Surveying aquatic life from the Great Lakes to small ponds, ecologists at Cornell University and the Institute of Ecosystem Studies (IES) have found that food-chain length -- the number of mouths food passes through on the way to the top predators -- is determined by the size of an ecosystem, not by the amount of available food energy.

The finding, which is reported in the June 29 issue of the journal Nature, should help resolve one of the oldest questions in ecological science: How long are food chains and what determines their length?

But the systematic accounting of food-chain links and lengths also serves as a warning: The so-called top predators -- such as eagles, falcons and most of the fish that anglers catch to eat -- are more likely to accumulate high concentrations of mercury, PCBs and other contaminants when they live in large ecosystems. And that puts humans, the ultimate predators, at risk.

"If your neighbor comes home with two fish -- a 10-pound trout from Lake Erie and the same size fish from a smaller lake -- ask for the fish from the small lake. Your risk of consuming bioaccumulated contaminants is probably lower with the trout from the smaller lake," said David M. Post in a prepublication interview.

A graduate student of ecology at Cornell and IES, Post is one of three authors (together with Michael L. Pace, aquatic ecologist at IES in Millbrook, N.Y., and Nelson G. Hairston Jr., professor of ecology and evolutionary biology at Cornell) of the report titled "Ecosystem size determines food-chain length in lakes." Their results show that larger lakes have longer food chains than smaller lakes. The same principle may apply to food chains in terrestrial ecosystems, such as large forests and smaller forest fragments, Post observed.

"The standard, textbook explanation for 70 years has been that food-chain length is determined by energy availability," Post said. "Our results show that it is not energy availability that constrains food-chain length, but rather something else related to the size of the ecosystem. We do not know what that something is just yet, but it's clearly not the amount of energy."

The study was conducted in large lakes (Erie and Ontario), medium-sized lakes (such as Champlain, between Vermont and New York, and some of the larger Finger Lakes in western New York) as well as small lakes and several ponds near Madison, Wis., and West Point, N.Y. Researchers studied aquatic food chains that begin with green photosynthetic algae and culminate in large fish like walleye, lake trout, northern pike and largemouth bass. Between the bottom and the top are dozens and perhaps hundreds -- depending on the size of the ecosystem -- of intermediary organisms, such as tiny invertebrates, little fish and medium-sized fish.

Because the researchers couldn't hold their breath long enough to follow food chains from the bottom to the top and count the mouths along the way, they essentially counted backward from the top. In tissue samples of fish in each lake and pond, the researchers measured the ratio of the stable isotopes of nitrogen, a common constituent of all organisms.

Most of the nitrogen in the world is nitrogen-14, but a small percentage is the heavier nitrogen-15. As nitrogen moves up the food chain, more nitrogen-15 than nitrogen-14 accumulates in the tissue of animals at higher "trophic positions." By knowing the ratio of nitrogen-15 to nitrogen-14 of fish at the top of the food chain, compared with the ratio of nitrogen-15 to nitrogen-14 for organisms at the bottom, Post and his colleagues could measure the length of the food chains.

In ecological terms, the researchers were trying to determine the trophic position of various organisms in the food chains, Post explained: "A vegetarian and a cow occupy roughly the same trophic position and would have the same amount of nitrogen-15 in their tissue when they're both eating the same plants; a carnivore who eats the cow that ate the plants is at a higher trophic position and would have more nitrogen-15 in his tissue than the vegetarian or the cow," he said.

What the researchers at Cornell and IES found surprised them because it showed that productivity and energy availability are less important to food-chain length than the size of the ecosystem: Small, nutrient-rich and highly productive lakes have shorter food chains than do the larger, low-productivity, crystal-clear lakes. And big lakes have longer food chains than smaller lakes with the same productivity levels. The 10-pound fish from the large lake is feeding at a higher trophic position than a 10-pound fish from the small lake.

The finding helps explain why a large forest will support greater diversity of life forms than will a forest fragment. But it probably won't change strategies for sport fishing, Post says. "You are still more likely to catch a big fish in a nutrient-rich, productive lake simply because there are more fish to catch. Just be aware," says the Cornell ecologist, "that the big fish from the large lake is at the top of a longer food chain and that more contaminants may have accumulated. The old saying, 'You are what you eat,' applies to whatever -- or whomever -- is at the top of the food chain."