A MATHEMATICAL MODEL OF REGIONAL VARIATIONS IN PLANKTON

Abstract

The relationships of phytoplankton, zooplankton, and a limiting nutrient (phosphate) arc examined in a simple two‐layered model in which the upper layer corresponds with the maximum limits of the euphotic zone and is regulated in its depth by the effect of the chlorophyll content of the phytoplankton upon light absorption. The model is applied to quasi‐steady state conditions in summer, when radiation values are similar over wide geographical areas, so that radiation is treated as a constant. Basic environmental variables include a) phosphate concentration in the deepwater mass underlying the euphotic zone, b) the rate of mixing between the two layers, and c) temperature, which is postulated to be important in affecting the sinking rate of phytoplankton. For any given array of environmental factors, steady state concentrations of phytoplankton, zooplankton, and phosphate in the upper layer can be calculated. In two areas in the Atlantic Ocean, one in boreal waters of the North Sea and one in the subtropics off Bermuda, environmental factors and populations have been determined with sufficient precision for a good test of the model, and the results are realistic. Chlorophyll data are then examined on a broader regional scale, and again there are indications that the model provides a reasonable fit for most of the observed variations. © 1965, by the Association for the Sciences of Limnology and Oceanography, Inc.