SUMMARY.1. In 1981–84 limnocorral (LC) experiments were performed in Lake Lucerne. Switzerland, to manipulate the planktonic community by varying P fertilization and by removing large zooplankton (with a 95 μm screen).2. The C:P ratios in both suspended and entrapped seston exceeded the ‘ideal’ C:P ratio of 106 proposed by Redfield, Ketchum&Richards (1963) when P was limiting algal growth.3. P fertilization could decrease the sestonic C:P ratio to 106 only when P did not limit algal growth; P additions far exceeding the P loading of eutrophic lakes were necessary to obtain this situation.4. Changes in epilimnetic C:P ratios were usually related to short‐ term changes in primary production, caused by variable insitulight conditions and turbulence, and subsequent variation in POC concentrations.5. Entrapped seston in the 95 μm‐filtered LCs showed C:P ratios slightly higher than those of suspended seston, indicating fast P release and slower C mineralization in the epilimnetic nutrient cycle.6. Removing large crustacean zooplankton enhanced epilimnetic P mineralization, and C:P ratios of entrapped seston in the 95 μm‐filtered LCs were increased.7 Detritus formed a relatively high proportion of the seston and amounted to more than two‐thirds of the measured POC concentration.8. Calculations of algal P uptake using information on sestonic C:P ratios and14C uptake rates are questionable, as long as detritus cannot be separated from algae and net carbon uptake cannot be acc