Effects of light level and N:P supply ratio on the competition between Phaeocystis cf. pouchetii (Hariot) Lagerheim (Prymnesiophyceae) and five diatom species

Abstract

Phaeocystis cf. pouchetii (Hariot) Lagerheim and five winter/spring diatom species from Cape Cod Bay/Massachusetts Bay were grown in semi-continuous culture in order to investigate the effects of irradiance and N:P supply ratio on their competitive ability. Experiments were conducted at two light levels under N- and P-limiting conditions. Phaeocystis grew fastest relative to the other algae when nutrients were non-limiting and irradiance was low, although it was not capable of dominating under these conditions within the time course of the experiment. In general, Phaeocystis was a poor competitor when N or P was limiting; however, its relative abundance under P limitation varied greatly with irradiance. The flagellated form of this organism predominated in these experiments and appeared to have an enhanced ability to compete for P under high light conditions. Differences in the chemical composition of the phytoplankton assemblages at the two light levels indicated an interactive influence of nutrients and irradiance on growth and biovolume. This interactive effect may play an important role in phytoplankton competition, since nutrient requirements may vary in unique ways for different species with changes in irradiance.