Abstract
We conducted laboratory experiments with 85 assembled phytoplankton communities composed of species from four predefined functional groups (chlorophytes, diatoms, cyanobacteria, chrysophytes) to distinguish the relative importance of species diversity from functional group diversity on carbon uptake. We separated the observed diversity effects on carbon uptake into those caused by species with particularly important traits (selection effect) and those caused by positive interactions among species (e.g., complementary resource use or facilitation [complementarity effect]). Additionally, we measured the composition of photosynthetically active pigments and light absorbance in communities and monocultures, and related them to species and functional diversity effects on carbon accrual. Biodiversity effects were weak or even absent in pure cyanobacterial and diatom communities compared to strong effects in chlorophytes. Complementarity effects and light absorbance increased as functional (i.e., phylogenetic) diversity increased. There was a positive correlation between complementarity effects on carbon accrual and light absorbance. These findings support hypotheses regarding biodiversity–productivity relationships in phytoplankton communities based on niche separation along spectral light gradients.