Phytoplankton diversity emerging from chromatic adaptation and competition for light

Phytoplankton absorption cross-section is a key quantity in biogeochemical ocean models that alters the underwater spectral light field and the photosynthetic response of phytoplankton. Phytoplankton taxa are characterized by absorption spectra with defined absorption bands in the visible region of the light spectrum that govern the capability of different taxa to exploit light as a resource and hence contribute to the selection among taxa. We use a medium-complexity biogeochemical model and observations from the Mediterranean Sea to simulate nine optically different phytoplankton functional types (PFTs) and a coupled radiative transfer model to simulate the spectral underwater light field where the PFTs interact. We investigate the competitive advantage provided by the different absorption spectra and the possibilities of coexistence emerging from the availability of different light niches. Different pathways of light competition among PFT’s can be achieved introducing different grazing formulations. Predator no-switching functional response allows to survive only the PFT with the absorption spectrum better matched to available light. Synechococcus and Cocolithophores won the competition in the pico and nanoplankton size-classes, respectively. The availability of different light niches lead to the coexistence of picoplankton PFTs but the extent of coexistence was minimal among nanoplankton. Predator active-switching functional response that promotes coexistence, allowed all PFTs to coexist but Cocolithophores were still more abundant than other flagellates in clear waters.