Eight different mathematical formulations of the photosynthesis—light curve for phytoplankton (up to and including light saturation) were recast in terms of the same two parameters: the initial slope α, and the assimilation number PmB. Each equation was tested for its ability to describe empirical data from natural populations of marine phytoplankton: the results of 188 light‐saturation experiments at three coastal locations in Nova Scotia over a 2‐year period. The most consistently useful mathematical representation of the data was found to be the hyperbolic tangent function.