Are fish made of maple leaves?

01/11/04

Scientists shed light on how terrestrial vegetation impacts aquatic life



Aerial view of Lakes Peter and Paul(photo: Steve Carpenter)

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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 create supporting aquatic life, from invertebrates to the largest sport fish. However, a new study shows that aquatic plants are receiving a little help from trees. 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 a significant part of the aquatic food chain is supported by terrestrial organic matter that originates on the shoreline.

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 organic carbon from land is also significant to aquatic life but this idea is difficult to demonstrate. By tracing the fate of carbon through large-scale lake manipulations, Pace, Cole, and colleagues have revealed that in some water bodies the aquatic food web is significantly subsidized by terrestrial organic carbon.



Institute of Ecosystem Studies Research Assistant Matt Van de Bogert and Jonathan Cole preparing NaH13CO3 on Peter Lake

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That maple tree leaves many eventually become perch, and that the vegetation around a water body can have profound impacts on the animal life within the body of water-- both blur the perceived ecological boundaries between aquatic and terrestrial systems.

The impetus behind the study, which involved manipulating two Michigan lakes, was a better understanding of the aquatic food chain. 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 chemical tracer, Pace and his colleagues set out to quantify this assumption. "The moral of the story," Pace comments, "is, yes, fish are made from algae, but fish are also partly made of maple leaves."



Emma Kritzberg (Lund University, Sweden) and Crystal Fankhauser (University of Wisconsin-Madison) on Peter Lake (photo: Steve Carpenter)

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By applying the tracer NaH13CO3 to Lakes Peter and Paul (located at the University of Notre Dame Research Center), the scientists tested if lake plant production was sufficient to support resident aquatic life. Similar to baking soda, NaH13CO3 has distinct carbon atoms (13C), with one neutron more than is typically found in the most abundant form of carbon (12C). During photosynthesis 13C is taken up by plants, providing a unique signature that can be traced as it moves through the food web. By labelling the carbon produced by aquatic plants with 13C, the scientists were able to use a mass spectrometer to determine if 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 terrestrial carbon fuels aquatic production. The carbon signatures of the zooplankton reflect their dependence on both internal plant production and terrestrial organic matter. Zooplankton are a dietary staple for many fish, especially very young life stages . "Our results," notes Cole, "tell us there is not nearly enough aquatic carbon to support these animals, they are dependent on terrestrial inputs."

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. "Organic matter from the watershed subsidizes lake food webs allowing more animal life than if they were simply isolated water bodies," Pace concludes.

Source: Eurekalert & others

Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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