Silicate as regulating nutrient in phytoplankton competition

The development of phytoplankton communities was studied in floating enclosures. The enclosures were supplied with either surface water or water from 40 m depth. Nutrients with or without silicate were added in some of the experiments, while others recieved no artificial fertilization. It is shown that diatom dominance occurred irrespective of season if silicate concentration exceeded a threshold of approximately 2 PM. Flagellate dominance changed to diatom dominance within a few days after nutrient addition resulting in silicate concentrations above this threshold. Dominance of Phaeocystis sp. appeared on several occasions after the bloom of another species, but never at high silicate concentrations. The success of the diatom group seemed to be due to a high inherent growth rate at non-limiting silicate concentrations. Calculations indicated that the inherent growth rate for the diatom group had to be 5 to 50 % higher than for the flagellate group in order to explain the outcome of our experiments.