Estimation of net uptake and leakage rates of orthophosphate from 32 P‐uptake kinetics by a linear force‐flow model

32 P‐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 32 P‐uptake data. The study period (January–May 1992) covered development, steady state growth, and collapse of a typical spring diatom bloom. 32 P incorporation of <3‐, 3–12‐, and > 12‐ µ 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, 32 P‐uptake curves, and size partitioning of 32 P incorporation can be recalculated from the constants of the linear force‐flow model. The initial orthophosphate concentration fluctuated around 0.1 µ 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‐ µ 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.