Stable isotopes of carbon and nitrogen as indicators of diet and trophic structure of the fish community in a shallow hypereutrophic lake

Abstract

Stable carbon (δ 13 C) and nitrogen (δ 15 N) isotopes were employed to elucidate energy flows and trophic interactions in Lake Apopka, a hypereutrophic lake in central Florida, U.S.A. Isotope compositions of lake biota ranged from −27·1 to −3·0‰ for δ 13 C, and from 3·7 to 13·9‰ for δ 15 N. The food web was based primarily on plankton production with diatoms, Microcystis and zooplankton dominating the diet of fish. Carbon isotope evidence showed that pico‐ and nano‐phytoplankton were not a direct carbon source for fish, but were important to zooplankton. δ 15 N mass balance estimates indicated that planktivorous fish obtained 48–85% of their diets from zooplankton. The ∼3‰ range of δ 15 N in gizzard shad reflected increasing dependence on zooplankton as fish grew whereas the positive relationship between total length and δ 15 N of largemouth bass reflected increasing predation on larger planktivorous fish with growth. The broad ranges of δ 13 C (−25·9 to −9·5‰) and δ 15 N (5·8 to 14·4‰) of blue tilapia were indicators of diet diversity. Two presumed omnivores (brown bullhead and white catfish) and piscivores (black crappie, largemouth bass and Florida gar) were found to depend on planktivorous fish. However, stable isotope data revealed no trophic links between blue tilapia, an abundant fish in the near‐shore area, and piscivores.

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Affiliated Institutions

• University of Florida US

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