Time-courses of size-fractionated phosphate uptake: are larger cells better competitors for pulses of phosphate than smaller cells?

Time-course experiments of phosphate uptake by size-fractionated phytoplankton were conducted in oligotrophic Kennedy and Sproat Lakes. The objective was to determine if large phytoplankton obtained more phosphate than smaller cells, when the nutrient was present at higher concentrations. Studies at Kennedy Lake revealed that uptake rates in the 0.2-3.0 μm fraction were very sensitive to the time they were exposed to elevated concentrations; rates determined over the 60-120 min interval were less than 30% of those recorded over the 0-60 min interval. In contrast, there was little difference in uptake rates over these intervals for cells>3.0 μm. At Sproat Lake phosphate incorporation into the two size fractions was followed after the aerial fertilization of the lake with inorganic nutrients. Following nutrient addition the proportion of phosphate entering the>3.0 μm size fraction increased from ca. 35% to ca. 85%. Despite these observations, it is doubtful that larger cells are able to sequester enough phosphate from pulses to realize the same specific growth rates as their smaller counterparts.