A little over a year ago, I posted some information about the rapid rate of ocean acidification now occurring. It’s time for a re-visit. Last week’s issue of Nature included not one, but two news items on ocean acidification, and the rate of appearance of reports and new science discoveries seems to be growing rapidly. The Arctic Monitoring and Assessment Program (AMAP) of the Arctic Council has just released a report specific to Arctic waters, Arctic Ocean Acidification Assessment. The UN General Assembly devoted three days in late June to discuss the issue. Even Canada’s Department of Fisheries and Oceans (DFO) released a report, Ocean Acidification, as part of its State of the Scotia Shelf report last October. True to form for the Harper government, Ocean Acidification appeared quietly on the DFO website, with no public announcement, however, an article this week by reporter, Mike De Souza, brought it to the surface, and to my attention. (Canada needs more reporters who dig and dig, and turn up things the government forgets to tell us when advertising its Economic Action Plan! Advertizing for the Economic Action Plan has cost more than $100 Million since 2009, and it continues.)
Ocean Acidification is written in a very dispassionate style, one that will give the casual reader or politician absolutely no cause for concern. Still, it includes an accurate summary of the causes and potential consequences of ocean acidification, and reports that, as part of DFO’s $16.5 Million Aquatic Climate Change Adaptation Services Program (ACCASP), there is expected to be research on possible ocean acidification impacts on fisheries of the Scotia Shelf. When I checked the list of projects under ACCASP, I found 4 regional risk assessments, 7 new tool development projects, and 24 research projects on aspects of climate change in Canadian waters. One of these 24, Effects of Ocean Acidification on Marine Fauna and Ecosystem Processes in the Northwest Atlantic, was the only one clearly centered on ocean acidification. No information on budget or scope of these projects seems to be available on the website, but as one of 4 + 7 + 24 projects, it is presumably receiving somewhere around $500K – being polite, I’d call this a modest effort by Canada.
Decline in pH on Scotia Shelf 1927 to 2009 (black line) compared to average global oceanic decline. Data points show the considerable variation in sample measurements. The North Atlantic is an area where ocean acidification is proceeding particularly rapidly. Figure © DFO
Ocean acidification is that other thing we are doing to the global environment. It’s the second string on a two-string banjo that plays a tune called “how to cause problems using CO2”. The first string is, of course, global warming; the second string exists because about a third of the CO2 we gleefully pump into the atmosphere dissolves in the surface layers of the ocean. Once there, CO2 dissociates, forms carbonic acid, and lowers ocean pH. The ocean has not turned into lemon juice yet. Indeed, the pH of surface waters is still on the basic (non-acidic) side of neutral, averaging about 8.1 on the pH scale from 0 to 14. Neutral is 7.0 and lemon juice is about 2.0 to 2.5. But the pH scale is a logarithmic scale, meaning that the decrease of about 0.1 units in ocean pH since the start of the industrial revolution represents a 30% increase in the degree of acidity of surface waters. As I noted in my earlier post, this is a rate of change in pH unsurpassed in the last 65 million years, and likely not exceeded in the last 450 million years. And projections of our future releases of CO2 suggest this rapid rate of change is going to continue over at least the next century, resulting in a dramatically large and fast shift in pH, especially when compared to the last 10,000 years when pH of surface waters has been essentially constant.
Very slow processes are now kicking in that will ultimately rectify this shift in ocean chemistry. Surface waters mix slowly with deeper water so the excess CO2 gets distributed more uniformly and pH increases slightly. Secondly, at least some of the organisms living in the ocean fall to the bottom when they die and become covered by sediments before they have a chance to decay. This takes their carbon out of circulation completely, sequestering it in sediments that subsequently become new sources of oil. I suppose the good news is that while we may be making a mess of surface waters just now, that mess will get cleaned up over the next several thousands of years.
