Response of phytoplankton photosynthesis and growth to the aquatic herbicide Hydrothol 191

The herbicide Hydrothol 191, the amine salt of endothall, has been widely used for the control of aquatic vegetation and filamentous algae for over 30 years. Recently, it has been proposed for use in secondary wastewater stabilization ponds for control of algal-related total suspended solids (TSS) prior to and during discharge. However, published information on the effect of Hydrothol on algal growth and species-specific sensitivity is limited. We performed 96-h unialgal toxicity tests to determine effects on growth and photosynthetic activity on a number of species commonly found in these wastewater ponds. Results indicated differences in sensitivity to the herbicide between chlorophytes and cyanophytes. The 48-h 50% effective concentration (EC50) values for the blue-green Microcystis were 0.04 to 0.08 mg/L for cell growth and 0.04 to 0.22 mg/L for photosynthetic capacity; values for Phormidium were 0.05 mg/L and 0.097 mg/L for the same endpoints. The Chlorococcales greens Chlorella and Scenedesmus and the flagellated green Chlamydomonas showed much lower sensitivity to Hydrothol. The 48-h EC50 values for Chlorella were >0.60 mg/L for both cell growth and photosynthetic capacity while values for Scenedesmus were >0.60 mg/L and 0.23 mg/L for the same parameters. The flagellate Chlamydomonas required 0.52 mg/L and 0.42 mg/L, respectively, to reduce cell density and photosynthetic capacity 50% below controls. These data suggest that reductions in the amount of Hydrothol needed to reduce algal-related TSS may be possible when the population is dominated by blue-greens that could be economically beneficial and allow for applications with reduced environmental concerns.