The effect of light on biomass and fucoxanthin (Fx) productivities was studied in two microalgae, Tisochrysis lutea and Phaeodactylum tricornutum. High and low biomass concentrations (1.1 and 0.4 g L-1) were tested in outdoor pilot-scale flat-panel photobioreactors at semi-continuous cultivation mode. Fluorescence spectroscopy coupled with chemometric modeling was used to develop prediction models for Fx content and for biomass concentration to be applied for both microalgae species. Prediction models showed high R-2 for cell concentration (.93) and Fx content (.77). Biomass productivity was lower for high biomass concentration than low biomass concentration, for both microalgae (1.1 g L-1: 75.66 and 98.14 mg L-1 d(-1), for T. lutea and P. tricornutum, respectively; 0.4 g L-1: 129.9 and 158.47 mg L-1 d(-1), T. lutea and P. tricornutum). The same trend was observed in Fx productivity (1.1 g L-1: 1.14 and 1.41 mg L-1 d(-1), T. lutea and P. tricornutum; 0.4 g L-1: 2.09 and 1.73 mg L-1 d(-1), T. lutea and P. tricornutum). These results show that biomass and Fx productivities can be set by controlling biomass concentration under outdoor conditions and can be predicted using fluorescence spectroscopy. This monitoring tool opens new possibilities for online process control and optimization.