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Ocean acidification: Some Winners, Many Losers

Posted on 10 June 2011 by Rob Painting

Numerous lab experiments have shown that ocean acidification is harmful to marine life. Creatures that build chalk-like shells (or skeletons) fare poorly under conditions which mimic the low ocean pH levels expected later this century. This isn't a universal response however; some starfish, brittle stars and sea urchins, seem relatively unaffected by ocean acidification, so it's likely there will be winners and losers as the world's oceans become less alkaline. 

Nature's own laboratory

Despite their usefulness, lab experiments are no substitute for the natural environment, and experiments tend to be of short duration too, so the long-term effects of elevated CO2 on marine communities is largely unknown. Luckily there are a couple of locations in the world which mimic ocean acidification. These locations are not perfect analogues for the future, seawater acidity is not held permanently low because of seasonal wind and current variation, for example, but they do give some insight into how marine life might adapt (or not) to ocean acidification over the long haul.

Fabricius 2011 examines the coral reefs in one such area, Milne Bay Province in Papua New Guinea, where cool CO2 bubbles up through natural seeps on the seafloor, thereby lowering seawater pH. The authors examined underwater sites which are equivalent  to 3 ocean pH scenarios: a control site, where seawater is at ambient (normal) pH (around 8.1), a low pH site (7.8-8.0), and a very low pH site (below 7.7).

Figure 1 -Seascapes at a, control site (‘low pCO2’: pH~8.1), b, moderate seeps (‘high pCO2’: pH 7.8–8.0), and c, the most intense vents (pH<7.7), showing progressive loss of diversity and structural complexity with increasing pCO2. d, Map of the main seep site along the western shore of Upa-Upasina. Colour contours indicate seawater pH, and the letters indicate the approximate locations of seascapes as shown in a–c.

What the authors found, is probably no surprise: as the level of seawater pH dropped from it's normal value of 8.1, the health of the reef began to deteriorate. Various hard and soft corals, and other hard-shelled marine life, such as cructose coralline algae, disappeared from the seafloor. Only one type of coral was able to tolerate ph as low as 7.8, which is equivalent to 750 ppm of atmospheric CO2: a figure we're on track to reach before the end of the 21st century. Below a pH of 7.7 however, coral reef development stopped dead in it's tracks. The beneficiaries, it turns out, are slime (macroalgae) and seagrasses, with their populations flourishing as seawater becomes more acidic.

Another discovery made by the authors, is that coral reef growth (calcification) was 30% lower than is the norm for coral reefs at similar latitude (distance from the equator), and both the low and ambient pH sites exhibited the same condition. The Milne Bay area has experienced repeated mass coral bleaching events in the last 20 years, so this has probably played a role in the low growth rates. This is consistent with the latest research showing coral growth rates have diminished worldwide in recent decades.

Natural CO2 seeps in the Mediterranean

The work of Fabricius 2011 builds on earlier work off the island of Ischia, Italy; another area where CO2 bubbles up from the seafloor. Hall-Spencer (2008) reached similar conclusions researching the waters off Ischia; lower pH inhibited the development of calcifying marine life, and favoured the growth of seagrasses and slime. See video of the area, featuring Dr Hall-Spencer, below:

Ocean Acidification - Ischia from UNEP WCMC on Vimeo.

Future implications - oceans of slime?

The potentially devastating impact of future ocean acidification is clearly obvious in the pictures above. Despite being exposed to acidic conditions for thousands of years, it seems many marine creatures can't handle ocean acidification, and the ones that do are not conducive to healthy coastal ecosystems. The loss of of cructose coralline algae is particularly important to corals, as they secrete chemicals that provide settlement cues for coral larvae, help consolidate the reef structure, and prevent the growth of slime. Without the complex structure and shelter that healthy reefs provide, many thousands of reef species, including many fish, may disappear.  

