Sources of variability in the factors used for modelling primary productivity in eutrophic waters

The accumulation of phytoplankton biomass, which often occurs when water bodies receive enhanced inputs of inorganic nutrients, causes large changes in the availability and spectral composition of underwater light. Quantitative descriptions of the variations in light available to phytoplankton are a prerequisite for modelling of aquatic primary productivity. As an example of the range of natural variation in irradiance, the main spectral, spatial and temporal changes in underwater light, which occur in a shallow eutrophic estuary in response both to increasing chlorophyll concentration and to wind-induced vertical mixing, are described. Additionally, phytoplankton were shown to exhibit changes in photosynthetic physiology which were triggered by changes in the quantity and spectra of light. The characteristics and kinetics of these responses are presented and discussed in relation to their impact on modelling of primary productivity.