Population dynamics of light-limited phytoplankton: Microcosm experiments

This paper investigates the extent to which the predictions of an elementary model for light-limited growth are matched by laboratory experiments with light-limited phytoplankton. The model and experiments link the population dynamics of phytoplankton species with changes in the light gradient caused by phytoplankton shading. The model predicts that a phytoplankton population should continue to grow until, at steady state, the light intensity at the bottom of the water column equals its critical light intensity. The experimental results were in good agreement with the theoretical predictions: (1) the steady-state population density increased with an increase of the incident light intensity, (2) the steady-state population density (per unit volume) was inversely proportional to mixing depth, (3) the steady-state population size (per unit area) decreased linearly with mixing depth, (4) the critical light intensity decreased with an increase of the incident light intensity, (5) the critical light intensity was approximately the same at each mixing depth, and (6) the time courses predicted by the model were in line with the observed time courses of population density and light penetration. Implications for phytoplankton ecology and aquatic production biology are discussed.