ESTIMATION OF NET UPTAKE AND LEAKAGE RATES OF ORTHOPHOSPHATE FROM P-32 UPTAKE KINETICS BY A LINEAR FORCE-FLOW MODEL

P-32-uptake kinetics of extremely P-deficient planktonic microorganisms were analyzed in shallow Lake Balaton by means of a conceptual model of P uptake. According to the model, net P uptake is a linear function of the driving force of the uptake and ceases below a substrate threshold for energetic reasons. We estimated net P-uptake and P-leakage rates from P-32-uptake data. The study period (January-May 1992) covered development, steady state growth, and collapse of a typical spring diatom bloom. P-32 incorporation of <3-, 3-12-, and > 12-mu m microorganisms was separated by postfiltration. On five occasions, net P-uptake rates were simultaneously obtained from chemical measurements. Chemically measured net P-uptake curves, P-32-uptake curves, and size partitioning of P-32 incorporation can be recalculated from the constants of the linear force-flow model. The initial orthophosphate concentration fluctuated around 0.1 mu g P liter(-1). The community-average P threshold exceeded the initial P concentration by a mean factor of 1.4, that of the > 12-mu m microorganisms by a mean factor of 3.9. Small-scale fluctuations of in situ P concentrations were necessary to maintain P uptake and growth of larger microorganisms. In a nonhomogeneous environment, sensitivity to the fluctuating nutrient concentrations, efficiency of leakage reduction, and lower maintenance cell quotas may provide a competitive advantage to more complex organisms.