STABLE ISOTOPES OF CARBON AND NITROGEN AS INDICATORS OF DIET AND TROPHIC STRUCTURE OF THE FISH COMMUNITY IN A SHALLOW HYPEREUTROPHIC LAKE

Stable carbon (delta(13)C) and nitrogen (delta(15)N) isotopes were emp loyed to elucidate energy hows and trophic interactions in Lake Apopka , a hypereutrophic lake in central Florida, U.S.A. Isotope composition s of lake biota ranged from -27.1 to -3.0 parts per thousand for delta (13)C, and from 3.7 to 13.9 parts per thousand for delta(15)N. The foo d web was based primarily on plankton production with diatoms, Microcy stis and zooplankton dominating the diet of fish. Carbon isotope evide nce showed that pico- and nano-phytoplankton were not a direct carbon source for fish, but were important to zooplankton. delta(15)N mass ba lance estimates indicated that planktivorous fish obtained 48-85% of t heir diets from zooplankton. The similar to 3 parts per thousand range of delta(15)N in gizzard shad reflected increasing dependence on zoop lankton as fish grew whereas the positive relationship between total l ength and delta(15)N of largemouth bass reflected increasing predation on larger planktivorous fish with growth. The broad ranges of delta(1 3)C (-25.9 to -9.5 parts per thousand) and delta(15)N (5.8 to 14.4 par ts per thousand) of blue tilapia were indicators of diet diversity. Tw o presumed omnivores (brown bullhead and while catfish) and piscivores (black crappie, largemouth bass and Florida gar) were found to depend on planktivorous fish. However, stable isotope data revealed no troph ic links between blue tilapia, an abundant fish in the near-shore area , and piscivores. (C) 1996 The Fisheries Society of the British Isles