THE PHYTOPLANKTON POPULATION AS A COMPOUND PHOTOSYNTHETIC SYSTEM

The ecological significance of the photosynthetic characteristics of planktonic algae has long been recognized. A simple application of these to the behaviour of natural populations is not possible, however, since the latter is normally governed by the average behaviour of cells continuously circulating within the strong vertical gradient of light intensity characteristic of natural waters. In this situation the unit of ecological significance can often be interpreted as the total population present below unit area of surface. The photosynthesis of this unit constitutes the total or integral photosynthesis in the water column concerned. Its properties are of direct ecological interest. Their relation to the photosynthetic characteristics of the algae present is discussed in this paper, and the influence of various environmental factors is considered. Here an analysis of the variation of photosynthetic rate with depth in the water column is first discussed in relation to an experimental study of the photosynthesis of a freshwater plankton diatom, Asterionella formosa Hass. A mathematical model (of which a preliminary account was given by Talling, 1955), describing the general case of integral photosynthesis in the water column, is attempted. Its application to ecological situations is illustrated by examples relating to the growth of Asterionella in Lake Windermere.

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