Food Chains: The Carbon Link; Borrowing a Technique from Geochemistry, Ecologists Move from Guesses to Numbers

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Food Chains: The Carbon Link

Apolar bear kills and eats a ring seal, forging one link in an Arctic food chain. From this simple observation, ecologists can draw a conclusion: Atoms from the seal will become part of the bear. But to link the bear all the way down the food chain to the one-celled phytoplankton building sugar molecules from carbon dioxide, two scientists resort to a mroe exotic technique: trimming the bear's toenails.

Ecologists have strung together food chains for decades, tracing food backward from consumers to primary producers such as phytoplankton -- microscopic algae thought to occupy the lower end of the chain in the oceans. Traditionally, such efforts have relied on observation and guesswork. But in recent years, a growing number of ecologists have added a quantitative technique, called stable isotope tracing, to their expanding kit of research tools. While geochemists have used the technique since the 1940s, ecologists have only recently recognized its potential in their field.

"In the past five years, it's taken off in terms of mapping carbon flow in food webs," says Charles A. Simenstad of the University of Washington in Seattle. "It certainly is changing a lot of our concepts of food webs." Before ecologists took up isotope tracing, he adds, "all we could do was sit down and play accountant. I don't think we considered it guesswork in those days, but in hindsight, it certainly was."

Scientists studying food webs can trace isotopes in bits of tissue -- bear-claw parings, for example -- to answer questions such as where a polar bear fed last winter, where a whale migrated last year, what kind of grass a bison grazed 10,000 years ago and whether phytoplankton supply most of the food in coastal marine food webs. Understanding where an animal spends its time and what it eats can help scientists protect fragile ecosystems from encroaching human habitation, manage hunting and assess the long-term environmental effects of pollution.

If ecologists could dye plankton with an indelible red food coloring, their jobs would be easier. As larger organisms devoured the plankton and the telltale red atoms made their way through the food chain, the red would remain, eventually tinting the polar bear's fur a delicate pink. Stable, or nonradioactive, isotopes have no color, but like the imaginary food coloring, they can serve as a label that persists through an ecosystem.

All elements come in various forms called isotopes, each isotope carrying a different number of neutrons. The number of protons -- the other type of particle in an atom's nucleus -- remains constant from one isotope to the next. Extra neutrons affect an atom's chemical activity only slightly, so living things can use common isotopes as well as the rarer variants. Isotopes of nitrogen, sulfur, oxygen and carbon all figure into biological processes and all leave their mark in tissues. Increasingly, ecologists analyze isotopes of more than one element to untangle knotty food webs. But they used carbon first, and for many it remains the element of choice.

Nearly all of Earth's carbon atoms contain 12 neutrons; about 1 percent have 13 and a vanishingly small number are even heavier with 14. In carbon, one extra neutron adds weight but two add instability. Carbon-14, a radioactive isotope, decays slowly, while both carbon-12 and carbon-13 remain stable.

To measure the relative amounts of isotopes in tissue, scientists use a mass spectrometer designed specifically for the task. After scrupulously cleaning a small sample, they burn it until nothing remains but carbon dioxide gas that the spectrometer can read. Finally, they compare the ratio of carbon-13 to carbon-12 in the gas to an international standard. Variations from the standard amount to only a few parts per thousand.

Organisms that capture the sun's energy, such as green land plants and phytoplankton, serve as the base of a food pyramid for all other living things on Earth. …