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Measurements of plankton δ¹³C from 28 temperate lakes show that zooplankton are depleted in ¹³C relative to smaller planktonic size fractions. A broad literature survey indicates that this is a general pattern in lakes but not in marine and estuarine plankton communities, where zooplankton are generally enriched in ¹³C relative to particulate organic matter (POM) or microplankton. Marine plankton thus conform to the assumptions that pelagic food webs are essentially driven by phytoplankton and that POM or small planktonic size classes largely reflect algal carbon, which is transferred to zooplankton with a slight enrichment in ¹³C. The plankton of lakes and possibly of estuaries, however, do not conform to this expectation. We show that there is a continuum in the δ¹³C differences between POM and zooplankton from open ocean to coastal areas and estuaries and that a similar pattern occurs from unproductive …