Effects of pigment richness and size variation on coexistence, richness and function in light limited phytoplankton

1. Trait diversity is traditionally seen as promoting species richness and ecosystem function. Species with dissimilar traits would partition available resources, increasing niche differences, facilitating coexistence and increasing ecosystem function.

2. Here we first show, using theory and simulations for light-limited phytoplankton, that combing photosynthetic pigments is indeed a necessary condition for coexistence and stimulates ecosystem function.

3. However, pigment richness does mostly not permit the coexistence of more than two species, and increases productivity at most 40\% compared to single-pigment communities. That is because blending in more pigments leads to coexistence of species with many pigments and therefore flat absorption spectra, which equalizes their fitness but decreases their niche differences. Similarly, seeding species with more variable size leads to an excess of large-celled species, which does not only decrease fitness differences, but also niche di...