SS3.10 Ecological Implications of Terrestrial Inputs into Lakes and Ponds
Date: Tuesday, June 11, 2002
Time: 10:15:00 AM
Location: Esquimalt
ColeJJ, Institute of Ecosystem Studies, Millbrook, NY, USA,
Carpenter, S, R, Center for Limnology, University of Wisconsin, Madison, WI, USA,
Hodgson, J, , St. Norbert College, De Pere, WI, USA,
Kitchell, J, F, Center for Limnolody, University of Wisconsin, Madison WI, USA,
Pace, M, L, Institute of Ecosystem Studies, Millbrook, NY, USA,
In many small aquatic ecosystems watershed loading of organic C exceeds autochthonous primary production. While this allochthonous organic C has long been thought of as refractory, multiple lines of evidence indicate that substantial portions are respired in the receiving aquatic ecosystem. This raises two questions. To what extent does this terrestrial C support secondary production of invertebrates and fish? Do current models adequately trace the pathways of allochthonous and autochthonous C through the food web? We evaluated the roles of allochthonous and autochthonous organic C, by manipulating 13-C content of dissolved inorganic C in a small, softwater, humic lake, thereby labeling autochthonous primary production for about 20 d. To insure rapid and sufficient uptake of inorganic 13-C, we enriched the lake with modest amounts of N and P. We constructed a carbon flow model based on the ambient and manipulated levels of 13-C in C compartments in the lake, along with information on key rate processes. Several results emerged from this combination of model and experiment: 1) fractionation of photosynthetically-assimilated 13C-CO2 by phytoplankton (epsilon) is lower (6 per mil) than physiologic models would estimate (> 20 per mil); 2) bacteria respire, but do not assimilate, a large amount of terrestrially-derived dissolved organic C (DOC) but pass little of this C to higher trophic levels; 3) the respiration of terrestrial DOC is the major source of dissolved inorganic C in the lake; and, 4) zooplankton production, a major food of young-of-year fishes, is primarily derived from autochthonous carbon sources and is essentially independent of the large terrestrial organic C subsidy to this ecosystem, under the conditions of this experiment.