How does rising atmospheric CO2 affect marine organisms?

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Great Barrier Reef - Summary
Many people are concerned about the health of earth's coral reefs and how they will respond to future environmental change.  Foremost among their worries is the postulated potential for CO2-induced global warming to cause deadly coral bleaching, since bleaching has often been observed to follow significant rapid increases in water temperature (Jones et al., 1997), although Toren et al. (1998) have countered that "there are a number of weak points in this argument."

Nevertheless, in a report posted on the Internet on 19 October 1998 (see our Editorial of 1 Jan 1999 for details), Thomas Goreau of the Global Coral Reef Alliance was quoted as saying that "corals are dying from heatstroke," while he and another scientist were said to have claimed that "reefs will rebound only through dramatic reduction of fuel consumption."

In reality, earth's coral reefs have been suffering far more from a host of other more mundane anthropogenic activities for centuries, as we note in our Editorial of 19 Sep 2001); and it is the cumulative effect of these other affronts to their wellbeing (which typically increase with time, as the planet's human population grows ever larger) that is making earth's corals more susceptible to warmer temperatures, to which they have been much more immune in years, decades and centuries the past.

In this Summary, we discuss the findings of several of the scientific journal articles we have reviewed that come to bear upon this issue, focusing on those that are based upon studies conducted on Australia's Great Barrier Reef.  Our reason for doing so is that to predict the future with any degree of confidence, especially on the basis of climate model simulations, it is necessary to determine what has occurred in the past, in the real world of nature, in order to have something solid against which to evaluate the model-generated projections and thereby assess the likelihood of their coming to pass.  Hence, we begin with a brief review of some of the pertinent environmental history of Australia's Great Barrier Reef or GBR.

In an overview of past and current threats to the vitality of the GBR, Murray Hogarth of The Sydney Morning Herald (see our Editorial of 1 Jan 1999 for context) lists the following: (1) rising nutrient levels caused by runoff from agricultural activity on land, (2) outbreaks of the coral-devouring crown-of-thorns starfish, (3) the barbed hooks and scything nets used in fishing, (4) tourists and the developers who build resorts and marinas for them, (5) increased sediment levels, (6) the nets of prawn trawlers stirring up the growing load of sediments, (7) the 6-10 tons of "bycatch" for each ton of prawns netted that are caught and die, which dramatically changes the composition of reef life, (8) sea life depleted to the point of exhaustion by over-fishing, (9) huge catamarans and dive boats that take thousands of visitors to the Barrier Reef each day and dump their sewage in the sea on the way home, (10) the live reef-fish trade, (11) fishermen using dynamite and cyanide, (12) coral diseases, and (13) pollution.

Most of these phenomena are associated with human activity; and as the human population of Australia has grown larger, so too have these several anthropogenic threats to the GBR become greater.  Hence, we will consider a number of them in more detail, beginning with increased sediment delivery to coastal waters, which, according to Umar et al. (1998), is itself attributable to a number of human activities, including deforestation, agricultural practices, coastal development, construction, mining, drilling, dredging and tourism.

As noted in our Editorial of 26 Mar 2003, sediments suspended in river water contain barium (Ba), which is desorbed from the particles that carry it as they enter the ocean, where growing corals incorporate it into their skeletons along with calcium (Ca).  Consequently, when more sediments are carried to the sea by periodic flooding and more gradual longer-term changes in land use that lead to enhanced soil erosion, the resultant increases in sediment load are recorded in the Ba/Ca ratio of coral skeleton material.

In light of these facts, McCulloch et al. (2003) measured Ba/Ca ratios in a coral core from Havannah Reef that covered the period of time from about 1750 to 1985, as well as in some shorter cores from Havannah Reef and nearby Pandora Reef that extended the proxy sediment record to 1998.  Prior to the time of European settlement, which began in the Burdekin catchment in 1862, they found there was "surprisingly little evidence for flood-plume related activity from the coral Ba/Ca ratios."  Soon after, however, land clearance and domestic grazing intensified, and the soil became more vulnerable to monsoon-rain-induced erosion.  A mere eight years later (1870), in fact, baseline Ba/Ca ratios had risen by 30% and "within one to two decades after the arrival of European settlers in northern Queensland, there were already massive impacts on the river catchments that were being transmitted to the waters of the inner Great Barrier Reef."  During subsequent periods of flooding, for example, the transport of suspended sediment to the reef increased by fully five to ten-fold over what had been characteristic of pre-European settlement times.

