Several weeks ago I posted an article which was very sceptical about ocean acidification.  After spending the last term writing this blog and researching ocean acidification, I will now attempt to explore some of their claims and see if there is any truth behind them.

One of their main claims was that ocean biodiversity will flourish under increasingly acidic conditions.  Whilst the last post I wrote suggested that some species of mussel and lobster will be able to survive in acidic oceans there has been no conclusive evidence to suggest that biodiversity will actually prosper.  In fact if you try searching species that will thrive in acidic waters in Google or Web of Science you get very few useful returns. However, if you search species which are threatened by acidifying waters the literature is vast.  As I have said previously, the species most at threat are those which calcify, due to dissolution and a shoaling of the carbonate and aragonite saturation horizon.  These species are often low down in the food chain, and so if they go extinct their predators will also be affected.  The evidence for dissolution is abundant, not only from scientific experiments but also just by looking at some of the world’s coral reefs. I posted this image before which shows how rapidly a shell can deform in acidic water. 

This cartoon shows the deformation of a shell in an acidic ocean over a period of 45 days (NAOO).

Anthony et al. (2011) created a model to see how ocean acidification affected the resilience of coral reefs.  Their main findings were that reefs which suffer from overfishing and eutrophication were more vulnerable to acidic waters.  They also found that as CO₂ levels rise, the resilience and growth rates of coral reefs will fall and mortality will increase.  The Great Barrier Reef has seen a rapid reduction in calcification rates and growth of large corals (Hoegh-Guldberg, 2009).  Hoegh-Guldberg (2009) has also shown that ocean acidification not only reduced the productivity of corals but also made them more sensitive to bleaching.  More recent work by Kurihara et al. (2008) and Mayor et al. (2007) has shown that ocean acidification reduces egg production in marine shrimps and also causes an increase in unsuccessful hatchings in copepods.  Earlier on in this blog I also pointed out larger marine animals were being affected by ocean acidification.  Whale numbers are falling due to an increase in ‘noise’ in acidic waters, which makes communication and navigation trickier.  Clown fish are losing their sense of smell, due to the acidic conditions, which means that they can’t smell their habitats. 

I believe that we cannot deny the fact that ocean acidification is having a detrimental impact on marine organisms.  The effects are most widely felt by calcifying organisms yet larger marine mammals such as Whales are not out of harm’s way.  Caution needs to be given and some people probably over emphasise the effects of ocean acidification, as it is evident that some species can prosper in acidic waters. However, if their prey cannot adapt and goes extinct then they will die too, whether they are adapted to acidification or not.  More work needs to be done to assess the impact of rising CO₂ levels on a wider range of species to see how many other species have been able to adapt.  A significant proportion of ocean species will be affected by ocean acidification but it cannot be claimed that every species will be negatively affected.  Similarly, at the moment the evidence is too large to suggest that creatures are unscathed from rising carbon dioxide levels.

References

Anthony, K, Maynard, J. Diaz-Pulido, G. Mumby, P. Marshalls, P. Cao, L. (2011). ‘Ocean acidification and warming will lower coral reef resilience’, Global change biology, 17, 1798-1808.

Hoegh-Guldberg, O. (2009). ‘Climate change and coral reefs: Trojan horse or false prophecy?’, Coral Reefs, 28, 3, 569-575.

Kurihara, H. Matsui, M. Furukawa, H. Hayashi, M. Ishimatsu, A. (2008). ‘Long-term effects of predicted future seawater CO2 conditions on the survival and growth of the marine shrimp Palaemon pacificus’, J Exp Mar Biol Ecol , 367:41–46.

Mayor, D. Matthews, C. Cook, K. Zuur, A. Hay, S. (2007). ‘CO2-induced acidification affects hatching success in Calanus finmarchicus’, Mar Ecol Prog Ser, 350:91–97.

Can we do anything to reverse ocean acidification?

