Simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] inhibited photosynthesis 50% at concentrations of 1.1 μM for Spirogyra jurgensii (Kutz), 3.0 μM for Pithophora oedogonia (Mont.) Wittr., and 3.8 μM for Cladophora glomerata (L.) Kutz. Photosynthesis of Ankistrodesmus braunii (Brun.), a nonfilamentous species, was inhibited the same amount by 4.7 μM simazine. The filamentous algal species had significant reductions in growth but no other phytotoxic symptoms when exposed to 5 μM simazine at light intensities of 100 μE·m–2·s–1 and below. Algicidal effects did occur at a light intensity of 400 μE·m–2·s–1 and were most severe in Spirogyra. The relationship between light intensity and simazine toxicity indicates algicidal effectiveness on these filamentous algae will be limited in habitats where light is reduced by turbidity, depth, or self-shading.
