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

Abstract

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.