Up until now I have spent quite a lot of time looking at what is going on in our oceans and what is likely to happen in the future. The picture hasn’t been too rosy so far, but now I will see if there is anything we can do to combat ocean acidification or if we are just destined to disaster.  Iron fertilisation has been put forward as one of the main mechanisms that could help to reverse the acidifying trend.  Ice cores show that during past glacials iron fertilisation has occurred naturally (for example, dust storms coming from the Sahara towards the Canary islands) and has caused large amounts of CO₂ to be drawn out of the atmosphere (Lampitt et al. 2008). Put simply, it is thought that by adding iron to high nutrient low chlorophyll (HNLC) areas of the ocean it will stimulate the growth of phytoplankton blooms which will take up large amounts of atmospheric carbon dioxide and eventually deposit it into the ocean sediments (Cao and Caldeira, 2010).  There has been quite a large amount of speculation as to whether iron fertilisation will actually cause a reduction in atmospheric and oceanic carbon dioxide levels and several experiments have been run, each reaching different conclusions.  Martin was the first to propose the idea of iron fertilisation and he proposed that 1 ton of carbon added to a HNLC region could cause the sequestration of up to 100,000 tonnes of carbon. More recent, high resolution studies have suggested that iron fertilisation is far less efficient and would only result in a 10ppm reduction in atmospheric CO₂ (Lampitt et al. 2008).  The drop in carbon dioxide is only really found in the surface waters and doesn't extend deeper than about 70m (Cao and Caldeira, 2010).  Cao and Caldeira (2010) also found that even if iron fertilisation was successful by the end of the century it would only reduce the pH change by 0.06.  It is thought that iron fertilisation could produce carbon credits which would be doubly bad news for our oceans as not only would the iron do little to prevent the oceans from becoming acidic, countries would be allowed to increase their carbon dioxide emissions. Many private sector companies are already planning to enrich the ocean in order to gain carbon credits, the moderating of such activities is very difficult, especially away from territorial waters.  There are also several ecological impacts that need to be considered, it is probable that there would be an increase in harmful algae which could cause environments to become toxic.  Microbial shifts could also result in the production of other greenhouse gases such as DMS and methane, both of which are far more potent than carbon dioxide.  Due to monitoring difficulties it is also hard to know if plankton blooms could cause the carbon dioxide to be taken into the deep ocean and sediments.  Iron fertilisation initially sounds like a good idea and some believe that it could cause a large reduction in atmospheric CO₂. There are wide scale ecological effects that could result And the actual reductions in ocean pH and carbon dioxide levels are very low.  More work needs to be done to work out the potential side effects of iron fertilisation and to develop a greater understanding of the movement of carbon dioxide to the ocean sediments.

References

Cao, L. Caldeira, K. (2010). ‘Can ocean iron fertilization mitigate ocean acidification?’, Climatic change, 99, 1-2, 303-311.

Lampitt, R. Achterberg, E. Anderson, T. Hughes, J. Lucas, M. Popova, E. (2008). ‘Ocean fertilisation; a potential means of geoengineering’, Philosophical transactions of the royal society A, 366, 1882, 3919-3945.

Did ocean acidification cause the world's largest mass extinction? (Permian-triassic boundary)

This week I am going to look back at the biggest extinction event in the history of the earth and see if ocean acidification played a significant role.  If it did, it intensifies the need for a rapid response to the current crisis.  The PTB occurred 251.4myaBP and saw the loss of over 90% of all marine species, it was also the only extinction event which caused the widespread loss of insects (Bowring et al. 1998, Erwiin, 1994).  Volcanic eruptions and ocean anoxia were both dominant during the PTB and both have been put forward as a possible cause.  Montenegro et al. (2011) have recently created a model to try and assess the role of ocean acidification in the PTB mass extinctions.  The article deduces that the high levels of carbon dioxide during the PTB are enough to make the oceans acidic and undersaturated in aragonite.  This is thought to have been biologically significant and caused widespread coral loss.  More work needs to be done in order to further assess the role of ocean acidification in the PTB mass extinction as the Monetnergo et al. (2011) model is the first to include an accurate portrayal of ocean circulation and seafloor bathymetry.  As ocean acidification is likely to have played a key role in the greatest mass extinction ever, it could suggest that we are on the brink of a 6^th mass extinction, the effects of which could be catastrophic. 

Bowring, S. Erwin, D. Jin, Y. Martin, M. Davidek, K. Wang, W. (1998). ‘U/pb zircon geochronology and tempo of the end-Permian mass extinction’, Science, 280, 1039–1045.

