Comparative effects of herbicides on photosynthesis and growth of tropical estuarine microalgae

Pulse amplitude modulation (PAM) fluorometry is ideally suited to measure the sub-lethal impacts of photosystem II (PSII)-inhibiting herbicides on microalgae, but key relationships between effective quan- tum yield [Y(II)] and the traditional endpoints growth rate (l) and biomass increase are unknown. The effects of three PSII–inhibiting herbicides; diuron, hexazinone and atrazine, were examined on two trop- ical benthic microalgae; Navicula sp. (Heterokontophyta) and Nephroselmis pyriformis (Chlorophyta). The relationships between Y(II), l and biomass increase were consistent (r2 P 0.90) and linear (1:1), validat- ing the utility of PAM fluorometry as a rapid and reliable technique to measure sub-lethal toxicity thresh- olds of PSII-inhibiting herbicides in these microalgae. The order of toxicity (EC50 range) was: diuron (16–33nM)>hexazinone (25–110nM)>atrazine (130–620nm) for both algal species. Growth rate and photosynthesis were affected at diuron concentrations that have been detected in coastal areas of the Great Barrier Reef.