Cascading trophic interactions and lake productivity

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.