Patterns of food chain length in lakes: A stable isotope study

Food web structure is paramount in regulating a variety of ecologic patterns and processes, although food web studies are limited by poor empirical descriptions of inherently complex systems. In this study, stable isotope ratios (delta(15)N and delta(13)C) were used to quantify trophic relationships and food chain length (measured as a continuous variable) in 14 Ontario and Quebec lakes. All lakes contained lake trout as the top predator, although lakes differed in the presumed number of trophic levels leading to this species. The presumed number of trophic levels was correlated with food chain length and explained 40% of the among-lake variation, food chain length was most closely related to fish species richness (r(2) = 0.69) and lake area (r(2) = 0.50). However, the two largest study lakes had shorter food chains than lakes of intermediate size and species richness, producing hump-shaped relationships with food chain length. Lake productivity was not a powerful predictor of food chain length (r(2) = 0.36), and we argue that productive space (productivity multiplied by area) is a more accurate measure of available energy. This study addresses the need for improved food web descriptions that incorporate information about energy flow and the relative importance of trophic pathways.