Representing the effects of diurnal variations in light on primary production on a seasonal time-scale

Rates of production by primary producers are strongly affected by light. Diurnal variations in irradiance produce characteristic diurnal patterns in primary production and respiration. In cases in which the processes of interest occur on a longer time-scale, it is not uncommon to ignore diurnal variations and use mean daily irradiance to force models of primary producers. This simplification reduces the computer processing time and input data requirements of models. However, this approach results in an error which may be significant if irradiance during part of the day is higher than that needed for growth at the maximum possible rate. Here, an alternative approach is presented and applied to a simulation of phytoplankton in a shallow lake. The model uses a semi-analytical calculation of mean daily growth rates that takes into account periods of supersaturation with respect to light to deliver results that conform closely to those of a model using hourly time-steps and irradiance forcing, but with a substantially smaller computational cost.