A Long Collapse: Coral reefs are threatened by global warming and ocean acidification, and so it is important to understand better how and why environmental changes have affected them in the past.Tothet al.(p.81) present a 6000-year-long record of coral reefs off the coast of Panama, Central America. The reefs effectively stopped growing for approximately 2600 years, beginning around 4000 years ago. This collapse of the coral reef system was probably caused by increased variability of ENSO, the El Nino–Southern Oscillation. If the strength or frequency of ENSO were to increase, the viability of these and other reef systems in the Pacific could be put further at risk.

Ocean acidification affects a wide diversity of marine organisms and is of particular concern for vulnerable larval stages critical to population replenishment and connectivity. Whereas it is well known that ocean acidification will negatively affect a range of calcareous taxa, the study of fishes is more limited in both depth of understanding and diversity of study species. We used new 3D microcomputed tomography to conduct in situ analysis of the impact of ocean acidification on otolith (ear stone) size and density of larval cobia ( Rachycentron canadum ), a large, economically important, pantropical fish species that shares many life history traits with a diversity of high-value, tropical pelagic fishes. We show that 2,100 μatm partial pressure of carbon dioxide ( p CO ₂ ) significantly increased not only otolith size (up to 49% greater volume and 58% greater relative mass) but also otolith density (6% higher). Estimated relative mass in 800 μatm p CO ₂ treatments was 14% greater, and there was a similar but nonsignificant trend for otolith size. Using a modeling approach, we demonstrate that these changes could affect auditory sensitivity including a ∼50% increase in hearing range at 2,100 μatm p CO ₂ , which may alter the perception of auditory information by larval cobia in a high-CO ₂ ocean. Our results indicate that ocean acidification has a graded effect on cobia otoliths, with the potential to substantially influence the dispersal, survival, and recruitment of a pelagic fish species. These results have important implications for population maintenance/replenishment, connectivity, and conservation efforts for other valuable fish stocks that are already being deleteriously impacted by overfishing.

Ocean acidification (OA) is expected to reduce the calcification rates of marine organisms, yet we have little understanding of how OA will manifest within dynamic, real-world systems. Natural CO₂, alkalinity, and salinity gradients can significantly alter local carbonate chemistry, and thereby create a range of susceptibility for different ecosystems to OA. As such, there is a need to characterize this natural variability of seawater carbonate chemistry, especially within coastal ecosystems. Since 2009, carbonate chemistry data have been collected on the Florida Reef Tract (FRT). During periods of heightened productivity, there is a net uptake of total CO₂ (TCO₂) which increases aragonite saturation state (Ω_arag) values on inshore patch reefs of the upper FRT. These waters can exhibit greater Ω_arag than what has been modeled for the tropical surface ocean during preindustrial times, with mean (± std. error) Ω_arag-values in spring = 4.69 (±0.101). Conversely, Ω_arag-values on offshore reefs generally represent oceanic carbonate chemistries consistent with present day tropical surface ocean conditions. This gradient is opposite from what has been reported for other reef environments. We hypothesize this pattern is caused by the photosynthetic uptake of TCO₂ mainly by seagrasses and, to a lesser extent, macroalgae in the inshore waters of the FRT. These inshore reef habitats are therefore potential acidification refugia that are defined not only in a spatial sense, but also in time; coinciding with seasonal productivity dynamics. Coral reefs located within or immediately downstream of seagrass beds may find refuge from OA.