By 2100 oceans, which are the largest store of carbon dioxide on the planet, are likely to be more acidic than at any time during the last 20 million years. Acidification increases the corrosiveness of the water and is also driving a decline in the amount of carbonate ion, needed to make aragonite and calcite, two forms of calcium carbonate that many marine organisms use to build their shells and skeletons. Plants and animals that have a shell or build their skeleton through calcium carbonate (calcifiers), like oysters, mollusks, corals and some algae, are therefore the ones most likely to be negatively affected first by ocean acidification.
Even if some calcifiers are able to protect their shell or skeleton, their resistance is diminished when organisms are exposed to extended period of elevated temperature (28.5°C). This is a result of an international study recently published on Nature Climate Change (Coral and mollusk resistance to ocean acidification adversely affected by warming) and part of the two ongoing European projects on Ocean Acidification EPOCA (European project on ocean acidification) and MedSeA (The European Mediterranean Sea Acidification in a changing climate).
Jean-Pierre Gattuso, research scientist at the Laboratoire d'océanographie de Villefranche (CNRS/UPMC) is EPOCA’s co-ordinator and a member of the scientific steering committeee of MedSeA.
The overall goal of EPOCA is to advance our understanding of the biological, ecological, biogeochemical, and societal implications of ocean acidification.
Could you summarize the main societal implications of ocean acidification?
There are potential impacts. For example, coral reefs are critical features of coastal environment in the Tropics; they protect the shores from devastating impacts of cyclones and storms and are the sites where lots of open ocean fishes come to reproduce. They are threatened by two processes: the increase in temperature and ocean acidification, which make the reefs growth more difficult and weaken the resistance of their structure. That has a dramatic impact on local populations and could also have an impact on tourism.
Isn’t tourism itself affecting negatively the status of the coral reefs?
Ocean acidification and the increase in temperature are global threats to coral reefs, tourism is a local one. The coasts are built up with big resorts and hotels and the release of sewage in lagoons is also affecting the condition of coral reefs. Other potential socio-economic implications regard fishery. Mollusks (including mussels and oysters) are dramatically affected by ocean acidification and this could have an impact on aquaculture. In the North West US along the Pacific Coast the oyster industry has been having a hard time for the last two or three years, partly because of ocean acidification, which is related with the upwelling of deep water. Deep water is more acidic than surface water and when deep water comes to the surface it brings acidified water, something that is causing a lot of problems to the oyster farms, since it blocks the recruitment of juveniles, the oyster larvae.
The recruitment is when the juveniles, after spending some time in the water, go down and settle, attach to a rock and then become oysters. With ocean acidification the larvae just don’t attach to the rock, bringing to a big decrease in the production of the farm. Two farms in the US have installed a system through which they stop pumping water when PH becomes too low and the water too acidic, they close the sea water intake and the oysters remain in a close system to avoid damaging the juveniles.
What about the possible consequences of ocean acidification at biology, ecology and biogeochemistry level?
Ocean acidification is a very recent science, so there is a lot that we don’t know and it is very difficult to predict what the future of the oceans in high CO2 conditions will be. Secondly, some organisms and processes are negatively affected while others are positively influenced so it is important to keep the balance, we do not want to paint a dramatic picture.
Which organisms are positively influenced and which are negatively affected?
The main positive aspect is related to seagrasses. We have a very emblematic type of these marine plants in the Mediterranean, Posidonia Oceanica, which is using CO2 for photosynthesis, so when it has more CO2 it grows faster and it is therefore benefitting from higher CO2 levels in contrast with most of the other algae and plants which use bicarbonate.
The main negative aspect is certainly for calcifiers, which undergo two processes. As ocean acidification proceeds, carbonate becomes less and less abundant, so at one point the carbonate concentration in the water is limiting the precipitation of calcium carbonate and organisms have a harder time to make their shell and skeleton since one of the bricks needed to make the wall is becoming less and less abundant. In parallel to that, water becomes more and more corrosive, therefore the wall is getting damaged and some organisms are becoming very fragile because they have been eroded or dissolved faster than their shell is being built.
Ocean acidification is a fact. You will not find anyone, here including the skeptics’ world, who will say it is an invention, because we do measure it and we do see the increase in ocean acidity since the Industrial Revolution. Where we still have a lot of uncertainties is on what the biological consequences will be and since we do not know exactly the impact for some organisms, it is difficult to estimate the biogeochemical and societal impact. There is a clear need for research on this and it is encouraging that the EU is leading it through the work of EPOCA and now MedSeA. The US programme, for instance, has not even started yet.
So it has been the need of better understanding biological, ecological and biogeochemical consequences that brought you to start these research projects?
Absolutely. When I started to work on ocean acidification in 1995 we were less than ten people in the world looking at this. We have recently published a book in the Oxford University Press called ‘Ocean acidification’ where I looked at the literature and in 2010 more than 600 authors have published on this topic!
Ocean acidification affects different oceans in a different way, right?
