23. Sources of Organic Carbon in the Food Webs of Two Florida Lakes Indicated by Stable Isotopes

Carbon cycling pathways in lacustrine systems are complex because there are often multiple sources of organic carbon available to the food webs. Among the techniques used to delineate carbon flows from organic matter to consumers, stable isotope analysis may be the most powerful one because isotope compositions of consumers reflect those of the dietary carbon assimilated and incorporated into their tissues and because no system manipulation is involved. The use of stable carbon isotopes in food web study is based on these premises: (1) there is a broad isotope range among different sources of organic matter (Peterson and Fry, 1987) and (2) consumer ODC closely resembles their diets within 1%0 (DeNiro and Epstein, 1978). Stable isotope analysis has been successfully applied to the investigations of carbon flows in some lacustrine food webs (e.g., Hecky and Hesslein, 1995). .The objective of this study was to determine carbon sources that supported the food webs of lakes Apopka and Okahumpka, Florida, phytoplanktonand macrophyte-dominated systems, respectively. We used stable carbon isotopes as natural tracers to illustrate carbon flows in each food web.