Food webs of tropical, temperate, and arctic lakes can be characterized by the carbon and nitrogen stable isotope ratios of their constituent organisms. After assigning trophic levels using δ ¹⁵N, a broad range of δ ¹³C is observed at the primary consumer level in nearly all lakes. The range of δ ¹³C is on the order of 20 ‰ in tropical lakes Kyoga and Malawi and lakes with low dissolved inorganic carbon in temperate Canada, but is narrower in shallow lakes of the Canadian arctic. This broad range exists in ecosystems in which terrestrial inputs and/or aquatic macrophytes are often minimal. The isotopically light end of the range results from phytoplankton photosynthesis whereas the isotopically heavy end represents benthic algae photosynthesizing within an unstirred boundary layer. This range is successfully predicted by an application of a simple isotopic model for photosynthetic fractionation, originally developed for aquatic macrophytes, which uses boundary layer thicknesses reported for benthic algal communities. When benthic photosynthesis becomes light-limited in very turbid lakes of the Mackenzie Delta, then phytoplanktonic carbon dominates the diet of the primary consumers. The organisms on the primary consumer trophic level appear from their δ ¹³C values to harvest preferentially either planktonic or benthic algal carbon but, in temperate and arctic lakes, higher consumer levels are increasingly omnivorous. Therefore top aquatic predators often have a narrow range of δ ¹³C. In temperate and arctic lakes these top predators have a δ ¹³C near the midpoint of the range at the primary consumer level, which would result from nearly equal dependence on planktonic and benthic algal carbon. This equal dependence would not be predicted from the relative magnitude of planktonic and benthic algal photosynthesis as currently estimated in these systems.

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