Fabricius 2011 adds to the growing body of scientific research which spell out a troubling future for our oceans. There will undoubtedly be some winners as the oceans become more acidic, but just how palatable is a menu consisting of slime, seagrass and brittlestars, and the occasional sea urchin? That is something future generations may have to to ponder.

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Comments 1 to 18:

  1. Hi Rob, Thanks for covering this. Two questions. 1) What is the natural low pH value around the PNG bubblers? Ie, how high does acidity get? The common criticism in the field of this "natural experiment" approach is that when flow (current velocity) is low, pH gets to extremely low values (due to reduced mixing with ambient water), which is when the damage is likely done to nearby organisms. Yet what often gets reported in these papers is the mean pH, which isn't as relevant as the extreme values. IOW, are they an analogue for 750 ppm or 3000 ppm? 2) Why label macroalgae "slime"? Id rather eat algae than coral (being partly facetious) and lots of people love to eat urchins! [you might add that it is the inhabitant fish, which are dependent on corals, that would be lost from dinner plates]
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  2. There is one point where I have to disagree: where the article says, "That is something future generations may have to to ponder" They don't have to ponder it -- we are not giving them any choice. We have already made sure that ocean acidification is here to stay, and can only get worse. We did this by failing to cut when we could, and dismantling the political/social structures that might have made cuts possible in the near future. War, pestilence and famine in a shrinking and degrading biosphere is the legacy we leave our descendants. Good thing their curses won't affect us.
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  3. Perhaps the most significant impact of ocean acidification is the decline of phytoplankton as discussed in the following paper. "Global phytoplankton decline over the past century Daniel G. Boyce, Marlon R. Lewis & Boris Worm Journal:Nature Volume:466,Pages:591–596 Date published: (April, 2011) In the oceans,ubiquitous microscopic phototrophs (phytoplankton) account for approximately half the production of organic matter on Earth. Analyses of satellite-derived phytoplankton concentration (available since 1979) have suggested decadal-scale fluctuations linked to climate forcing, but the length of this record is insufficient to resolve longer-term trends. Here we combine available ocean transparency measurements and in situ chlorophyll observations to estimate the time dependence of phytoplankton biomass at local, regional and global scales since 1899. We observe declines in eight out of ten ocean regions, and estimate a global rate of decline of ~1% of the global median per year. Our analyses further reveal interannual to decadal phytoplankton fluctuations superimposed on long-term trends. These fluctuations are strongly correlated with basin-scale climate indices, whereas long-term declining trends are related to increasing sea surface temperatures. We conclude that global phytoplankton concentration has declined over the past century; this decline will need to be considered in future studies of marine ecosystems, geochemical cycling, ocean circulation and fisheries." “The data is good in the northern hemisphere and it gets better in recent times, but it’s more patchy in the southern hemisphere – the Southern Ocean, the southern Indian Ocean, and so on. The higher quality data available since 1950 has allowed the team to calculate that since that time, the world has seen a phytoplankton decline of about 40%." In addition to being the basis of the ocean food chain, plankton generate half of the worlds oxygen. I don't know why these two facts don't put the world into a panic in view of a 40% decline in plankton.
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  4. John Bruno - 1. valid points, and I don't know that I've seen them adequately addressed, however: see figure 1 d above - less than 5% of the measurements find pH drops as low as 7.09 at the most intense CO2 sites (avoiding the term percentiles here). Hopefully we'll see some further studies resolving these issues. 2. Slime?, seen the same term used by coral reef experts, and a lot of it feels pretty darned slimy to me too!. Macroalgae, although technically correct, doesn't mean much to Joe and Jane Public, but if you can came up with a better term......... 3. Me, I'd rather not eat slime nor coral, but different strokes (I can do facetious too!). I've amended the text to mention the loss of fish.
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  5. MattJ - I was referring to pondering the palatability of what's left in the oceans. That the oceans will be greatly depleted, seems a given on our current course. Bibasir - it's a concern alright, but other scientists have questioned the results of that study, so we'll have to see how that plays out in the end. As far as oxygen in the oceans, warming reduces the solubility of oxygen, and we're consequently seeing an expansion of 'dead zones'. There's actually a whole bunch of other stuff taking place in the oceans, that we'll get around to discussing in the future.
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  6. I do have some questions. Linking two articles together, the question is how come coral reefs and other shell creatures survived for several million years when CO2 levels were much higher than today? Why wouldn't a quadruple amount of CO2 cause more acidification than anything we can possibly reach this century? Yet Coral and other shell creatures were able to survive and thrive...why? Links: Graph of atmospheric CO2 a few million years ago to today. Source aticle for graph. Coral was alive during high atmospheric CO2 levels.
