Stoichiometric Constraints vs. Algal Defenses: Testing Mechanisms of Zooplankton Food Limitation

The nature of resource limitation for zooplankton grazers is strongly debated. We provide a test of stoichiometric constraints and three alternative mechanisms of resource limitation in six lakes that represent a natural gradient in resource quality. Growth assays measured the responses of juvenile Daphnia to concentrated natural resources and to supplements of phosphate (P), essential fatty acids (EFA), and P-deficient and P-rich algae. Assimilation experiments quantified the digestibility of 32P-labeled resources. Daphnia growth was much lower on resources from deep compared with shallow lakes (0.13 ± 0.01 d–1 and 0.30 ± 0.03 d–1, respectively) but was independent of resource concentration. Growth was negatively correlated with the resource C:P ratio, but the relationship was not causal as responses to the P supplement were very weak. Adding EFA also produced weak responses. In contrast, supplements of readily digested algae stimulated moderate (P-deficient) and strong (P-rich) increases in growth that accounted for growth variation among the lakes. Changes in Daphnia P balance were also contrary to P limitation. In all, our growth assays implicate energy limitation unrelated to food concentration and, therefore, support the digestion resistance hypothesis. Digestion resistance is further supported by a positive correlation between assimilation efficiency and growth. Our results show that stoichiometric theory needs to be integrated with theory on plant and algal defenses against herbivores.