Body size and mass flow in freshwater plankton: models and tests

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Abstract
Observations of radiotracer (32P) flow in size-fractionated lake plankton were compared with the predictions of size-dependent models of material flow. The models assume that the metabolic activity and turnover of the size fractions can be described as power functions of size (Y=aMb) One model, called size-structured, is an application of the Platt and Denman formalism: it assumes that materials are incorporated preferentially by the smallest organisms and flow continuously toward the larger organisms. In this model, the trophodynamic flow from a size fraction is constrained to the next larger fraction. The tracer data indicated that metabolic activity could indeed be described as a continuous function of size, at least within the microplankton (< 75 μm), but it did not support the hypothesis of a size-hierarchical trophodynamic flow. Another model limited the size range of predators to the larger (> 10 μm), size fractions while relaxing the size dependence constraint on the trophodynamic flow. This ‘unstructured’ model agreed better with the 32P dynamics and generated estimates of seston P uptake and of the rates of biomass turnover that compared favorably with published estimates for oligotrophic lakes. Some problems with the fit to the data remain but might be dealt with by distinguishing a priori among functional types of planktonic organisms through fluorescence (flow cytometry) or metabolic properties (inhibitors).

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