Effects of ocean acidification on Mediterranean coastal habitats: lessons from carbon dioxide vents off Ischia

Ocean chemistry is changing at an unprecedented rate and magnitude due to anthropogenic carbon dioxide emissions. The oceans currently absorb over 25 million tons of carbon dioxide from the atmosphere every day which has caused the average pH of surface waters to decrease from about 8.2 to 8.1 with a further decrease to pH 7.8 predicted by the end of the century. Lowering seawater pH reduces the concentration of calcium carbonate raising concerns about the knock-on effects on calcification in key organisms such as coccolithophores, reef-forming corals and commercially important molluscs. Current research into ocean acidification is mainly laboratory based (using aquaria or mesocosms) and whilst this has been invaluable in determining effects on a range of biological and physical processes it is nevertheless difficult to extrapolate the findings to predict effects on whole marine ecosystems.
Here we describe how volcanic carbon dioxide vents are being used as natural laboratories to study the effects of ocean acidification on a variety of coastal ecosystems as well as individual organisms. Off Ischia Island, Italy, vents enrich seawater carbon dioxide levels and alter calcification, recruitment, growth, survival and species interactions in the acidified waters. 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 the Ischia vents (e.g. min 309, mean 854, max 1908 μatm pCO2). However, a fall in mean pH 8.1 to mean pH 7.8 has detrimental effects on the recruitment of benthic organisms from the plankton with 30% fewer species in adult populations at mean pH 7.8 than in adjacent areas at mean pH 8.1. Important groups, such as coralline algae, calcified foraminiferans and sea urchins are common outside the vents but absent from areas with mean pH ≤7.8, probably due in part to widely variable carbon dioxide levels.

Ongoing transplant experiments have revealed that unusually high sea surface temperatures can act synergistically with ocean acidification. This strengthens concerns, based on model predictions and laboratory experiments, that ocean acidification will likely combine with other stressors to cause declines in Mediterranean marine biodiversity and lead to shifts in ecosystem structure.

Hall-Spencer J. M. & Rodolfo-Metalpa R., 2012. Effects of ocean acidification on Mediterranean coastal habitats: lessons from carbon dioxide vents off Ischia. In: Stambler N. (ed.), Life in the Mediterranean Sea: A Look at Habitat Changes, Chapter 25. Nova Science Publishers. Article (restricted access).

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