In this study, a microfluidic device with a light modulation system was developed to analyze microalgal photosythesis and respiration via real-time monitoring of oxygen production/consumption rates (OPR and OCR) based on based on phase-based phosphorescence lifetime detection within a microfluidic device. Besides, we also demonstrated our proposed device for water toxicity analysis and more particularly Diuron herbicide detection in water. The basic detection principle was based on monitoring disturbances in metabolic photosynthetic activities of algae induced by the presence of Diuron herbicide. Algal response, evaluated through variations in oxygen production rates (OPR) of algae, was different due to inhibition effect of the herbicide on photosynthesis in the presence of Diuron herbicide at 0.2–1 µM in the examined sample.