Another parameter that has proven useful for reconstructing histories of sediment delivery to portions of the GBR are the luminescent yellow lines in coral cores that are readily discerned by the human eye when the cores are viewed under long-wave UV light.  What is more, Barnes and Taylor (2001) have determined that these lines are "the result of reduced calcification as the coral responds to low salinity conditions associated with coastal runoff," which adds to the associated deleterious effects of the sediments carried by the low-salinity water.

Documenting the relative locations of yellow luminescent lines seen in multi-century Porites cores obtained from eight different locations on the central GBR, Hendy et al. (2003a) developed "a 373-year chronology by cross-dating techniques adapted from dendrochronology."  This master chronology, dating back to AD 1615, proved to be a good proxy for both Burdekin River runoff and Queensland summer rainfall; and, in Hendy et al.'s words, the data demonstrate that "low-salinity runoff regularly reached the main reef tract during the relatively wet decades of the late 1890s, 1910s, 1950s and 1970s."

Other factors associated with low-salinity runoff and the sediments it transports to coastal reefs are toxic substances of human origin that move with the suspended sediments.  One rather unique such substance is the s-triazine herbicide Irgarol 1051, an antifouling agent used in certain paints applied to marine craft.  Scarlette et al. (1999) measured its abundance in seagrasses off the east coast of Queensland within the GBR Marine Park, as well as in a green alga from the Outer Barrier Reef, finding potentially toxic concentrations of it at nine of ten sampling locations.  These results, in their words, raise the possibility that this toxin "could be affecting the endosymbiotic microalgae of coral polyps, upon which the health of the reef system depends."  Hence, it is only logical to expect that the presence of this debilitating substance - and many others - would weaken various coral components of the GBR and make them more susceptible to other stresses, such as periodic increases in water temperature, possibly leading to serious coral bleaching.  Indeed, it is amazing that corals do not bleach more than they do, considering the many direct onslaughts of humanity that tend to weaken them.

One of the most widespread episodes of coral bleaching to occur in recent years, which may have been exacerbated by some of the factors discussed above, was that associated with the major El Niņo of 1998.  In discussing this subject, Hendy et al. (2003b) note that "it has been suggested that since massive corals, some as old as 700 years, died as a result of the 1998 bleaching event, it must have been the most severe bleaching event to hit the Great Barrier Reef over the last seven centuries (Hoegh-Guldberg, 1999)," which many likely hoped would be interpreted to mean that the 1998 El Niņo was also the strongest El Niņo of that time period, and, hence, that it was likely due to the occurrence of what many have claimed are the warmest temperatures of the past millennium.

Questioning this string of assumptions, Hendy et al. devised a plan to determine "the likelihood of observing, in cores taken from Porites colonies, past mass coral mortality events equivalent in intensity and scale to the 1998 bleaching event," noting that "an historic record of past coral mortality events is needed to gain some perspective on current events and the impact of recent environmental change."  Based on the work of Marshall and Baird (2000), who studied the bleaching responses of different coral taxa to the environmental conditions that produced the 1998 event in the GBR, they calculated the probability of sampling an event of equivalent severity that may have occurred in the more distant past, finding that "the chance of seeing an event across [eight] cores is exceedingly unlikely, even for one as dramatic as the 1998 bleaching event."  In fact, they calculated that a growth discontinuity of that magnitude "is most likely to be observed in only one of the cores in any sample population size smaller than 17 cores."

Not content with a good explanation for the absence of evidence, however, Hendy et al. carefully examined eight long Porites cores extracted from inshore and midshelf reefs in the central GBR, discovering in the process two hiatuses in coral skeletal growth that were accurately dated to 1782-85 and 1817 AD, the telltale "die-off scars" of which were observed in only one core for each event.  Contemporary historical and proxy-climate records additionally indicated that El Niņo conditions occurred at the times of both growth discontinuities, with those of 1782-83 being termed "exceptional" by Whetton and Rutherfurd (1994), while other data indicated that low salinity from river runoff was an additional contributor to the bleaching event of 1817, just as it was in the El Niņo-triggered bleaching of 1998.  Hence, not only would it appear there was nothing unusual about the 1998 bleaching event within a multi-century timeframe, it would appear that recovery from such an event is normal.

