According to the results, paraquat at different concentrations in every microcosm adversely affected attached algae by inhibiting the synthesis of chlorophyll a and c of attached algae. In agreement with this present study, Wong [10] clearly stated that paraquat in concentrations of 0.02 to 200 mg/l could lead to the inhibition of chlorophyll a synthesis of Scenedesmus quadricauda Berb 614 – a species of green algae. A study by Butler (as cited in [7]) reiterated the destructive effect of paraquat on some species of diatoms. This researcher reported that 0.1 mg/l of paraquat inhibited growth of Navicula osteraria, while 100 mg/l could stop the growth of Phaeodactylum tricornutum.