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  7. Norman, changes to carbonate compensation depth is actually part of the tools for determining past atmosphere. Shallow reef corals all but went extinct during PETM and we are now increasing CO2 at rate nearly 10 times faster than this event. Read up on CCD.
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  8. Norman, The NOAA information page says, "Appearing as solitary forms in the fossil record more than 400 million years ago, corals are extremely ancient animals that evolved into modern reef-building forms over the last 25 million years." I take that to mean that the corals that synthesize calcium carbonate (CaCO3) evolved during the last 25 million years. Now look at the CO2 level in your graph, as of 25 million years ago. It was already lower than at any earlier time in the past 500 million years. Putting these together, it appears to me that, while solitary corals were alive during the high CO2 levels, the _modern reef-building, (CaCO3)-synthesizing_ forms did not evolve (nor have lived) during especially high CO2 conditions, according to the graph you present.
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  9. What about shelled zooplankton, coccolithophores? Aren't they a major carbon sink, which acidification could threaten?
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  10. In my preceding comment, "I take that to mean that the corals that synthesize calcium carbonate (CaCO3) evolved during the last 25 million years." should have been "I take that to mean that the corals that synthesize enough calcium carbonate (CaCO3) to build reefs evolved during the last 25 million years" since the NOAA article further says, "Although all corals secrete CaCO3, not all are reef builders. Some corals, such as Fungia sp., are solitary and have single polyps that can grow as large as 25 cm in diameter. Other coral species are incapable of producing sufficient quantities of CaCO3 to form reefs."
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  11. Norman @ 6 - in addition to the advice you have received above, note that the rate of CO2 increase is important to global ocean pH. In the past, increases in atmospheric CO2 have often happened over much longer timescales than is currently taking place (thousands, or tens of thousands of years vs hundreds), therefore silicate weathering, is able to buffer the extra CO2 dissolved into the oceans. The boost in alkalinity provided by silicate weathering is able to offset much of the acidification if the rise in CO2 is slow enough, because the weathering process operates on timescales of tens to hundreds of thousands of years. An additional consideration is, if the process happens slowly enough, the extra CO2 is able to be distributed to the deep ocean, offsetting the effects of acidity in surface waters. What is happening today is that humans are adding so much CO2 to the atmosphere, so fast, that the natural buffering processes, and circulation to deeper waters, cannot keep up. CO2 is building up in the surface ocean; causing pH to fall rapidly. The oceans have not been this acidic for at least 20 million years, probably much longer, and the rate of decline in ocean pH is probably unprecedented. Hope this helps. We'll have some posts touching on this within the next month.
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  12. Of interest to this topic: SAHFOS page on acidification, and paper linked therein.
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  13. Hi Rob, how about "seaweed". My point is that labeling a critical and normal component of reef ecosystems "slime" to telegraph a message of a yucky, unwanted protoplasm is not exactly helpful or scientific. And yes, a few misguided coral reef scientists subjectively refer to "slime" on reefs, but they are referring to microbes not seaweed/algae. Regarding the extreme pH values, indeed, they make up a small amount of the total measured, but are the driving force of the extant community state. Same with climate change; eg, it is the very rare extreme high values that cause coral bleaching, so it would be misleading to look at mean temp and relate that to reef state. Ill dig up the papers on the extreme values at some of the world's CO2 bubblers.
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  14. John, I have seen reefs that have degraded and slime is the correct discription. It is usually blue green algae or red slime algae. Sometimes it is hair algae, which is also slimy. If you have citations that the replacement algae is seeweed and not slime you need to present them, not assert without any evidence that seeweed is the replacement algae. Suggesting that "a few misguided coral reef scientists subjectively refer to "slime" on reefs," without citations is not a scientific argument, it is unsupported opinion.
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  15. Pardon my ignorance, but I don't understand this: "The higher quality data available since 1950 has allowed the team to calculate that since that time, the world has seen a phytoplankton decline of about 40%." If phytoplankton generate 50% of the oxygen, then a 40% decline would appear to imply a 20% decline in oxygen generation. Surely we would notice if the concentration of oxygen in the atmosphere were falling? Is there a lag time before atmospheric concentration falls? How would reduced generation be reflected in overall concentration? How low would it go? Thanks in advance for any replies.
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    [DB]  IIRC, any reduction in seawater oxygen content due to the phytoplankton decline will be interspersed thoughout the various layers of the worlds oceans.  That's a lot of volume.  Given the volume of the atmosphere, any reductions in oxygen content due to that loss may have been offset by oxygen coming from melting ice sheets (Canadian Archipelago, Greenland, Antarctica as well as the global decline in alpince glaciation).  Speaking off-the-cuff, as I haven't studied that particular aspect.  Don't lose any sleep over it.


