Part of the complication is that the exact ways in which experiments
with animals have modeled ocean acidification vary.  Exact future
conditions are, of course, uncertain.

Deep ocean water is unsaturated with respect to aragonite, which is
what most mollusks, all modern corals, and some other organisms use
for their skeletons (and very deep ocean water is also unsaturated
with respect to calcite, which is what most other animals, except for
vertebrates, and many other organisms use).  This is because, as stuff
decays, carbon dioxide and organic acids are released.  Current ocean
circulation patterns make the main sinking point of surface water in
the north Atlantic and mixing back into shallower water about three
hundred years later in the north Pacific, based on a complex interplay
of water temperature and salinity (colder or saltier=denser water that
sinks).  If the water dissolves the shells faster than they can grow,
the animal has problems.  Increasing CO2 levels tends to make the
water more acidic-better at dissolving shells and harder to make
shells.  However, if the acid is neutralized, that CO2 ends up as
(bi)carbonate ions, which is what is needed for making shells.

Volcanoes occasionally give a small boost to the levels of various
acid-making gases.  Major volcanic eruptions in the geologic past may
have caused significant changes of that sort.  However, under ordinary
conditions, the blip caused by the volcano settles back down.  Right
now, the rapid, multidecade increase in CO2 levels makes it hard for
compensating factors to keep up.  The oceans are generally getting
more acidic, though the particular incident off the west coast may be
more related to climate promoting more upwelling of deep water than to
changing ocean chemistry.

--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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