Consumer control of lake productivity

There is no common currency for ecological interaction. For example, the consumption of a small fish by a larger one entails all the following characteris­tics: behavioral interplay during pur­suit and capture, an instantaneous reduction of the prey population, greater reproductive potential for the predator, a flux of organic energy, and a transfer of mineral nutrients such as phosphorus and nitrogen. Thus the same event is viewed differ­ently by behavioral, population, evo­lutionary, physiological, community, and ecosystem ecologists. Many eco­logical problems involve complexes of interactions that transcend the boundaries among traditional subdis­ciplines. Complex interactions arise when system components are linked by multiple types of pathways (e.g., predation, behavioral cues, and trans­fers of energy and nutrients) (Car­penter 1988a). Consequently, new combinations of approaches are often necessary. This article summarizes the evi­dence that a set of complex interac­tions regulates lake ecosystem productivity. The maJor components involve species interactions such as predation and competition, tradition­ally studied by population and com­munity ecologists, plus physicochem­ical processes traditionally studied by limnologists and ecosystem ecolo­gists. Nutrient input, water-turnover time, and vertical mixing are the ma­jor physicochemical factors. These have demonstrable effects on lake productivity (measured as total an­nual carbon fixation by the phyto­plankton), but they explain only about half of the observed variability in productivity (Carpenter and Kit­chell 1987, Schindler 1988, Schindler et al. 1978). A few years ago, we offered the hypothesis that much of the unex­plained variance in lake productivity is due to food web dynamics (Car­penter et al. 1985). Changes in top carnivores are transmitted to lower trophic levels through a trophic cas­cade. For example, an increase in biomass of large piscivorous fishes should cause a decrease in biomass of small planktivorous fishes, causing in­creased biomass of herbivorous zoo­plankton and decreased biomass ofphytoplankton (Carpenter et al. 1985, 1987).