Erwin, D. (1994). ‘The Permo-Triassic extinction’, Nature, 367, 231–235

Monetnergo, A. Spence, P. Meissner, K. Eby, M. (2011). ‘climate simulations of the Permian-traissic boundary: ocean acidification and the extinction event’, Paeleoceanography, 26, 3.

http://www.guardian.co.uk/environment/interactive/2009/dec/10/acidification-oceans-copenhagen?INTCMP=SRCH

This interactive link nicely shows how aragonite concentrations have changed globally and also how this is effecting certain marine species.  The dark orange colours in the Arctic are quite scary as it shows just how much aragonite is thought to have been lost between 1865 and 2095, with some areas seeing a decrease of up to 75%.  I think the impact that ocean acidification is having on Whales and Clown fish is quite surprising, and unlike most other marine animals it is not caused by a reduction in aragonite and calcium carbonate.  Acidic oceans are more 'noisy' as sound can travel further, this is what is causing Whale numbers to fall as they cant communicate or navigate as easily.  The problem for clown fish is that the acidifying oceans are affecting their ability to smell the particular species of anemone that they use as a habitat.  If the pH of oceans continues to fall it is likely that they (and other fish species) will lose their sense of smell altogether.  This proves that ocean acidification is having a more devestating impact on ocean ecosystems than you would initially think.

Why is ocean acidification a problem?

Why should we be concerned about ocean acidification?  Well, one of the main threats is that ocean acidification will cause wide spread extinctions of ocean fauna.  This will then have consequences that will work up the food chain and could potentially be catastrophic.  The rate of change of ocean pH is particularly problematic as it means that many organisms (especially those which calcify) will not have enough time to adapt (Guinotte and Fabry, 2008).  The increase of carbon dioxide into the oceans causes an increase in H⁺ and a decrease in CO^2-₃ which in turn causes a fall in the calcium carbonate saturation state.  Fewer CO^2-₃ will make it harder for calcifying organisms to make calcium carbonate.  Biogenic calcium carbonate is required by a huge amount of marine organisms- most notably the majority of coral species, foraminifera and coccolithopores.  Calcite and Aragonite are also used by calcifying organisms and the abundance of both is influenced by CO₂ levels.  The positions of the calcite and aragonite saturation zones are crucial as CaCO₃ precipitation can only occur above this horizon and below it dissolution will occur (Guinotte and Fabry, 2008).  As oceans acidify the horizons are moving towards the surface, with those of the North Pacific shoaling at a rate of 1-2m/year (Feely, 2007).   Orr et al. (2005) predict that the surface waters of the southern ocean will become undersaturated in Aragonite by 2050 and by 2100 this could include the subarctic pacific as well as the whole of the southern ocean.  Calcifying organisms appear to be incapable of maintaining their shells and skeletons when the oceans are undersaturated in aragonite.  Once the organisms exist below the aragonite/calcite saturation horizon dissolution occurs and so, they become deformed (Fabry et al. 2008).

This cartoon shows the deformation of a shell in an acidic ocean over a period of 45 days (NAOO).

A reduction in calcification and increase in dissolution is not the only detrimental influence that ocean acidification will have on these creatures; ion capacity, buffering capability; acid base regulation and metabolism are all also affected (See Fabry et al. 2008).  In my next post I will take a closer look at how the earth’s coral reefs are responding to these changes in calcite, aragonite and biogenic CaCO₃ levels.

References:

Fabry, V. Seibel, B. Feely, R. Orr, J, (2008). ‘Impacts of ocean acidification on marine fauna and ecosystem processes’, ICES journal of marine science, 65, 414-432.

Guinotte, J. Fabry, V. (2008). ‘Ocean acidification and its potential effects on marine ecosystems’, Annals of the new york academy of sciences, 1134, 320-342.

Orr, J. Fabry, V. Aumont, O. Bopp, L. Doney, S. Feely, R. et al. (2005). ‘anthropogenic ocean acidification over the twenty first century and its impact on calcifying organisms’, Nature, 437, 681-686.

Natural trend or Anthropogenic change?

This blog post will explore some of the past changes in ocean pH to try and evaluate whether the current change is part of a natural trend or solely anthropogenic.  It is, however, hard to compare current CO₂ induced changes with those of the distant past because of continental drift, changes in the elevation and orientation of the continents and the location of mountains.  All of which alter the oceanic and atmospheric circulations (Goodwin et al. 2009). 

