THE IMPACT OF LIGHT-INTENSITY AND DAYLENGTH ON SILICATE AND NITRATE COMPETITION AMONG MARINE-PHYTOPLANKTON

I tested the extent to which differences in light supply could influen ce the outcome of nutrient (Si and N) competition between marine phyto plankton. Competition experiments were performed with 11 species of ma rine phytoplankton at Si:N ratios from 16 to 124: 1, light intensities from 28 to 225 mu mol quanta m(-2) s(-1), and three different dayleng ths. Thus, light supply was the composite result of two components: ph otoperiod and intensity. Diatoms were dominant competitors at higher S i:N ratios, nonsiliceous flagellates at lower ones. Light had no impac t on the transition from flagellate to diatom dominance along the Si : N gradient. However, species within those groups were separated along the light gradient. Contrary to theoretical expectations, changes in l ight intensity and changes in daylength led to similar shifts in speci es dominance. Therefore, it was possible to describe the light climate by the integral parameter ''daily light dose.''