ARE MARINE DIATOMS FAVORED BY HIGH SI-N RATIOS

Competition experiments were performed first with 4, then with 11 spec ies of marine phytoplankton at various ratios of silicate:nitrate and various light intensities. Diatoms became dominant at Si:N ratios > 25 :1 while flagellates were the superior competitors at lower ratios. Th e light supply did not influence the competitive position of diatoms a nd non-siliceous flagellates in general, while ii: was important in de termining the outcome of competition at the species level, In the 11 s pecies experiments, Stephanopyxis palmeriana was the dominant diatom a t high light intensities. It shared dominance with Lauderia annulata a t medium and low light intensities and high Si:N ratios. Pseudo-nitzsc hia pungens was the dominant diatom at low light intensities and relat ively low Si:N ratios. The green alga Dunaliella tertiolecta was the d ominant flagellate at high light intensities, while at low light inten sities the prymnesiophycean Chrysochromulina polylepis and the cryptop hyte Rhodomonas sp. were also important.