Light spectrum matters: Interactive effects of light and nutrients on phytoplankton communities and trophic transfer

Abstract Phytoplankton is highly sensitive to both the quantity (intensity) and quality (spectrum) of available light, due to their wide range of light‐absorbing pigments. Light is a major driver of trophic energy transfer, and while the impact of light intensity is well investigated, it is unclear how changes in light spectrum may influence this mechanism. The distribution of light underwater is strongly dependent on the optical properties of the water column, and both the available light intensity and wavelength spectrum change with increasing depth. In this study, we investigated the effect of different light spectra on the community structure and biochemical composition (nutrient stoichiometry and fatty acids) of natural freshwater phytoplankton, additionally including two different levels of water depth and nutrient availability in a full‐factorial design. The field experiment was combined with a subsequent laboratory experiment using the lake seston as food to identify potential nutritional constraints for the zooplankton species Daphnia pulex in response to the treatments. We found that light spectrum influences phytoplankton species composition and seston biochemistry, with these effects being highly dependent on the available light intensity (through depth) and nutrients. In particular, the spectral quality of light was important when light, but not nutrients, limited primary producers. In addition, we could observe that the growth performance of D. pulex differed when grazing on the different seston communities because abiotic factors altered the prevalent growth constraints of Daphnia , affecting both mineral and biochemical growth constraints. This indicated that the effects of different light spectra were propagating from the primary producer level to the consumers. Synthesis. Overall, our results showed that the spectral quality of light has a significant impact on phytoplankton community dynamics and the biochemical composition of lake seston. Moreover, the results suggest that light intensity, light spectrum, and nutrients interactively act at the base of aquatic food webs with propagating effects to herbivores, particularly under scenarios of light limitation. Thus, we demonstrate that the light spectrum is a significant component of the abiotic environment that influences plankton dynamics and the trophic transfer in aquatic ecosystems.