The ratio of nitrogen to phosphorus resupplied by herbivores: zooplankton and the algal competitive arena

One hallmark of pelagic herbivory is the rapid and tight cycling of nutrients among algae, herbivores and the dissolved phase. An implication of this cycling is that the ratio of elements released by the herbivore is a "supply ratio' as defined in resource-competition theory. Three alternative models (a constant, mass-specific basal metabolic release of N and P; a constant efficiency of accumulating N and P in grazer biomass; and an adjusting efficiency of N and P accumulation that maintains a homeostatic ratio of N to P (N:P) in the bodies of the grazers) all predict a positive correlation between the N:P in the algal pool and that released by the animals. Data provide strong supportive evidence for this relationship. The model of basal metabolism is rendered unrealistic by a lack of relationship between the N:P released by grazers and that in the zooplankton pool. The other 2 models differ only in whether grazers maintain a homeostasis of the N:P in their bodies. Data indicate that homeostasis of the N:P is a reasonable constraint but that homeostasis is not strict. Constant accumulation predicts a direct proportionality between the N:P released and the N:P in the algal pool. Homeostasis predicts a curvilinearity in this relationship, such that imbalances of the N:P in the food compared with that in the zooplankton are accentuated. Comparison of the N:P released calculated from alternatives 2 and 3 with measured values showed good agreement with both models, but especially for the homeostatic one. Homeostatic grazers appear to establish an unstable equilibrium in the N:P in the algal pool and thus, presumably, in the nutrient-limitation patterns of the algae. Whichever element appears in the food in low relative proportion will be recycled by the grazer with relatively low efficiency. The model of strict homeostasis thus predicts a role for grazers not predicted by other theoretical approaches: grazers cause the nutrient-limitation patterns of their algal prey to diverge. -from Author