Of Cod and Climate

SCIENCE

Ancient cod bones unearthed at an Alaska archaeological site carry a very modern warning for a world with a rapidly changing climate—as sea levels rise, so do levels of mercury in the food web. (Alaska Dispatch News)

Use this activity to learn more about marine food webs.

Teachers, scroll down for a short list of key resources in our Teachers’ Toolkit.

The healthy, modern specimens of Pacific cod in this spectacular photo were caught around Alaska's Lake and Peninsula Borough, not far from where scientists discovered mercury-rich bones of Pacific cod dating from just after the last Ice Age. Love this image. Photograph by Nick Rahaim, courtesy Flickr. CC-BY-NC-SA 2.0

The healthy, modern specimens of Pacific cod in this spectacular photo were caught around Alaska’s Lake and Peninsula Borough, not far from where scientists discovered mercury-rich bones of Pacific cod dating from just after the last Ice Age. Love this image.
Photograph by Nick Rahaim, courtesy Flickr. CC-BY-NC-SA 2.0

Discussion Ideas

 

Click here for more information on this and other marine communities—including a labeled guide to this illustration. Illustration by Doris Dialogu, National Geographic

Click here for more information on this and other marine communities—including a labeled guide to this illustration.
Illustration by Doris Dialogu, National Geographic

  • Pacific cod are intermediate predators in Arctic or near-Arctic marine food webs. What other organisms might be part of a Pacific cod’s food web—producers, first-order consumers, intermediate predators, and top predators? What about scavengers and decomposers? Read through our activity “Marine Food Webs” and take a look our Arctic Community illustration above for some ideas.
    • The Pacific cod’s Arctic food web might include producers such as phytoplankton.
    • First-order consumers might include jellies and krill, as well as small fish such as herring or sardines.
    • Intermediate predators (such as the Pacific cod) might include other fish, such as pollock.
    • Top predators may include polar bears and seals.
    • Scavengers might include gulls and crabs.
    • Decomposers might include species of bacteria and copepods.

 

  • Scientists say mercury bioaccumulation and biomagnification were linked to sea level rise and climate change. What is bioaccumulation? What is biomagnification?
    • Bioaccumulation is the process by which chemicals (such as mercury) are absorbed by an organism, either from exposure to a substance with the chemical or by consumption of food containing the chemical.
    • Biomagnification is the process in which the concentration of a substance (such as mercury) increases as it passes up the food chain.

 

  • How was bioaccumulation of mercury linked to sea level rise and climate change in ancient Alaska?
    • Thousands of years ago, at the end of the last Ice Age, glaciers melted and caused sea levels to rise. “In Alaska, the Bering Sea nearly doubled in size and stretches of coastline to the south were flooded,” according to the ADN. The rising sea unlocked natural pockets of mercury that were trapped in land that was dry or frozen during the Ice Age.
    • The process of bioaccumulation allowed this seaborne mercury to be absorbed by phytoplankton and sea plants. As first-order consumers such as sardines consumed the plankton, the process of biomagnification increased the concentration of mercury in the consumer’s body. The bones and (presumably) muscle of intermediate predators—the cod—developed an even greater concentration of mercury as they ate the sardines. The people of the ancient Ocean Bay culture at what is now the Amalik Bay Archeological District consumed cod and even top predators such as seals and whales. Mercury accumulated at the highest concentration in their bodies.

 

  • Why do mercury levels in fish matter to people?
    • People eat fish, and mercury is highly toxic. According to the USGS, mercury “affects the immune system, alters genetic and enzyme systems, and damages the nervous system, including coordination and the senses of touch, taste, and sight.” People are exposed to methylmercury (the most toxic form of mercury, where it has combined with carbon and hydrogen) almost entirely by eating contaminated fish and wildlife that are at the top of aquatic food chains, and “[e]ven at very low atmospheric deposition rates in locations remote from point sources, mercury biomagnification can result in toxic effects in consumers at the top of these aquatic food chains.”

 

  • Why is this study of 4,000-year-old fish bones relevant today?
    • We are currently living in a warming period when sea levels are once again rising. Mercury trapped in frozen or dry areas may again be released through coastal flooding and permafrost thaw.
      • “With the predicted increase in rainfall along the Aleutians and in western Alaska in the next 50 to 100 years,” says once expert in the ADN article, “with glaciers melting and ice levels melting, mercury will become more mobile and go into the food chain.”

 

TEACHERS’ TOOLKIT

Alaska Dispatch News: Ancient cod bones carry modern warning about mercury, climate change

Nat Geo: Marine Food Webs activity

Nat Geo: Marine Communities illustrations

(extra credit!) Frontiers in Environmental Science: Biogeochemical analysis of ancient Pacific Cod bone suggests Hg bioaccumulation was linked to paleo sea level rise and climate change

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