DeMott WR, Pape BJ, Tessier AJ
C:P ratio, Ecological stoichiometry, phosphorus, Zooplankton, mesocosm, 3 m3, nutrient addition, freshwater, land based, Kellog Biological Station, Michigan, USA
It has recently been shown that Daphnia can vary in the phosphorus P -content of their body tissues, but therelative importance of genetic versus environmental causes for this variation is unexplored. We measured variationin P-content as % body mass of Daphnia from eight lakes and conducted experiments to contrast threesources of variation: interspecific variation, clonal variation and phenotypic plasticity. Daphnia P-content decreasedwith 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:Pseston, 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 toseston C:P. Evidence for phenotypic plasticity came from experiments with D. pulicaria and D. dentifera collectedfrom a lake with P-deficient seston and fed a P-sufficient diet in the laboratory. In addition, populations ofD. dentifera differed in P-content even after 7 d of feeding on P-sufficient resources, suggesting within-speciesclonal variation. However, mesocosm experiments revealed broad and surprisingly continuous variation in theP-content of individual clones of D. pulex range 1.54–1.05% and D. mendotae 1.51–1.07% over a gradient indietary C:P. The broad range in P-content exhibited by individual clones, acclimated for generations, suggeststhat variation in Daphnia P-content from laboratory experiments needs to be interpreted with caution. Theseresults also show that phenotypic variation in response to environment can be a larger source of variation inP-content than genetic differences within or among species.