Ocean acidification, another climate issue

When you make changes in a self-regulating system, everything else changes. Dr Anjani Ganase wonders about how we are intensifying the acidic ocean.

 

 

Chemistry behind ocean acidification

 

Our ocean absorbs about 40 % of carbon dioxide pumped into the atmosphere. When carbon dioxide dissolves into the ocean it reacts with the water releasing hydrogen ions to form carbonic acid making the ocean acidic. Having more carbon dioxide in the water means that the ocean becomes more acidic as carbon emissions increase. Ocean acidification limits the availability of carbonate ions for the formation of calcium carbonate shells and skeletons (think of clams, crabs, conchs, lobster).

 

Shells eventually break down to form sand. Shells collected at Pigeon Point. Photo by Anjani Ganase

 

The ocean, the largest body of water on the planet, has absorbed over 525 billion tonnes of carbon dioxide since the industrial revolution. Under more acidic conditions, carbonates are more likely to bind with hydrogen ions than calcium. Which means that formation of calcium carbonate skeletons become much more difficult. This has resulted in a decline in average pH (pH is actually measured as concentrations of hydrogen ions) from 8.2 to 8.1 of the ocean. This drop means that there has been a 30 % increase in hydrogen concentration (in the form of carbonic acid). We are already seeing significant ecological impacts on the marine life.

 

Between 2006 – 2008, oyster farmers in the Pacific northwest suddenly experienced large scale die off (70 – 80 %) of oyster larvae. This sparked concerns over possible pollutants, but investigations revealed that the water being pumped to the farms had lower pH conditions. The waters along the Pacific northwest well up to the ocean surface from the deep. The last time this deep water might have reached the surface was between 30 – 50 years ago and would therefore reflect the acidic ocean or carbon dioxide chemistry. This is startling considering how much more carbon dioxide has been absorbed between then and now. Today, oyster farmers have to buffer their water supply with antacids to regulate the pH conditions.

 

Ocean acidification and coral reefs

 

Many ocean organisms derive their skeletons and shells from calcium carbonate. Coral reefs provide habitat to over 25 % of marine life in the ocean making it the most biodiverse ecosystem in the ocean, however the foundation of this habitat is calcium carbonate. While some coral species fare better than others in adapting to the changing ocean chemistry, some species are quite sensitive owing to the formation of their skeleton and their morphology.  Ocean acidification can limit coral growth, weaken the skeleton integrity and in some cases erode the fine structures.

 

To gain some insight into what the future of coral reefs might look like, scientists explored coral reefs growing in natural volcanic seeps in Papua New Guinea where carbon dioxide bubbles through the water. What scientists saw was a transition from the finely branching species of corals common to the area to a reef dominant in boulder and dome shaped corals with much denser skeletons. Finer branching corals are more sensitive to eroding acid and therefore the reef lost the structural complexity (similar to the branches of the trees) that formed intricate spaces for organisms to utilise (Fabricius et al 2014). They also noted the lower abundance of crustaceans at these seeps. Ocean acidification is also likely to make reef recovery harder for coral: fleshy algae, a known competitor to coral, thrives under more acidic conditions, while the calcifying algae (crustose coralline algae) - which encourages coral recruitment – is likely to be limited by ocean acidification.  It is predicted that by 2080, acidic ocean conditions would be high enough to result in net erosion of corals. Depending on the rate of acidification, some organisms may be able to adapt but many organisms are likely to become extinct.

 

Fine branching corals vulnerable to ocean acidification. Houtman Abrolhos, Western Australia. Credit: Matt Curnock / Ocean Image Bank

 

Ocean acidification and the ocean food web

 

Plankton – zooplankton and phytoplankton – form the basis of the ocean web. Many of the zooplankton are composed of foraminifera (single-celled shelled organisms) and pteropods (tiny sea snails). While it was thought that the life cycle of these tiny organism may be quick enough be able to adapt to ocean acidification, some studies reveal otherwise. Foraminifera are also major sequesters of carbon, drawing from the atmosphere into the skeleton which then falls to the deep ocean. More acidic conditions have shown a nearly threefold decline in zooplankton populations likely because of the inability to form their shells. One study predicts that foraminifera will be completely removed from the environment by 2100 unless there is drastic reduction in carbon emissions. Furthermore, scientists have also observed in the Southern Ocean - another ocean system that receives deep sea upwelling – actively eroded cold water corals and pteropods in 2008.  

 

Even non-calcifying marine organisms will be affected by drops in pH. If the pH of blood in humans drops by 0.2 – 0.3, we experience seizures and even death; ocean acidification is likely to impact fish and all marine life.  The ocean affects all life on earth. The changing condition of the ocean is changing human life as we know it. How quickly are we able to adapt? Will we change (reduce, reduce, reuse, recycle) enough to restore the natural ecosystems that sustained us for centuries? Will we adapt to changing and more turbulent weather conditions? Or will we be eradicated slowly.. or rapidly?

 

Reference:

https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification

 

https://e360.yale.edu/features/northwest_oyster_die-offs_show_ocean_acidification_has_arrived

 

Fabricius KE, De’ath G, Noonan S, Uthicke S. 2014 Ecological effects of ocean acidification and habitat complexity on reef-associated macroinvertebrate communities. Proc. R. Soc. B 281: 20132479. http://dx.doi.org/10.1098/rspb.2013.2479

 

Bednaršek, N., Tarling, G., Bakker, D. et al. Extensive dissolution of live pteropods in the Southern Ocean. Nature Geosci 5, 881–885 (2012). https://doi.org/10.1038/ngeo1635

 

Uthicke, S., Momigliano, P. & Fabricius, K. High risk of extinction of benthic foraminifera in this century due to ocean acidification. Sci Rep 3, 1769 (2013). https://doi.org/10.1038/srep01769