Effects of Si : P supply ratio, supply variability, and selective grazing in the plankton: An experiment with a natural algal and protistan assemblage.

Semicontinuous cultures were used to examine the effects of Si and P supply on a natural algal community. Three Si : P supply ratios were used, with different patterns of supply variability imposed by various schedules of Si and P pulses delivered 1, 4, or 10 d apart. All cultures with a given Si: P supply ratio received the same time‐averaged supply of Si and P. The Si: P supply ratio had a strong effect on the species composition of the algae. Chlorophytes were most abundant at the low Si: P supply ratio (20 : 1), diatoms were most abundant at the intermediate Si : P supply ratio (80 : 1), and the siliceous chrysophyte Mallomonas sp. was most abundant at the high Si : P supply ratio (160 : 1). Diatom species showed differentiation in their ability to compete for Si and P. On average, algal cells became smaller as the Si: P supply ratio decreased. Higher algal species richness was associated with increased variability in P supply. Competitive ability of some algae was increased by variability in P supply, but that of others was decreased. A flagellated protist grazed selectively on two Nitzschia spp., and thereby altered their competitive dynamics. The biovolume of this protist, and that of other heterotrophic flagellates, decreased with increasing variability in P supply. Despite selective grazing and nutrient supply variability, patterns in the algal communities were generally consistent with expectations based on resource competition theory and previous experience with algal competition in cultures.