Low recruitment due to altered settlement substrata as primary constraint for coral communities under ocean acidification.

The future of coral reefs under increasing CO2 depends on their capacity to recover from disturbances. To predict the recovery potential of coral communities that are fully acclimatized to elevated CO2, we compared the relative success of coral recruitment and later life stages at two volcanic CO2 seeps and adjacent control sites in Papua New Guinea. Our field experiments showed that the effects of ocean acidification (OA) on coral recruitment rates were up to an order of magnitude greater than the effects on the survival and growth of established corals. Settlement rates, recruit and juvenile densities were best predicted by the presence of crustose coralline algae, as opposed to the direct effects of seawater CO2 Offspring from high CO2 acclimatized parents had similarly impaired settlement rates as offspring from control parents. For most coral taxa, field data showed no evidence of cumulative and compounding detrimental effects of high CO2 on successive life stages, and three taxa showed improved adult performance at high CO2 that compensated for their low recruitment rates. Our data suggest that severely declining capacity for reefs to recover, due to altered settlement substrata and reduced coral recruitment, is likely to become a dominant mechanism of how OA will alter coral reefs.

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 dependent upon sediment type and diuron concentration. Exposure to fine (<63 microm grain size), nutrient-rich estuarine sediments reduced effective quantum yields (Delta F/F(m')) of photosystem II in CCA species more than exposure to the same amount of fine (<63 microm grain size) calcareous sediments. Significant inhibition of photosynthesis (Delta F/F(m')) was also observed at diuron concentrations > or =2.9 microg L(-1). Fine estuarine sediments in combination with 0.79 microg L(-1) dissolved diuron, caused yields (Delta F/F(m')) to drop by 60% compared with controls after 24 h. The combined exposure to sediments and diuron also retarded recovery, thus Delta F/F(m') 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.