It has recently been shown that
Daphnia can vary in the phosphorus (P)-content of their body tissues, but the relative importance of genetic versus environmental causes for this variation is unexplored. We measured variation in P-content (as % body mass) of
Daphnia from eight lakes and conducted experiments to contrast three sources of variation: interspecific variation, clonal variation and phenotypic plasticity.
Daphnia P-content decreased with increasing seston C:P ratio across lakes. This relationship reflected both inter- and intraspecific variation.
Daphnia parvula and
D. dubia exhibited high P-content and were found in shallow lakes with low C:P seston, whereas
D. pulicaria had low P-content and was found in deep, stratified lakes having high C:P seston. Populations of
D. dentifera spanned this lake gradient and exhibited P-content that was negatively related to seston C:P. Evidence for phenotypic plasticity came from experiments with
D. pulicaria and
D. dentifera collected from a lake with P-deficient seston and fed a P-sufficient diet in the laboratory. In addition, populations of
D. dentifera differed in P-content even after 7 d of feeding on P-sufficient resources, suggesting within-species clonal variation. However, mesocosm experiments revealed broad and surprisingly continuous variation in the P-content of individual clones of
D. pulex (range 1.54–1.05%) and
D. mendotae (1.51–1.07%) over a gradient in dietary C:P. The broad range in P-content exhibited by individual clones, acclimated for generations, suggests that variation in
Daphnia P-content from laboratory experiments needs to be interpreted with caution. These results also show that phenotypic variation in response to environment can be a larger source of variation in P-content than genetic differences within or among species.
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