Abstract
We consider several phytoplankton competition models in a well-mixed water column to explore which mechanisms promote bio-diversity. The dynamics of phytoplankton communities under photo-inhibition shows that alternative stable states exist, and coexistence does not occur. The dynamics of two microbial species competing for nutrient and light exhibits competitive exclusion, coexistence, and bi-stability. The competition of phytoplankton species for light with wavelength can also promote bio-diversity. The competition of two microorganisms for nutrient with seasonal succession in chemostat can exhibit coexistence, reversing competition outcomes, but not bi-stability.