PARENT SESSION
Session #22: Observation, manipulation, experimentation and extrapolation: Limitations and opportunities in ecosystem-level studies. Organized by: R.H. Gardner and J.E. Petersen.
Friday, August 10, 2001. Lecture Hall
Connecting aquatic food webs to organic C from the watershed: Do maple leaves feed fish?
Cole, Jonathan1, Pace, Michael1, Carpenter, Stephen2, Kitchell, James2, 1 2
ABSTRACT- In many aquatic ecosystems the loading of organic C from the watershed exceeds the supply from autochthonous primary production. While this externally-supplied organic C has been thought of as refractory, multiple lines of evidence indicate that substantial portions are decomposed in the receiving aquatic ecosystem. In many lakes, the combined respiration of all organisms (R) often exceeds gross primary production (GPP) causing net ecosystem production (NEP = GPP-R) to be negative. Such systems are net heterotrophic; respiration is subsidized by inputs from the watershed. This subsidy can be large. In a series of small lakes in N. Wisconsin, we found that the respiration of terrestrial materials accounted to from 13 to 40% of total lake respiration, varying inversely with GPP. To what extent does this large metabolism of terrestrial C support the secondary production of invertebrates and fish? Comparative studies, using ambient levels of stable isotopes suggest that terrestrial organic C comprises a large portion of secondary production. A preliminary experiment in which we manipulated the 13C content of dissolved inorganic C, thereby labeling autochthonous production, suggests that zooplankton, the major food of small fish, are essentially independent of the terrestrial subsidy. The combination of the experiment and modeling is necessary to reconcile these apparently contradictory results.
KEY WORDS: lake, carbon, stable isotope, food web