Synergistic effects of diuron and sedimentation on photosynthesis and survival of crustose coralline algae

Effects of short-term exposure to sedimentation and diuron, separately and in combination, on the photophysiology and survival of crustose coralline algae (CCA) were examined in controlled time-course experiments, using pulse-amplitude modulation (PAM) chlorophyll fluorometry. These experiments indicated that the effects of sediments and diuron, when applied in isolation, were often reversible, with recovery time dependant upon sediment type and diuron concentration. Exposure to fine (<63 μm grain size), nutrient-rich estuarine sediments reduced effective quantum yields (ΔF/Fm′) of photosystem II in CCA species more than exposure to the same amount of fine (<63 μm grain size) calcareous sediments. Significant inhibition of photosynthesis (ΔF/Fm′) was also observed at diuron concentrations ⩾2.9 μg L−1. Fine estuarine sediments in combination with ⩾0.79 μg L−1 dissolved diuron, caused yields (ΔF/Fm′) to drop by 60% compared with controls after 24 h. The combined exposure to sediments and diuron also retarded recovery, thus ΔF/Fm′ values were still only 60% of the controls after 9 days recovery in clean seawater. Mortality of CCA was observed in some fragments treated with combinations of sediment and diuron. Our results suggest that sediment deposition and exposure to diuron can negatively affect the photosynthetic activity of CCA, with sedimentation stress being significantly enhanced by the presence of trace concentrations of diuron.