    I have a question.  I have heard that the oceans are under saturated by CaCO3 because the average concentration of Ca++ in the oceans is much lower than the sum of all carbonate, bicarbonate, and carbonic acid concentrations.  However,  when concerned with the solubility of the CaCO3 shells of living organisms, I would expect that the more relevant question is - is the sea water in the immediate vicinity of these living species saturated with respect to CaCO3?   If so, one would observe a gradient of Ca++ concentration as one moves from the region of shelled species out towards the depths am remote regions of the oceans.  Certainly such measurements have been made.    


    Note that Matt Ridley has suggested that increased atmospheric CO2 levels will actually facilitate the growth of CaCO3 shells (in his misnamed book, Realistic Optimist) - an argument that has meaning only if the sea water in the vicinity of shelled critters is not saturated w.r.t. CaCO3.   


    Any insight on this point would be apprecited.


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  17. Sorry, I meant to say  because" average concentration of Ca++ in the oceans is much HIGHER than the sum of all carbonate, bicarbonate, and carbonic acid concentrations."  Thus according to Ridley, increasing the latter three would assist in CaCO3 formation.  

    (as we also know, of course, increased acidity serves to decrease the conc of carbonate ion relative to bicarbonate and carbonic acid - thus working against CaCO3 (s) formation -  an important point not mentioned by Ridley).

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  18. Eric - It shouldn't surprise you to learn that Matt Ridley is wrong. The oceans are well saturated with calcium ions, so they are not a consideration. The concentration of calcium ions dissolved into the oceans only changes on geological timescales (typically millions of years) - hence the shift between aragonite and calcite seas over these long periods.

    It seems for many marine life the concentration (activity) of carbonate ions is the key because carbonate ions serve as one of the building blocks of the calcium carbonate (chalk) shell/skeleton. One of the reactions that takes place when extra CO2 is dissolved into the oceans is:

    Seawater currently favours the left-hand side of that equation, so adding CO2 to the oceans is actually decreasing the activity of carbonate ions, which in turn makes shell-building ever more energetically expensive. Decrease the carbonate ion concentration sufficiently (calcium carbonate undersaturation) and we end up with seawater that is physically corrosive to marine calcifiers.

    This corrosiveness is already occurring in the waters of the American Pacific Northwest where oyster larvae are now largely unable to survive in the wild, because they dissolve (Barton [2012]). The Antarctic is also seeing highly corrosive surface water too. See the photo below of a pteropod (sea butterfly) which was caught (alive) several years ago: 

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