Putative future increase in atmospheric CO2 is expected to adversely affect herbivore growth due to decrease in contents of key nutrients such as nitrogen and phosphorus (P) relative to carbon in primary producers including plant and algal species. However, as many herbivores are polyphagous and as the response of primary producers to elevated CO2 is highly species-specific, effects of elevated CO2 on herbivore growth may differ between feeding conditions with monospecific and multiproducer diets. To examine this possibility, we performed CO2 manipulation experiments under a P-limited condition with a planktonic herbivore, Daphnia, and three algal species, Scenedesmus obliquus (green algae), Cyclotella sp. (diatoms) and Synechococcus sp. (cyanobacteria). Semibatch cultures with single algal species (monocultures) and multiple algal species (mixed cultures) were grown at ambient (360 ppm) and high CO2 levels (2000 ppm) that were within the natural range in lakes. Both in the mono- and mixed cultures, algal steady state abundance increased but algal P : C and N : C ratios decreased when they were grown at high CO2. As expected, Daphnia fed monospecific algae cultured at high CO2 had decreased growth rates despite increased algal abundance. However, when fed mixed algae cultured at high CO2, especially consisting of diatoms and cyanobacteria or the three algal species, Daphnia maintained high growth rates despite lowered P and N contents relative to C in the algal diets. These results imply that algal diets composed of multiple species can mitigate the adverse effects of elevated CO2 on herbivore performance, although the magnitude of this mitigation depends on the composition of algal species involved in the diets.
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