Influence of inter- and intra-daily light-field variability on photosynthesis by coastal phytoplankton

An investigation was carried out to determine the relationships between the principal photosynthetic parameters of natural marine phytoplankton and the properties of the irradiance of the preceding five days and the intra-daily ambient irradiance. Samples used in the study were collected from Strangford Lough, Northern Ireland during both winter and summer between 1979 and 1981. Photosynthetic parameters were determined for both constant and ambient incubation light fields. The fluctuation properties of the irradiance field exerted little influence on the photosynthetic parameters. In summer populations, the parameters obtained from morning samples were strongly influenced by the total irradiance recorded on the fourth day previous to experimentation, irrespective of the incubation irradiance. In contrast, the corresponding parameters from afternoon samples were most influenced by the total irradiance received during incubation. The influence of these two separate factors over a day may contribute to variability in the magnitude of diel changes in marine phytoplankton photosynthesis.