A lone fish swims through a veil of carbon dioxide bubbles in the extreme low acidity zone. The brown landscape below is devoid of the sea urchins, gastropods, and worms, found in areas with a normal level of acidity in the Mediterranean Sea. Photo: Kristy J. Kroeker |
Stanford University researchers have
gotten a glimpse into an uncertain future where increasing levels of carbon
dioxide in the Earth’s atmosphere will lead to higher levels in the ocean as
well, leaving the water more acidic and altering underwater ecosystems.
The glimpse
comes from waters near Ischia, Italy, where unusual shallow-water volcanic
vents in the floor of the Mediterranean Sea
bubble carbon dioxide into the water, creating a local underwater neighborhood
that may resemble the ocean of the future.
If the results
are a prediction of the future, “you are left with a dramatically
different ecosystem that is likely going to be less able to deal with stress
and is going to have less biomass available to feed organisms higher up the
food chain,” says Kristy Kroeker, a graduate student in biology at Stanford’s
Hopkins Marine Station.
The special
significance of this research site is that, unlike most hydrothermal vents, it
spouts just carbon dioxide, without hot, sulfurous brews that are lethal to all
but the most highly adapted extremophile organisms. The carbon dioxide comes
out of the vents at the same temperature as the surrounding water.
The carbon
dioxide vents cause a local gradient in the seawater chemistry, with the
greatest acidity closest to the vents. While the researchers found that various
species reacted differently to the more acidified waters—some suffered, some
prospered—their overall findings do not bode well for the biological community
as a whole.
“The
types of organisms that were winners in this acidified environment are
functionally very different from those that were lost,” Kroeker says.
Kroeker is the
lead author of a paper describing the research published recently in Proceedings of the National Academy of Sciences.
Fiorenza Micheli, professor of biology at Hopkins Marine Station, is Kroeker’s
thesis adviser and a coauthor of the paper.
Kroeker and
her colleagues looked at invertebrate seafloor communities spread along a range
of acidity, from the most acidic water right around the vents out to distances
of 200 m, where the acidity of the water was that of the ambient Mediterranean Sea.
The biggest
losers were organisms with shells made of calcium carbonate, which dissolves in
acidic water. Snails, clams, mussels, and scallops were all absent from the
extremely acidic zone. Small crabs, sea urchins, shrimps, and species of worms
that live in tubes of calcium carbonate attached to the rocks were also
missing. Some of the worms were also absent from the intermediate zone, where
the acidity was moderately greater than the ambient water.
“If you
were to go snorkel along this gradient, you would see some variability within
each zone, but the changes in the makeup of the ecological community are very
clear when you move from one area to another,” Kroeker says. Moving into
the most acidic zone, “The change is obvious within a single meter,”
she says.
The winners
tended to be very small-bodied organisms, in particular some shrimp-like
crustaceans and small worms.
But ecology is
not a zero-sum game and in terms of the overall ecosystem, winners and losers
don’t even out. Even with an increased abundance of the smaller organisms, the
total biomass of the community is decreased because of the loss of the larger-bodied
creatures, according to Kroeker. That could reduce the amount of food available
for organisms higher in the food web.
There is also
less diversity in the biological community. The organisms that prospered tended
to be generalists, while the number of specialist species dwindled in the
extreme high acidity zone.
“It is a
simplified community and you have fewer species, so each species plays a
disproportionately more important role,” Kroeker says. “If something
happens to one of those species, you are more likely to have larger effects in
the ecosystem as a result, likely making it less stable.”
An ecosystem
may be able to withstand the loss of different species up to a point, but
eventually, like a table that’s had too many of its legs knocked off, it will
collapse.
The biggest
drop in species diversity was observed in moving from the zone of intermediate
acidity to the extremely acidic zone, suggesting that many species may be able
to tolerate a modest increase in acidity before having to exit or expire.
Most computer
models for the effects of global warming are designed to predict trends on a
global scale and generally are based on conditions on the open ocean, which is
an extremely stable environment showing little acidity (pH) variation on a
daily cycle.
“The
ocean as a whole might have one pH, but on the scale at which an organism
experiences the acidification, things could look very, very different,”
Kroeker says.
In areas where
plant and animal life are abundant, pH would be influenced by algae absorbing
carbon dioxide from the water during photosynthesis and animals releasing it
during respiration, potentially making habitat-level acidity more variable.
Areas of
upwelling—where highly acidic water from the deep ocean is brought to the
surface—can experience rapid changes in pH values. Along the coast of the
western United States,
for example, upwelling can cause the pH range over the course of a single day
to fluctuate almost twice as much as the predicted increase in acidity for the
ocean between now and 2100.
“Near-shore
ecosystems that are affected by upwelling and by biological processes are quite
likely to see acidification values that are more extreme than what we actually
predict for the global oceans,” Kroeker says.
That could bode ill not just for the marine organisms living in near-shore
waters, but also for the humans who have gotten used to feeding on that rich
biota.
