Are diatoms poor competitors at low phosphate concentrations?

Growth and succession of natural phytoplankton communities in enclosed water-masses fertilised with variable nutrient compositions have been investigated for several years. The aim of these experiments have either been to investigate effects of nutrients supplied in different compositions, or to initiate blooms of a certain species or group of phytoplankton. The experiments have in general been carried out for 4 w and the results show diatom dominance when nutrients, including silicate were in excess. Diatoms, however, were not able to dominate when phosphate was deficient, although silicate and nitrate were in excess. From a dominance of 72 and 74%, for the entire experimental period, in enclosures supplied with all three nutrients, the diatom dominance was reduced to 45 and 55% in enclosures where phosphate was not supplied. Growth of the flagellate community appears to be less affected by low phosphate concentration. Confirming observations have been made both at high and low silicate concentration in experiments conducted between 1991 and 1993. A hypothesis for nutrient competition between diatoms and flagellates, based on morphology, is presented along with the experimental findings.