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Limnologists have been studying patterns in lake primary productivity for more than 60 years (Elster 1974). More recently, concern about eutrophication has focused attention on nutrient supply as a regulator of lake productivity. However, nutrient supply cannot explain all the variation in the primary productivity of the world's lakes. Schindler (1978) analyzed a sample of 66 lakes that were likely to be limited in productivity by phosphorus because their nitrogen/phosphorus ratios exceeded five. Phosphorus supply, corrected for hydrologic residence time, explained only 48% of the variance in primary production, and lakes with similar phosphorus supply rates differed nearly a thousandfold in productivity. Phosphorus loading explains 79-95% of the variance in chlorophyll a concentration (Dillon and Rigler 1974, Oglesby 1977, Schindler 1978), but chlorophyll a concentration is a poor predictor of primary production (Brylinsky and Mann 1973, Oglesby 1977). The concept of cascading trophic interactions, on the other hand, explains differences in productivity among lakes with similar nutrient supplies but contrasting food webs. The concept reflects an elaboration of long-standing principles of fishery management based on logistic models (Larkin 1978). Simply put, a rise in piscivore biomass brings decreased planktivore biomass, increased herbivore biomass, and decreased phytoplankton biomass (Figure 1). Specific growth rates at each trophic level show the opposite responses. Productivity at a given trophic level is maximized at an intermediate biomass of its predators. Productivity at all trophic levels, and energy flow through the food web, are highest where intensities of predation are intermediate at all trophic levels (Kitchell 1980). Although this simple conceptual model is heuristically useful, real ecosystems exhibit nonequilibrium dynamics that result from different life histories and variable interactions among the major species. Cascading trophic interactions and nutrient loading models are complementary, not contradictory. Potential productivity at all trophic levels is set by nutrient supply. Actual productivity depends on the recycling of nutrients and their allocation among populations with different growth rates. The phosphorus availability to phytoplankton, for example, is determined by processes that operate over a wide range of temporal and spatial scales (Harris 1980, Kitchell et al. 1979). Nutrient excretion by zooplankton is a major recycling process (Lehman 1980) that is strongly influenced by selective predation on zooplankton by fishes (Bartell and Kitchell 1978). Thus, by regulating recycling rates, consumers regulate primary production.