Abstract Spirulina is one of the most extensively used microalgae for animal and human nutrition; its main interest is centered in its high protein content, 60 -65% on a dry weight basis. In this study, Spirulina was grown in open raceway ponds, and several physicochemical (e.g., pH, dissolved oxygen concentration, temperature, conductivity and irradiance) and biological (e.g., biomass concentration and yield) variables were studied. The variables were correlated in order to implement a mathematical model to predict algal yield. Dissolved oxygen concentration in the cultivation ponds ranged between 10 mg l À 1 in winter (115% of O 2 saturation) and 30 mg l À 1 in summer (375% of O 2 saturation); a clear decrease of biomass concentration was found when dissolved oxygen was >25 mg l À 1 . Neither biomass concentration nor productivity was saturated at the maximum temperature achieved in the open pond during this study (approximately 28 jC). The pH seemed to control both the maximal algal density in the pond and the productivity that were found to be maximum at pH values below 10.5. Finally, all the variables were positively correlated with irradiance. Principal component analysis (PCA) allowed recognition of different sets of samples characterized by a combination of temperature, dissolved oxygen concentration, pH, biomass, productivity, irradiance and conductivity. This method helped to predict a significant loss of productivity in the open ponds in mid-summer due to high pH and high-dissolved O 2 concentration.
