The underwater light climate as a controlling factor in the production ecology of freshwater phytoplankton

Summary

The underwater light climate for phytoplankton is discussed in terms of three groups of factors — surface solar radiation input, vertical attenuation, and depth of mixing — and their varying combinations. Their quantitative assessment is considered from the standpoint of current mathematical models of light factors in photosynthetic productivity, and illustrated by situations drawn particularly from the English lakes. These include the range in optical diversity of English lake waters, the spring phytoplankton increase associated with rising radiation input, and the autumn-winter decline associated with a declining input and increased vertical mixing. A dimensionless parameter (q) is proposed which expresses, for vertically homogeneous populations, the combined action of the three factor-groups in the light climate. It is also used to restate conditions for the “critical depth” situation. Self-regulation of the effective light climate is discussed with reference to the self-shading behaviour and active vertical migrations of some dense populations of the dinoflagellate Ceratium hirundinella.