The role of light quality and species richness in shaping phytoplankton communities

In natural water bodies, the light spectrum changes with depth, often toward a higher proportion of blue light. While spectral niche partitioning and functional redundancy are important concepts, our understanding of how light spectrum changes affect phytoplankton communities is limited. To understand how phytoplankton respond to changes in spectral light availability, we studied the effects of light quality on species growth, community composition, and biomass production. In a controlled laboratory experiment, we assembled up to 7 phytoplankton species into 13 communities with 5 different initial species richness levels. Communities were exposed to three light quality treatments simulating the red light reduction across water depths (full spectrum, reduced red proportion, and no red). Community biomass was positively influenced by the initial number of species, and this was most pronounced if red light was reduced (shown by a significant interactive effect of light conditions and initial species richness). The growth rate responses were highly species specific, and among the species tested, only Chlamydomonas showed increased growth rates with higher blue light levels, while most others exhibited negative trends. Initial species richness significantly influenced these outcomes. By the end of the experiment, Chlamydomonas had increased in proportion within the community, demonstrating its competitive strength and ability to affect the growth of other species. Our study highlights the sensitivity of certain species to specific wavelengths of light and how competition can shape these responses, contributing to a better understanding of phytoplankton dynamics in changing aquatic environments.