Meanwhile, of course, there is this matter of acidifying waters, and the possible impacts of this pH shift on oceanic ecosystems. The first of the news items in Nature was a call for more research on the impacts of ocean acidification on marine organisms and ecosystems. We have known for many years that many biological processes are sensitive to pH, and recent research has revealed numerous instances in which aspects of the metabolism of some species are shown to be impacted by declining pH. Most studies of this problem are done on single species in aquaria, where it is possible to regulate (and change) pH and measure what happens. As the Nature article points out, those studies are revealing considerable complexity in the responses – some species are more sensitive, some processes are more sensitive – but they are not revealing the overall, ecosystem level response, when more and less sensitive species live together and respond differently to changed pH, and they are not revealing the impacts of long-term change in pH when it might be expected that adaptation or acclimation processes would lessen the overall impact over time. What is really important is the long-term consequences for ocean ecosystems of long-term change in pH, acting together with long-term warming, and all sorts of other insults like overfishing and pollution. Nature called for more, and more complex research projects to sort out the real risks.
Nature’s second news item was really a PR announcement for a new, large-scale, multidisciplinary project getting under way in European waters, plus a suggestion that this was a good example of the kind of large-scale science that needs to be undertaken if we are going to really understand the full import of our acidification of the oceans. The European study is a much scaled-up version of what lake ecologists have been doing for 40 years – mesocosm experiments. (They are called mesocosm experiments because they are performed on small, captive parts of the aquatic ecosystem – not small enough to be called microcosms, but not so big they could be called macrocosms… at least I guess that’s what the scientists were thinking when they invented the word.)
The large mesocosm being used in Swedish waters to study ocean acidification effects. It extends 20 meters below the surface.
Photo © Maike Nicolai/GEOMAR
The original mesocosm experiments in lakes used plastic bags the size of refrigerators that could be floated in a lake, surrounding a patch of lake water with its phytoplankton and zooplankton community, its natural cycle of day and night, natural variation in temperate and so on. Then the scientists could monitor what happened to the populations of the many species in the mesocosm if they changed something by adding a planktivorous fish, or removing certain types of plankton. The European study uses a giant mesocosm about 3 meters in diameter and 20 meters tall; a clear plastic tube surrounding a mass of water about the same volume as a double-decker bus, and extending from the surface down into the ocean. The tube is initially open at both top and bottom allowing water inside to equilibrate with that outside. Then it is closed during the experimental procedure when acid is added to create a new, more acidic ocean inside. While this mesocosm is clearly designed to study mid-water ecosystems, chiefly of phyto- and zooplankton, it is big enough to also accommodate fish and other creatures. At the present time six such mesocosms are moored in the Gullmar Fjord of Sweden, and there is one month left to go of a six month experiment in which three of the mesocosms have been given the pH anticipated for 2100, while three have ambient pH. The results should tell us something interesting about what happens to plankton communities under changed pH. (And since phytoplankton are responsible for 40% of all photosynthesis on Earth, how they fare in acidifying oceans is definitely of interest to humanity.)
The various effects of lowered pH on marine organisms include effects on growth and development, on physiology, on reproduction, and on behavior. Effects have been detected on plankton, on shellfish, on sea urchins and starfish, on crustaceans and fish, and effects have tended to be particularly pronounced during larval life. Calcification is one metabolic process that is critical for a broad range of marine species because so many of them use calcium carbonate as a primary material of their skeletons, and the process of building calcium carbonate out of calcium and carbonate ions is known to be energy-demanding and pH sensitive. Reducing pH increases the energetic cost of building calcium carbonate structures.
A magnificent photo of a Pteropod, taken by R. Hopcroft, U. Alaska Fairbanks/Census of Marine Life.
In May 2008, the U.S. Senate’s Commerce, Science, and Transportation Committee’s Subcommittee on Oceans, Atmosphere, Fisheries, and the Coast Guard met at the Seattle Aquarium. They were there to hear from six scientist experts on the issue of ocean acidification. They met in Seattle at least partly because acidification was proceeding far more quickly than scientists had anticipated in that part of the world. The scientists reported on the causes of ocean acidification and the likely impacts on fisheries. One story that caught the attention of the news media was the description by NOAA scientist Chris Sabine of watching pteropods swimming about as their shells dissolved. Pteropods, or marine butterflies, are minute, pelagic snails, a part of that wonderfully diverse and largely unknown world called the plankton. Pteropods, being very small and pelagic, have very light shells, and low pH water can cause the shells to dissolve. The point Sabine was trying to make was that the oceans are changing in ways that make life difficult for many creatures that live there. Sabine also predicted that declining pH might become a problem for the $100 Million shellfish aquaculture industry in the US Northwest. Oysters, like pteropods are mollusks, and at least when they are small, their shells are thin and delicate too.