With different timing. To put things simply, ocean acidification is progressing faster in colder water and it is very easy to understand, because every gas is more soluble in cold water and it dissolves much faster. Therefore, as CO2 increases in the atmosphere and goes into the water, the corrosiveness of water or the impact of the increasing acidity will appear first in the polar oceans and later in temperate and tropical ones. This is why EPOCA has done a lot of work in the Arctic both in terms of chemistry and of biological response and we have organized two major expeditions in Spitsbergen (Norway) in 2009 and 2010. The Arctic is clearly a hot spot for ocean acidification as it is for global warming and ice melt, of course.
Besides water temperature, are there other elements determining different effects of acidification in different oceans?
Not for the chemistry aspect. As for biology, other factors are obviously important. If we go back to the coral reefs, even if I said ocean acidification will progress slower in the tropics, the combination of ocean acidification and warm temperature is a deadly recipe for corals.
Besides the Arctic, what other areas are you studying in EPOCA?
The Mediterranean and the North Atlantic. We do a lot of work along the coasts of UK, France, Spain and Portugal, the Canary current close to the Canary islands, the Gulf of California, the Pacific and Fiji.
What does your group do within the MedSeA project?
First of all we are going to conduct what we call the mesocosm experiment, i.e. we are going to deploy in Corsica and in Crete nine huge plastic bags which will enclose about 60 cubic meters of water each. We will enclose in these mesocosms all the organisms that live in the sea water, including bacteria and plankton and will subject each bag to a different CO2 level in order to mimic what will happen up to 2100. We will follow the response of all the organisms that have been enclosed in these mesocosms for a period of 6 to 8 weeks. We are also involved in the response of mollusks, especially commercial ones, to ocean acidification, and this will be done with colleagues from Barcelona’s CSIC. We are going to grow mussels in the lab and also put them through tests at various CO2 levels. MedSeA will try to translate research obtained in the lab and in the field from the biology to the socio-economic impact. For this reason, there are socio-economic partners who will try to put the information generated by biologists and biogeochemists into socio-economic perspective.
What tools do you use to investigate ocean acidification and its consequences?
If we look at the chemistry, we go to the sea and collect two kinds of samples. First of all in time-series stations, that is repeat measurements at the same place for a long time. For example here in Villefranche we have a place where every week we collect water from the surface to 80 meters, bring it back to the lab and analyze the acidity level. Moreover, we do cruises and take samples of waters in different locations in order to study the regional distribution of ocean acidification. Then we have paleoceanographers who look at the impact of ocean acidification events that occurred in the past. They collect corals of sediments, they take samples of the various calcareous organisms in them and do measurements for instance of the isotopic composition of the shells. This way they are able to reconstruct what happened even sometime back 55 million years ago. We also have modellers that project future changes of ocean chemistry and biology in the next decades and century. If we move to biology, there are two main approaches used to investigate ocean acidification. The first is the one used in the Nature paper and it consists in making observations at sites that are naturally acidified. The waters around the island of Ischia in Italy, for example, have been acidified for a long time because there is the Vesuvio volcano nearby; there are a lot of volcanic activities and CO2 sources, that is there is CO2 from the bottom to the surface, which is acidifying the water. It is of course a different process than the one that generates ocean acidification, since this one is coming from the surface. What is interesting is that in the CO2 vents (sources) we are not looking at isolated organisms kept artificially in the lab but at an open system where all the organisms are subject to the same level of acidity. CO2 vents are therefore very good locations to investigate long term effects of ocean acidification.
The other approach is collecting organisms in the field and then investigate them in the lab under controlled conditions, where we add CO2 (perturbation experiment). There is also a third approach which is a mixture between the two and it is the mesocosm experiment I explained before.
Is there anything else relevant on EPOCA you would like to highlight?
The fact that we have a very strong programme, dissemination and outreach, to communicate the results on ocean acidification to policy makers, to the general public and we have a school programme for teachers and pupils.
In the Nature study you state that previous work has not determined the impact of acidification on the ability of individual species to calcify because they measured net calcification (that is, gross calcification minus dissolution) thus failing to disentangle the relative contributions of gross calcification (the amount of carbonate deposited by an animal over time) and dissolution rates. Since you state that a decrease in net calcification could result from a decrease in gross calcification, an increase in dissolution rates, or both, you distinguish between these responses and get to the conclusion that the impact of ocean acidification on a creature’s net calcification may be largely controlled by the status of its protective organic cover and that the net slowdown in skeletal growth under increased CO2 occurs not because these organisms are unable to calcify, but rather because their unprotected skeleton is dissolving faster. Why is it relevant to understand this?
The gross calcification is very much controlled by the organisms, so biology is involved. The dissolution is mostly controlled by chemistry. You can have a differential impact on biology and chemistry, so if you really want to assess what will be the status of calcifying organisms in 2100 there is one part, the chemistry, for which the organisms have no control but for the biology they can perhaps adapt and there might be a way for the organisms to mitigate the negative impacts of ocean acidification.
For the chemistry part, instead, there is nothing we can do. Adaptation is not possible, therefore it is important to differentiate the various processes because they will behave differently in the future.
How can differentiating the various processes help finding solutions to the negative impacts of ocean acidification?
Humans will not do anything, this is a natural process, but it is important to understand what it is happening if we want to make projections up to 2100 and make a general conclusion of whether calcification will be badly affected and will come to a stop or not at all. I don’t believe that local, small-scale manipulation of the species will solve the problem, it has to be a large scale solution, i.e. decrease CO2 emissions.
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