The study of Webster and Davies (2003) also demonstrates that the GBR is extremely resilient.  Analyzing variations in lithology and coral assemblages of two long drill cores made in the northern GBR by the International Consortium for GBR Drilling that yielded a good record for much of the Pleistocene, they found that "the repeated occurrence of similar coral assemblages in both drill cores suggests that the Great Barrier Reef has been able to re-establish itself and produce reefs of similar composition again and again over hundreds of thousands of years [including several glacial and interglacial periods], despite major environmental fluctuations (i.e. sea-level and temperature changes)."

But one does not have to go that far back in time to find the GBR successfully coping with temperatures equivalent to, or actually warmer than, those of today.  Gagan et al. (1998), for example, used a double-tracer technique based on Sr/Ca and 18O/16O ratios in the skeletal remains of corals from the GBR to infer that temperatures there approximately 5350 years ago were about 1.2°C warmer than the mean that prevailed throughout the early 1990s, which accords well with terrestrial pollen and tree-line elevation records elsewhere in the tropical southwest Pacific for the entire period from 7000 to 4000 years ago.  And, of course, the GBR was not destroyed by that lengthy period of greater-than-current heat.

Even closer to the present, Hendy et al. (2002) reconstructed a 420-year sea surface temperature (SST) history based on Sr/Ca measurements of several coral cores taken from massive Porites colonies in the central portion of the GBR.  The earliest portion of this temperature history, from 1565 to about 1700, includes some of the coldest periods of the Little Ice Age, where five-year blocks of mean SST during this South Pacific cold period were sometimes 0.5 to 1.0°C or more below the region's long-term mean.  Over the following century, however, the reconstructed SSTs were consistently as warm as, and many times even warmer than, those of the early 1980s, where the coral record ended, just as they also were at Rarotonga in the Cook Islands (Linsley et al., 2000).  And, of course, the corals in both places handily survived the warm temperatures.

In another pertinent study, Lough and Barnes (1997) derived a history of annual density, linear extension and calcification rates from several coral cores retrieved from 35 sites along the GBR from 9 to 23°S, thereby obtaining information about long-term variability in coral growth there since the late 15th century.  Their record did indeed indicate a recent decline in calcification rates for this region, concurrent with what some have described as unprecedented 20th-century global warming.  However, their data also showed that "a decline in calcification equivalent to the recent decline occurred earlier this century and much greater [our italics] declines occurred in the 18th and 19th centuries."  Furthermore, analyses of annual density banding from the coral samples indicated, in their words, that "the 20th century has witnessed the second highest period of above average calcification in the past 237 years."  As a result, they concluded that "the observed decline in coral growth in recent decades may be, simply, a return to more 'normal' conditions."

Building upon these observations, Lough and Barnes (2000) assembled and analyzed the calcification characteristics of 245 similar-sized massive colonies of Porites corals obtained from 29 reef sites located across the length and breadth of the GBR, and to these data they added data from the Hawaiian Archipelago (Grigg, 1981, 1997) and Phuket, Thailand (Scoffin et al., 1992), thereby extending the latitudinal range of the expanded data set to 20° and the annual average SST range to 23-29°C.

The two scientists found that the GBR calcification data were linearly related to the average annual SST data, such that "a 1°C rise in average annual SST increased [our italics] average annual calcification by 0.39 g cm-2 year-1."  Results were much the same for the extended data set, enabling them to report that "the regression equation [calcification = 0.33(SST) - 7.07] explained 83.6% of the variance in average annual calcification," noting that "this equation provides for a change [increase] in calcification rate of 0.33 g cm-2 year-1 for each 1°C change [increase] in average annual SST."