Mussel harvesting at Taylor Shellfish Farms, Shelton WA.
Photo © Ted S Warren/AP
Another of the experts present, Brian Bishop, owner of Little Skookum Shellfish Growers in Shelton, WA, drew attention to the value of the industry, and the growing risk. As the largest grower of ‘seed’ shellfish in Washington, he had much at stake, but so, of course, did the other businesses that grew his seed shellfish into marketable products. It turns out, of course, that even before this committee hearing in Seattle occurred, the first signs of impacts of ocean acidification on the shellfish industry had occurred. The Whiskey Creek Shellfish Hatchery in Oregon suffered a serious die-off of seed oysters in 2007 and was only able to fill a third of its orders. The same thing happened in 2008. The same thing happened at some other hatcheries, and to natural populations in Willapa Bay, Washington. All of the failures may not have been due to acidification, but collectively they caused a 22% drop in production btween 2005 and 2009. Nowadays, hatchery managers in the Pacific Northwest keep a close watch on NOAA and Fish & Game predictions of coastal ocean pH, and have instrumentation in place to monitor and adjust pH in their rearing tanks. Contrary to the early attention to the dissolving of thin and fragile shells on larval shellfish, it is now recognized that the main impact of the acidification is on the process of building the shell in the first place. Creatures that have difficulty building something as fundamental as their skeletons (the shell of most mollusks is really the only solid structure they possess, although snails also have a well-calcified radula – sort of like a file-like tongue – and octopus and squid possess a well-calcified beak). While the aquaculture industry can get animals through the most critical larval stages by carefully controlling pH in their tanks, this option is not available to the millions of wild mollusks living out off the coast, or to pteropods and other armored plankton who continue to suffer or disappear as the oceans become more acidified.
Ocean acidification is also, obviously, a potential problem for coral reefs – ecosystems that depend entirely on calcification by the corals and a whole suite of other creatures that build shells or skeletons to create the rocky substratum that is the reef. Without continued calcification, reefs cannot grow, and existing reefs are slowly eroded away. One cannot suspend a coral reef inside a giant floating mesocosm, but there is an urgent need to determine how sensitive reefs are, and therefore what the likely impacts of continued ocean acidification will be on reefs. A number of studies have involved rearing single coral species in water of differing pH over periods of days to months. These lab studies reveal quite a bit of variation among species, but it is difficult to move from these results to a projection of likely impacts on whole reefs over years.
One indirect, but very interesting study of corals of the genus Porites, examined cores taken from colonies of this genus sampled across the entire Great Barrier Reef region. Corals form annual growth bands in their skeletons as they grow, similar to the annual growth bands in trees, and the width of these bands is an index of growth rate, and therefore of rate of skeleton formation. In 2009, Glenn De’ath and colleagues at Australian Institute of Marine Science reported their results in Science. The colonies sampled ranged in age from 10 to 436 years, and De’ath and colleagues had measured growth rates for each year of life for young colonies, and for each year after 1900 for older colonies. They found that growth rates increased slightly during the period from 1900 to 1980, a fact they suggested might be due to the warming that was taking place. But growth rates slowed markedly after 1990, and by 2005 had fallen about 13%. They proposed that this was a consequence of ocean acidification slowing calcification despite temperatures being even warmer than in 1990. Some other studies obtained similar results, while others showed little apparent effect on growth – different species of coral respond in different ways.
Porites asteroides, a widespread Caribbean coral. Photo © RealReefs, Coastal and Marine Spatial Research Program PIESACOM, UMDI-Sisal, F. Ciencias, UNAM
Some other researchers took to sampling corals growing in places where the waters were naturally more acidic because of some underground seepage of lower pH water. The most recent use of this approach is in a paper that was posted on-line in Proceedings of the National Academy of Sciences, US, on 23rd May, 2013. Elizabeth Crook, from University of California at Santa Cruz, together with four US and Mexican colleagues, took advantage of subtidal springs off the coast from Puerto Morelos, Mexico that delivered water of lower than ambient pH to nearby coral reefs. They used underwater drills to extract cores from colonies of Porites astreoides, a widespread Caribbean species. Then they used the cores to measure growth rates and skeletal density, and to assess the extent of burrowing by organisms that slowly erode dead coral skeletons from the inside out, and contribute to overall rates of erosion. Their findings are interesting in three respects.
