A general theory of steady state phytoplankton growth in a nutrient saturated mixed layer

Abstract

An improved theory relating steady‐state phytoplankton density, primary production, and growth rate to environmental variables and to phytoplankton parameters is presented and developed in detail for a nutrient saturated, well mixed layer. For this case, mathematical statement of the theory requires seven equations: two expressing crop density and primary production as functions of environmental variables and phytoplankton parameters; one stating the dependence of the upper limit for daily production on incident light and phytoplankton parameters; three stating the dependence of phytoplankton parameters on growth rate; and one describing phytoplankton losses to primary consumers. The first six can be quantitatively stated on the basis of existing experimental information. The seventh cannot, and it is tentatively assumed that consumption is regulated in a way that maximizes the steady‐state harvest. For a nutrient saturated layer that absorbs all incident light, calculations arc relatively simple. Graphs of predicted crop density and primary production, as general functions of nonphytoplankton absorption and sedimentation rate, are used to predict the quantitative consequences of changes in seasonal date, sedimentation rate, nonphytoplankton light absorption, and mixed layer depth.