A warning from Washington State scientists that ocean acidification is endangering mussels has been very widely picked up in the press, with even the United Kingdom’s Daily Telegraph on the other side of the world announcing that mussels will be “off the menu due to climate change” by the end of the century.
But what have the researchers actually found, and is this the final word on mussels and acidifying oceans?
A press release from the University of Washington last week reported laboratory evidence that ocean acidification caused by carbon emissions could make it harder for a particular species of mussel to establish its grip on rocks or ropes. If mussel attachments are not strong enough to prevent them from being washed away, the shellfish become easier targets for predators. “A strong attachment is literally a mussel’s lifeline,” said researcher, Emily Carrington. The release added that weaker attachments could have “severe implications for aquaculture”.
The Washington researchers engaged in a series of laboratory experiments, away from the ocean itself, to test whether lowering seawater pH would affect the curing process of the adhesive plaque cementing threads to a surface.
The global average ocean pH level is 8.1. The researchers took threads made by mussels at this “normal” pH and then kept them at levels of 8, 7 or 5 and left to cure for 12 days. Then, using a materials testing machine—a device used to test the tensile strength—they found that mussels make weaker attachment threads once seawater pH drops below 7.6.
They noted that Washington coastal seawater is currently at a baseline pH of 7.8, the level to which the global average is predicted to drop by the end of the century. This sounds like a small difference, but the pH scale is logarithmic; this means that a difference of just one pH unit on the scale is equivalent to a tenfold difference in the substance.
However, the researchers themselves note that these are just “early lab studies”.
Last July a commentary in the journal Nature pointed out that the experimental protocols used in lab-based studies on ocean acidification are facing an uphill battle to match the actual conditions of oceans. Many such lab experiments have shown negative effects from acidification on sea-dwelling organisms via manipulation of seawater chemistry. Few involved real-world studies and in only 27 of the 465 studies did the authors of the commentary find appropriate experimental design.
“Ocean acidification does not stand alone,” said PICS executive director Sybil Seitzinger, herself a coastal biogeochemist, reacting with caution to the news from Washington. “Its interaction with other conditions are complex, and the threats to marine organisms are varied, from other agricultural and industrial pollutants to temperature and oxygen levels.
“If we focus on acidification on its own, we may be letting other culprits, including other human factors, off the hook.”
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