Plankton Abundance and Dynamics across Nutrient Levels: Tests of Hypotheses

In lakes and reservoirs in which Daphnia is able to suppress the biomass of edible algae far below the level set by nutrients, the interaction is stable across the range of nutrient-poor to nutrient-rich environments. This phenomenon contradicts standard consumer–resource models, which predict that dynamics should become increasingly unstable with enrichment. We test four hypotheses that might account for stability at high-nutrient levels: (1) greater abundance of inedible algae with enrichment interferes with Daphnia’s feeding; (2) Daphnia’s death rate increases with enrichment; (3) Daphnia’s death rate increases with Daphnia density; (4) Daphnia’s functional response depends on Daphnia’s density. All hypotheses are rejected because they predict much higher biomass of edible algae at high-nutrient levels than is observed. Additional evidence on Daphnia death rates strengthens the case against hypotheses (2) and (3). We consider other hypotheses and conclude that three in particular would repay further investigation. (a) Inedible algae act as a nutrient ‘‘sponge,’’ reducing the effective carrying capacity for edible algae; (b) limited spatial movement can enhance stability through a metapopulation-like effect, and (c) stochastic variation among individuals can be stabilizing. The central problem investigated here is a general one, with implications for many consumer–resource systems.