Suttle, C.A. and P.J. Harrison. 1988. Ammonium and phosphate uptake kinetics of size-fractionated phytoplankton from an oligotrophic freshwater lake. Journal of Plankton Research 10:133-149.

Experiments examined ammonium and phosphate uptake kinetics of size fractionated plankton (< and >3 µm) from an oligotrophic lake during May, July and September. The <3- µm fraction accounted for 65-84% and 50-90% of the measured saturated phosphate and ammonium uptake rates respectively. Maximum saturated ammonium uptake rates were highest when chlorophyll (Chl) was least (July), and lowest when Chl was greatest (May), whether expressed on a nutrient or volume specific basis. The proportion of ³²P entering each size fraction was concentration dependent; slight increases in phosphate resulted in more isotope entering the <3- µm fraction while still higher concentrations favoured uptake by the >3- µm size class. The highest specific uptake rate observed for phosphate was 31.8 /h which was associated with the <3- µm fraction in September; therefore in 1 h of exposure to saturating phosphate levels enough P is accumulated to meet the requirements of at least 45 new cells. Our results imply that the <3 µm cells are effective competitors for phosphate and ammonium in Kennedy Lake. Significant quantities of ³²P (40-70%) were found to remain in the filtrate when 'apparent' isotopic equilibrium had been reached, although estimated turnover times were relatively short (6-26 min). As well, a survey of the literature indicated a poor correlation between isotope partitioning into 'dissolved' and particulate components, and estimates of phosphate turnover times. These results indicate that a high proportion of ³²P remaining in the filtrate is not necessarily indicative of low phosphate demand. The results of these studies emphasize the importance of small-sized cells in the phosphorus and nitrogen dynamics of this lake.