Natural Carbon Dioxide Gradients Show Ecosystem-Wide Effects of Ocean Acidification

Areas with naturally high CO₂ (and/or low pH and low calcium carbonate saturation states) are increasingly being used as ‘natural laboratories’ to investigate how organisms and ecosystems might respond to the long-term consequences of ocean acidification. These areas are already revealing that some marine species can tolerate periods of high CO₂/low pH in upwelling zones, that estuaries have pH gradients that can corrode mollusc shells, and that some parts of the ocean have such low levels of calcium carbonate that coral reefs dissolve.  Volcanic vents are also proving to be particularly useful ‘natural laboratories’ for the study of ocean acidification as they reveal tipping points in calcification, recruitment, growth, survival and species interactions along CO₂ gradients. Our studies show that many species of macroalgae, seagrass, foraminiferans, corals, polychaetes, crustaceans, molluscs and bryozoans are remarkably tolerant of long-term exposures to high and variable carbon dioxide levels at tropical and temperate vents.  However, a fall in mean pH from 8.1 to 7.8 (which corresponds to predictions for the open ocean in the next 90 years) has detrimental effects on larval recruitment and causes a 30% loss in the biodiversity of sedimentary habitats, rocky shores, coral reefs  and seagrass beds.  In addition, transplant experiments show that unusually high sea surface temperatures can act synergistically with ocean acidification at these sites to cause further stress on marine species, strengthening evidence that rising global CO₂ levels could cause declines in coastal marine biodiversity and shifts in ecosystem structure.