic creatures at the very bottom of the food web. So McMahon and his colleagues created a tool to make this distinction — and ultimately to track Antarctic environmental changes through time.
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The team focused on the isotopic values of individual amino acids, the building blocks of proteins. Those values reveal “a lot about the biochemistry happening inside the body,” McMahon says. Some of these values are significantly altered as food is digested and incorporated into an animal’s body; others are little changed.
To understand what the wild penguins had been eating through time, the team first developed a set of “chemical fingerprints” for the isotopic values of a dozen different amino acids by mapping how the values found in Atlantic herring, a dietary staple, changed in the penguins’ bodies after digestion. The researchers acquired these data through a controlled feeding study in collaboration with the Omaha Henry Doorly Zoo and Aquarium in Nebraska that monitored precisely what, when and how much a population of Gentoo penguins ate. Comparing the chemical fingerprints with wild Gentoo penguin tissues revealed what the wild penguins must have eaten in the past.
Over the past 80 years, the penguins have shifted from eating mostly fish to eating primarily krill and then back to fish, the team reported. There is probably a straightforward historical explanation, McMahon said: In the late 1800s to the mid-1900s, whalers extensively hunted marine mammals that tend to dine on krill. Penguins likely took advantage of the resulting krill surplus. But from the 1970s to 1990s, krill harvesting ramped up, and penguins shifted back to a fish-dominated diet.
But there’s more to the story. Certain amino acids in the penguins’ food are known to pass through the body with their isotopic values essentially unchanged. In fact, the isotopes in those amino acids are thought to reflect the original isotopic values of the creatures at the very base of the Antarctic food web: phytoplankton floating in the Southern Ocean. Because climate drives the isotopic values of those phytoplankton, the isotopes in those amino acids are a record of climate change.
In the case of nitrogen, higher nitrogen-15 values relative to nitrogen-14 in phytoplankton are likely linked to more open water spaces within the sea ice, McMahon said. Such open spaces, called polynyas, have appeared in recent decades around Antarctica due to changes in wind directions linked to climate change.
This study highlights the power of this amino acid isotope technique to track environmental change through animal tissues, says Seth Newsome, an animal ecologist at the University of New Mexico in Albuquerque who was not involved in the study. The technique is becoming popular because it can detect both diet and baseline changes in the food web from the same tissue, he says.
“This 80-year record is just part of a much broader record of change,” McMahon said. The team plans next to look further back in time. Excavations have revealed penguin eggshells as old as 10,000 years that have a story to tell.