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STOICHIOMETRIC CONSTRAINTS VS. ALGAL DEFENSES: TESTING MECHANISMS OF ZOOPLANKTON FOOD LIMITATION
Author(s) -
DeMott William R.,
Tessier Alan J.
Publication year - 2002
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/0012-9658(2002)083[3426:scvadt]2.0.co;2
Subject(s) - daphnia , zooplankton , biology , assimilation (phonology) , algae , juvenile , branchiopoda , ecological stoichiometry , herbivore , ecology , cladocera , food web , nutrient , trophic level , philosophy , linguistics
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 32 P‐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.