Can physicochemical factors predict lipid content in phytoplankton?

1. We hypothesized that a large collection of reasonably standardized data for natural algal assemblages would reveal the influence of environmental factors on the fraction of recently produced photosynthate allocated to lipid. 2. Our analysis of photosynthate allocation in fresh‐ and saltwater systems showed that allocation of carbon to lipid was not well correlated with any one environmental factor. 3. Allocation to lipid increases with temperature up to 12 °C (lipid allocation = 11.7 + 1.70*temperature (°C), n = 48, r 2 = 0.50, P < 0.001) but not above. 4. The relationship between temperature and lipid allocation appears to be caused by a unique convergence of nutrient limitation (nitrate or silicate) in the presence of facultative lipid‐producing algae (diatoms or other chrysophytes) which occurs at or below 12 °C during stratification of the water column.