Rate of calcification for Porites asteroides measured at sites off Mexico having three levels of acidification (measured as aragonite saturation state (normal sea water is to the right), and compared to results from other studies done in aquaria. The results make clear that there is little if any acclimation or adaptation by corals over long periods of living under low pH conditions (otherwise the short-term aquarium studies would give different results to the field studies). Figure from Crook et al. PNAS 2013.
First, as expected, they found that rates of skeleton formation differed among corals growing in waters of different pH, with the rate slower near springs yielding the lowest pH water. Second, they found that growth rates showed the same relationship to pH as in several published laboratory studies – in other words the short-term lab experiments provide an accurate picture of rate of calcification under different levels of pH, despite the fact that corals in the field will have been exposed to these lower levels of pH for many years or decades. Third, they found that the corals, even in the most acidic water still managed to grow, maintaining their rates of growth by building a less dense skeleton. However, this finding had a distinct downside – these less dense skeletons were much more heavily eroded by boring organisms than were skeletons from corals grown at higher pH. The conclusions we can draw, at least for this species, are that pH does affect rate of skeleton formation as expected. That corals manage to survive by building weaker skeletons, but that these weaker skeletons are broken down far more rapidly (meaning that reefs made of them will be less durable). Put these results together and one does not get a good feeling about coral reefs in 2100.
A second paper on coral reefs, that appeared on-line in Current Biology late in May also deserves comment here although it was concerned with the full suite of changes being imposed on reefs, not just on acidification. Emma Kennedy, of the University of Exeter, UK, led a multinational team of 11 from UK, Mexico, Germany, Israel, USA, and Australia in a complex modeling study using Caribbean coral reefs and their processes as the basis for modeling. They used ecological models to derive carbonate budgets for a hypothetical reef and subjected this reef to the suite of environmental conditions expected into the future using current climate models. When a carbonate budget becomes negative (more reef is being lost to erosion than is being built), a reef system is on the way to disappearing, so they focused on the time periods and circumstances that would permit their reef to retain a positive carbonate budget.
This study, obviously dependent on the quality of the underlying models, is interesting in two ways. First, they were able to model, with reasonable accuracy, the history of Caribbean reefs over the last 50 years. This has been a time of profound change for reefs in this region, driven by overfishing, several diseases including a pathogen that nearly caused extinction of the very abundant Diadema sea urchin throughout the Caribbean in 1983, pollution, climate change and acidification. Second, they were also able to show that more effective local reef management had definite positive outcomes for reef persistence into the future. By managing to conserve herbivores, which play a major role in preventing Caribbean reefs degrading to become algal dominated benches, they prolonged the time when carbonate budgets remained positive. This was so regardless of whether the reef was relatively high or low in coral cover to start with and whether GHG emissions were continuing at present rates or being strongly and aggressively reduced. In fact, their modeling showed a possibility of having reefs persist, with positive carbonate budgets to 2100 if reef coral cover was reasonably good to begin with, and there was both a concerted effort to mitigate GHG emissions and strong local management keeping herbivores present.
Results of simulations assuming either weak local management (A,C,E,G) or strong local management sufficient to keep herbivores present and abundant (B,D,F,H), either a starting situation with low (10%) coral cover (A,B,E,F) or a better reef condition (20% coral cover)(C,D,G,H), and either a BAU approach to GHG mitigation (A,B,C,D) or an aggressive effort to reduce emissions (E,F,G,H). Better local management buys time in all cases (the vertical blue bar, marking dates when carbonate budgets cross zero to become negative, shifts right), and good local management, better starting conditions, and strong effort to reduce emissions is the only future in which reefs persist beyond 2100. Figure from Kennedy et al Current Biology 2013.
This final point is of course exactly what coral reef ecologists have been saying for some years now. Reefs can be kept present and functioning on tropical shores if we simultaneously work to reduce greenhouse gas emissions and work to put in place strong local management of all those impacts that can be managed locally.
And on that sliver of good news, I rest my case. Ocean acidification is bad news for the oceans and the goods and services they supply. It is one more piece of bad news all of which are coming at us at once. We will be doomed to a diminished future if we do not stop staring at the bad news and start doing something about it.