Press Release

Embargoed until 1:00pm EST of January 14, 2004

CONTACT
Lori M. Quillen, Public Information
Telephone (direct dial): 845-677-7600 x 321
E-mail Lori Quillen

Are Fish Made of Maple Leaves?

Read the complete article:
Whole-lake carbon-13 additions reveal terrestrial support of aquatic food webs (pdf format, 214 KB)

Many of us learned about the aquatic food web in high school. Our teachers explained that aquatic plants form the base of the food web, with the energy they capture from the sun supporting aquatic life, from invertebrates to the largest sport fish. However, a new study shows that aquatic animals are receiving a little help from trees and plants surrounding the lakes they live in. A recent Nature paper authored by Institute of Ecosystem Studies scientists Drs. Michael L. Pace and Jonathan J. Cole, along with colleagues from Wisconsin and Sweden, indicates that terrestrial organic matter that originates on the shoreline supports a significant part of the aquatic food chain.

A building block of life, organic carbon is essential to aquatic food webs. In lakes, aquatic plants produce organic carbon by harnessing the sun's energy (photosynthesis); some of this carbon supports the growth of fish and invertebrate populations. Scientists have long suspected that land-grown organic matter is also significant to aquatic life, but this idea is difficult to demonstrate. By tracing the fate of carbon through large-scale lake manipulations, Drs. Pace, Cole, and colleagues, have revealed that in some water bodies the aquatic food web is significantly subsidized by terrestrial organic carbon.

That maple tree leaves may eventually become perch, and that the vegetation around a water body can have profound impacts on the animal life within the body of water, blurs the perceived ecological boundaries between aquatic and terrestrial systems. The study confirms that the health of lakes is tied to the condition of their watershed, and that aquatic health cannot be assessed without considering surrounding terrestrial systems.

The impetus behind the study, which involved manipulating two Michigan lakes, was a better understanding of the aquatic food chain. Dr. Pace explains, "We wanted to reveal what many ecologists have long thought- aquatic life is partly dependent on organic matter produced in the watershed." Using a motorboat and a chemical tracer, Dr. Pace and his colleagues set out to quantify this assumption. "The moral of the story," Dr. Pace comments, "is, yes, fish are made from algae, but fish are also partly made of maple leaves."

Terrestrial carbon and aquatic carbon are virtually identical. To determine the percentage of aquatic life supported by the two types of carbon, the researchers needed to differentiate between them. By applying a baking soda-like solution (NaH¹³CO₃) to Lakes Peter and Paul, located at the University of Notre Dame Research Center, they were able to test if lake plant production was sufficient to support resident aquatic life.

Unlike regular baking soda, NaH¹³CO₃ has distinct carbon atoms with one neutron more than is typically found in the most abundant form of carbon, ¹²C. The unique ¹³C signature of the mixture allows it to be traced when it is taken up by aquatic plants during photosynthesis and then moved through the food web.

The NaH¹³CO₃ powder was applied to the lakes over the course of 43 days. Each day, micro-animals, as well as larger animals such as fish, were gathered from the lakes for testing. Using a mass spectrometer, the researchers were able to determine if the lake organisms were using aquatic-produced carbon or terrestrial-produced carbon.

Their findings, that 40-55% of particulate carbon and 22-50% of zooplankton (small animals that live in the water column) in the lakes are derived from terrestrial sources, confirm that lake life is dependent on inputs from outside the aquatic system. The carbon signatures of zooplankton, a dietary staple for many fish, reflect their dependence on both internal plant production and terrestrial organic matter. A future paper will explore the ¹³C signatures found in fish. "Our results," Dr. Cole remarks, "tell us there is not nearly enough aquatic carbon to support these animals; they are dependent on terrestrial inputs."

Dr. Pace comments, "We now have direct experimental evidence to confirm that aquatic food chains are supported not just by the production of plants in the water but also by the production of plants on the land surrounding lakes and ponds. The leaves and organic matter that enter lakes are ultimately incorporated into aquatic animals." These findings challenge traditional views of the aquatic food web and may help inform watershed management. Dr. Pace concludes, "Organic matter from the watershed subsidizes lake food webs allowing more animal life than if they were simply isolated water bodies."

Jim Morris, the program director of the National Science Foundation's Division of Environmental Biology, which funded the study, comments, "These scientists have found an ingenious method of teasing apart the carbon cycles of lakes." Adding, "Their study findings reinforce the concept that the ecology of lake ecosystems is tightly coupled with the terrestrial landscape."

sign-up for email updates