PLANKTON ABUNDANCE AND DYNAMICS ACROSS NUTRIENT LEVELS - TESTS OF HYPOTHESES

In lakes and reservoirs in which Daphnia is able to suppress the bioma ss of edible algae far below the level set by nutrients, the interacti on is stable across the range of nutrient-poor to nutrient-rich enviro nments. This phenomenon contradicts standard con sumer-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 Da phnia 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. Addi tional evidence on Daphnia death rates strengthens the case against hy potheses (2) and (3). We consider other hypotheses and conclude that t hree in particular would repay further investigation. (a) Inedible alg ae act as a nutrient ''sponge,'' reducing the effective carrying capac ity for edible algae; (b) limited spatial movement can enhance stabili ty 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 s ystems.