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In response to recent reports of contamination of the nearshore marine environment along the Queensland coast by herbicides (including areas inside the Great Barrier Reef Marine Park), an ecotoxicological assessment was conducted of the impact of the herbicides diuron and atrazine on scleractinian corals. Pulse-amplitude modulated (PAM) chlorophyll fluorescence tech- niques were used to assess the herbicide effects on the symbiotic dinoflagellates within the tissues (in hospite) of 4 species of coral (Acropora formosa, Montipora digitata, Porites cylindrica, Seriatopora hystrix) in static toxicity tests, and in freshly isolated symbiotic dinoflagellates from Stylophora pistil- lata. Using change in the effective quantum yield (∆ F/Fm') as an effect criterion, diuron (no observ- able effect concentration, NOEC = 0.3 µg l -1 ; lowest observable effect concentration, LOEC = 1 µg l -1 ; median effective concentration, EC50 = 4 to 6 µg l -1 ) was found to be more toxic than atrazine (NOEC = 1 µg l -1 , LOEC = 3 µg l -1 , EC50 = 40 to 90 µg l -1 ) in short-term (10 h) toxicity tests. In the tests with isolated algae, significant reductions in ∆F/Fm' were recorded as low as 0.25 µg l -1 diuron (LOEC, EC50 = 5 µg l -1 ). Time-course experiments indicated that the effects of diuron were rapid and reversible. At 10 µg l -1 diuron, ∆ F/Fm' was reduced by 25% in 20 to 30 min, and by 50% in 60 to 90 min. Recovery of ∆ F/Fm' in corals exposed to 10 µg l -1 diuron and then transferred to running sea- water was slower, returning to within 10% of control values inside 1 to 7 h. The effect of a reduction in salinity (35 to 27 ‰) on diuron toxicity (at 1 and 3 µg l -1 diuron) was tested to examine the poten- tial consequences of contaminated coastal flood plumes inundating inshore reefs. ∆ F/Fm' was reduced in the diuron-exposed corals, but there was no significant interaction between diuron and reduced salinity seawater within the 10 h duration of the test. Exposure to higher (100 and 1000 µg l -1 ) diuron concentrations for 96 h caused a reduction in ∆ F/Fm', the ratio variable to maximal fluo- rescence (Fv /Fm), significant loss of symbiotic dinoflagellates and pronounced tissue retraction, caus- ing the corals to pale or bleach. The significance of the results in relation to diuron contamination of the coastal marine environment from terrestrial sources (mainly agricultural) and marine sources (antifouling paints) are discussed.

Effects of herbicides diuron and atrazine on corals of the Great Barrier Reef, Australia