With funding from the Ecosystem Studies Program of the National Science Foundation (DEB), Drs. Michael Pace (IES) and Jon Cole, along with colleagues from the University of Wisconsin (Drs. Steve Carpenter and Jim Kitchell) and St. Norbert College (Dr. Jim Hodgson), are experimentally testing the hypothesis that terrestrial organic carbon supports a significant proportion of the secondary production of consumers, including fish, in lakes. We measured the contribution of internal primary production (autochthony) and external organic carbon (allochthony) to food webs by altering the ¹³C concentration of dissolved inorganic carbon (DIC), thereby causing in-lake primary production to be very enriched in ¹³C relative to organic matter from terrestrial inputs. In many lakes the isotopic composition of the CO₂ moiety of DIC (the proximate substrate for photosynthesis), and fractionation of that CO₂ during photosynthesis causes C of phytoplankton origin to be nearly identical in ¹³C to organic C of terrestrial origin Our experiments overcome this problem that often confounds natural abundance studies. We conducted one pulse experiment (Cole et al., 2002) in which a single addition of ¹³C was made and four press experiments (continuous daily additions; Pace et al., 2004; Kritzberg et al., 2004) in lakes that contrasted in colored (i.e. chromophoric) dissolved organic carbon, nutrients and food webs. These experiments increased total DIC < 1% but greatly increased d¹³C of DIC.

The experimental enrichments of ¹³C allow tracing of flows and estimates of sources that are otherwise difficult to determine. We found there was large variation among lakes in terrestrial C support of the food web and that allochthony decreased with nutrient enrichment in two experimental lakes. In unenriched lakes:

• About 50% of the POC has an allochthonous origin.
• 20-50% of the C in Daphnia (a herbivore) is ultimately derived from allochthonous C with the balance from within-lake primary production.
• DOC became enriched with ¹³C during the additions demonstrating that the ambient DOC pool is a dynamic mixture of terrestrial and aquatic sources even in tea-colored lakes with moderately high concentrations of DOC.
• Planktonic bacteria derive about 40-60% of their carbon from allochthonous sources.
• Young bass are a mixture of allochthonous and autochthonous C through feeding on Daphnia and other zooplankton indicating that fish are partially made of terrestrial C.

These experiments suggest that the lake food webs of small lakes are indeed coupled to watershed carbon sources. This coupling clearly varies with nutrient loading, and may vary as a function of a number of other lake features including organic matter and loading and possibly lake size.

Figure Legend: Small lakes in Northern Wisconsin. Small lakes such as these are numerically dominant on the landscape and an important place for the processing of organic C derived from the surrounding forest.

Figure Legend: Direct measurement of allochthony in small lakes. "Allochthony" is the fraction of a component supported by terrestrial inputs. In this case we show the fraction of zooplankton C supported by terrestrial inputs. The data were obtained by adding inorganic ¹³C to whole lakes and tracing this through the food web using two different modeling approached: a dual isotope flow box model "BOX"; and a statistical process model (STAT). The lakes marked "fertilized" received additions of N and P to stimulate phytoplankton growth; the lakes marked "ambient" did not. To learn more see the following publications: Pace et al., 2004; Cole et al., 2002, Kritzberg et al., 2004 or read more in the Cascade website.