EFFECTS OF LIGHT LEVEL AND N-P SUPPLY RATIO ON THE COMPETITION BETWEEN PHAEOCYSTIS CF. POUCHETII (HARIOT) LAGERHEIM (PRYMNESIOPHYCEAE) AND 5 DIATOM SPECIES

Phaeocystis cf. pouchetii (Hariot) Lagerheim and five winter/spring di atom species from Cape Cod Bay/Massachusetts Bay were grown in semi-co ntinuous culture in order to investigate the effects of irradiance and N:P supply ratio on their competitive ability. Experiments were condu cted at two Light levels under N- and P-limiting conditions. Phaeocyst is grew fastest relative to the other algae when nutrients were non-li miting and irradiance was low, although it was not capable of dominati ng 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 the se experiments and appeared to have an enhanced ability to compete for P under high light conditions. Differences in the chemical compositio n of the phytoplankton assemblages at the two light levels indicated a n interactive influence of nutrients and irradiance on growth and biov olume. This interactive effect may play an important role in phytoplan kton competition, since nutrient requirements may vary in unique ways for different species with changes in irradiance. (C) 1998 Elsevier Sc ience B.V.