So how do corals do it?  How do they survive, or successfully recover from, warming-induced bleaching?  In a study of the acquisition of symbionts by juvenile Acropora tenuis corals growing on tiles attached to different portions of the reef at Nelly Bay, Magnetic Island (an inshore reef in the central section of the GBR), Little et al. (2004) found that "initial uptake of zooxanthellae by juvenile corals during natural infection is nonspecific (a potentially adaptive trait)," and that "the association is flexible and characterized by a change in (dominant) zooxanthella strains over time."  These observations, in turn, led them to conclude that this symbiont shuffling "represents a mechanism for rapid acclimatization of the holobiont to environmental change," which may enable corals to grasp victory from the jaws of death, as it were, in the aftermath of a severe bleaching episode, which is also implied by the fact - cited by Lewis and Coffroth (2004) - that "corals have survived global changes since the first scleractinian coral-algal symbioses appeared during the Triassic, 225 million years ago."

In spite of these assuring observations from the real world of nature, people continue to devise new coral doomsday scenarios based on theoretical considerations that often underestimate the power of life to overcome obstacles that arise from purely physical considerations.  Concurrent with the global warming-induced coral bleaching storyline, for example, climate alarmists are simultaneously promoting the concept of a CO2-induced decline in coral calcification rate.  This theoretical threat has its origin in the study of Kleypas et al. (1999), who describe physical-chemical calculations that suggest the rising CO2 content of earth's atmosphere could lower the saturation state of the carbonate mineral aragonite in the surface waters of the world's oceans.  They state that this phenomenon could result in reduced calcification rates in coral reefs, which could in turn lead to weaker coral skeletons, reduced coral extension rates, and increased coral susceptibility to erosion, noting that these primary effects could lead to "a host of secondary changes in community structure, reproduction, and overall community functions."  They further note that aragonite calcite precipitation in the tropics should have already decreased by 6 to 11% since 1880, as a result of the increase in atmospheric CO2 we have experienced to date, and that these reductions could reach 17 to 35% by 2100, as a result of expected increases in the air's CO2 content over the current century.  As a result, in the frenzy of media attention that followed the publication of their paper, we were treated to headlines trumpeting "CO2 Could Kill Coral" and "Great Barrier Reef Faces Death Knell."

Interestingly, no one seemed to notice the caveats mentioned by Kleypas et al., who stated that (1) "calcification versus saturation state data are scarce," (2) the data that do exist were derived from experiments that "were conducted over [only] days to weeks," and (3) it may be possible for reef-building organisms to adapt to gradual changes in ocean-water carbonate chemistry via "mitigative physiological effects, such as CO2 fertilization of calcareous algae or the symbiotic algae within coral tissues."

In commenting on these findings, Lough and Barnes (2000) wrote that their own results actually "allow assessment of possible impacts of global climate change on coral reef ecosystems," noting that between the two 50-year periods 1780-1829 and 1930-1979 they observed a mean calcification increase of 0.06 g cm-2 year-1.  They further note that "this increase of ~4% in calcification rate conflicts with the estimated decrease in coral calcification rate of 6-14% over the same time period suggested by Kleypas et al. (1999) as a response to changes in ocean chemistry."  Even more stunning was their observation that between the two 20-year periods 1903-1922 and 1979-1998, the SST-associated increase in calcification was estimated to be on the order of "5% in the northern GBR, ~12% in the central GBR, ~20% in the southern GBR and to increase dramatically (up to ~50%) to the south of the GBR."

Observations of Bessat and Buigues (2001) made at the French Polynesian island of Moorea demonstrated much the same thing, i.e., that a 1°C increase in water temperature increased coral calcification rate by approximately 4.5%.  This finding led them to remark that "instead of a 6-14% decline in calcification over the past 100 years [as] computed by the Kleypas group, the calcification has increased, in accordance with [what] Australian scientists Lough and Barnes [found]."  They also observed patterns of "jumps or stages" in the record, which were characterized by an increase in the annual rate of calcification, particularly at the beginning of the past century "and in a more marked way around 1940, 1960 and 1976," which led them to state once again that their results "do not confirm those predicted by the Kleypas et al. (1999) model."

In light of these several observations, it would appear that Australia's Great Barrier Reef is well adapted to dealing with large-scale changes in the global environment, but that it could well come to an ignominious end as a result of a number of direct anthropogenic affronts to its local environment.  It is this latter group of problems to which we must turn our attention if we are to preserve this incomparable marine ecosystem.  To say we can save it "only through dramatic reduction of fuel consumption," as suggested by Goreau, is to ignorantly consign it to oblivion.

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