It is widely believed that the ocean is currently more acidic than it has been at anytime over the last 2my and δ¹¹B levels show that the predicted pH for 2100 has not been experienced since about 40mya (Pelejero et al. 2010).  Around 55mya, during the Paleocene-Eocene thermal maximum, there was a significant ocean acidification event.  There was a huge carbon input into the ocean, causing a 2km shift of the calcite saturation horizon towards the surface in less than 2ky (Doney et al. 2009).  The effects of which were catastrophic.  For a full review there is an excellent article by Zachos et al. (2008).  There were two epochs during the Cenezoic era which are thought to have seen extensive ocean acidification however, δ¹¹B levels have not yet been able to quantify this.

During the Cretaceous CO₂ levels were extremely high (up to 2000ppm) and it is believed that 100mya ocean pH was around 0.8 units lower than present (Kump et al. 2009).  Diatoms survived the event but calcified species were generally wiped out (Pelejero et al. 2010).  The early Aptian Oceanic Anoxic event happened around 120mya, a series of major eruptions along the Java plateau were thought to release enough CO₂ to cause a 2km shoaling of the calcite saturation zone.  This coincided with the demise of the nanoconids which were one of the most dominant species of the time and were heavily calcified (Kump et al. 2009).

Carbon isotopes suggest that there was a substantial decrease in ocean pH at the Triassic-Jurassic boundary (Hautmann et al. 2008).   Extensive volcanism was thought to release large amounts of CO₂, resulting in a temperature increase which initiated the release of further CO₂ from marine sediments (Palfy, 2003).  The CO₂ levels were high enough to inhibit the precipitation of CaCO₃ resulting in a subsaturated and acidified ocean (Hautmann et al. 2008).  Calacerous phytoplankton suffered very badly, whereas phytoplankton with organic walls actually benefitted from the acidic conditions (Van de Schootbrugge et al. 2007). 

A major ocean acidification event was thought to have occurred during the “super-greenhouse” state which followed the late Neoproterozoic and models have estimated that the pH could have fallen below 6.0 (Le Hir et al. 2008).  However, it is thought that this acidification occurred gradually, over a period of up to 30 my (Ibid.).  Calcifying organisms had not evolved during this time so it is hard to tell the impacts the acidification had on ocean biology and also how long the acidification lasted (Kump et al. 2009).

Several studies have looked back over the last 500my and reinforce the fact that ocean acidification played a major role in 5 mass extinctions, suggesting that the current ocean acidification event could contribute to the 6^th mass extinction.  It is clear that ocean pH has varied hugely in the past, with some acidification events being far more extreme than what we are currently experiencing.  This could suggest that the current acidification is part of a long term trend; however the rate of change suggests that humans are playing a significant role.

References:

Doney, S. Fabry, V. Feely, R. Kleypas, J. (2009). ‘Ocean Acidification: the other CO₂ problem’, Annu. Rev. Marine. Sci., 1, 168-192.

Goodwin, P. Williams, R. Ridgwell, A. Follows, M. (2009). ‘Climate sensitivity to thecarbon cycle modulated by past and future changesin ocean chemistry’, Nature Geoscience, 2,145–150.

Hautmann, M. Benton, M. Tomasovych, A. (2008) ‘ Catastrophic ocean acidification at the Triassic-Jurassic boundary.’, Neues Jahrbuch Geol. Palaontol. Abhand.,  249,  119–127.

Le Hir, G. Ramstein, G. Donnadieu, Y. Goddéris, Y. (2008). ‘Scenario for the evolution of atmospheric pCO2 during a snowball Earth’, Geology, 36,47–50.

Kump, L. Bralower, T. Ridgwell, A. (2009) ‘Ocean acidification in deep time’, Oceanography, 22, 4, 94- 107.

Palfy, J. (2003). ‘Volcanism of the Central Atlantic MagmaticProvince as a potential driving force in the end-Triassic extinction’, Geophysical Monograph, 136:255-267.

Pelejero, C. Calvo, E. Hoegh-Guldberg, O. (2010). ‘Paleo-perspectives on ocean acidification’, Trends in ecology and evolution, 25, 6, 332-344.

van de Schootbrugge, B. Tremolada, F. Rosenthal, Y. Bailey, T. (2007) ‘End-Triassic calcification crisis and blooms of organic-walled ‘disaster species’’, Palaeogeogr. Palaeoclimatol. Palaeoecol.,  244, 126–141.

Zachos J. Dickens G. Zeebe R. (2008) ‘An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics’, Nature, 451, 279–283.