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The data presented in this investigation revealed that the gain in the dry weights of all algae tested was suppressed in herbicide treatments, particularly, at higher concentrations. However, at the lowest dose of the herbicide, the growth inhibition was statistically significant for Anabaena and insignificant for the other organisms. The chlorophyll a content in Anabaena was slightly, however, insignificantly increased at lower concentrations; a phenomenon that was enhanced by increasing the herbicide dose. On the other hand, the pigment biosynthesis was suppressed in all treatments, in Tolypothrix and Aulosira, being more effective and statistically significant in the latter. The pigment accumulation in Nostoc was slightly and insignificantly promoted by all herbicide doses except at the highest concentration where a slight drop was noticed. The herbicide was most efficient at lower doses, in stimulating glucose absorption by either Nostoc and Anabaena up to 0.1 ppm. Applying higher doses, a remarkable drop was maintained. It induced significant retardation in the sugar uptake by Tolypothrix and Aulosira. This trend was confirmed by increasing the herbicide concentration. In all treatments with atrazine the total carbohydrate accumulation in all test organisms was suppressed and was more effective at the lowest concentration in Tolypothrix and Aulosira, however, of less significant effect in Anabaena and Nostoc. It may be concluded that the toxic effects of atrazine are partially due to its inhibitory effects on the synthesis of carbohydrates.

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