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

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Abstract
32P‐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 32P‐uptake data. The study period (January–May 1992) covered development, steady state growth, and collapse of a typical spring diatom bloom. 32P incorporation of 12‐µm microorganisms was separated by postfiltration. On five occasions, net P‐uptake rates were simultaneously obtained from chemical measurements. Chemically measured net P‐uptake curves, 32P‐uptake curves, and size partitioning of 32P incorporation can be recalculated from the constants of the linear force‐flow model. The initial orthophosphate concentration fluctuated around 0.1 µg P liter−1. The community‐average P threshold exceeded the initial P concentration by a mean factor of 1.4, that of the > 12‐µm 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.

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