Marine and Freshwater Acidification: Effects on Marine Animals

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In the introductory material to their paper on potential effects of predicted near-future increases in CO2-driven ocean acidification on shell-producing calcification in a species of oyster, Watson et al. (2009) report over the past two centuries CO2 emissions from deforestation and the burning of fossil fuels have increased atmospheric CO2 concentrations from 280 to 380 ppm, citing NOAA/ESRL records produced and maintained by Pieter Tans. They additionally state the portion of this extra CO2 that has been taken up by the planet’s oceans has caused a 0.1 unit drop in the pH of their surface waters, which would appear to be correct. However, they predict there will be a further reduction in ocean pH of 0.3 to 0.5 units by 2100, citing the work of Haugan and Drange (1996), Orr et al. (2005), and Caldeira and Wickett (2005), while noting these predicted changes in ocean pH “are not only greater but far more rapid than any experienced in the last 24 million years,” citing Blackford and Gilbert (2007), or “possibly the last 300 million years,” citing Caldeira and Wickett (2003). But how likely are such predictions?

Consider the findings of Tans himself, who Watson et al. approvingly cite in regard to the CO2 history they mention. In a paper published in Oceanography, Tans (2009) concluded the future trajectory of oceanic pH will likely be significantly different from that suggested by the scientists cited by Watson et al., while at the same time bravely criticizing the IPCC reports that also have accepted the highly inflated acidification predictions of those scientists. Indeed, whereas Watson et al. and the IPCC accept the claims of those who project a decline in pH somewhere in the range of 0.3 to 0.5 between now and the end of the century, Tans’ projections yield a pH decline somewhere in the range of 0.09 to 0.17, which would be expected to have significantly reduced biological impacts compared to those suggested by the experimental work of Watson et al.

Based on the results of their experiments and the maximum decline in ocean-water pH they accept, Watson et al. predict a significant decline of 72 percent in Sydney rock oyster (Saccostrea glomerata) larval survival by the year 2100. However, utilizing Watson et al.’s data but with the maximum ocean-water pH decline calculated by Tans, one obtains a non-significant larval survival decline of only 14 percent, based on interpolation of the graphical results portrayed in Watson et al.’s paper. Similar assessments of changes in antero-posterior measurement yield a significant decline of 8.7 percent using Watson et al.’s assumptions about ocean pH, but a non-significant decline of only 1.8 percent according to Tans’ pH calculations. Corresponding results for dorso-ventral measurement were a significant decline of 7.5 percent with Watson et al.’s pH values, but a non-significant decline of only 1.5 percent with Tans’ values, and for larval dry mass there was a decline of 50 percent in Watson et al.’s analysis, but an actual increase (albeit non-significant) of 6 percent using Tans’ pH analysis. Last, for empty shells remaining there was a significant decline of 90 percent in the Watson et al. study, but a non-significant decline of only 6 percent when Tans’ pH projections were used.

Based on their experimental data and the ocean pH projections for the end of the century that are promoted by them and the IPCC, Watson et al. find what they characterize as “a dramatic negative effect on the survival, growth, and shell formation of the early larval stages of the Sydney rock oyster.” On the other hand, employing the pH values projected by Tans, there are no statistically significant reductions in any of the five biological parameters measured and evaluated by Watson et al.

In a separate effort designed to project potential CO2-induced changes in estuarine calcification in the years ahead, larvae of two oyster species—the Eastern oyster (Crassostrea virginica) and the Suminoe oyster (Crassostrea ariakensis)—were grown by Miller et al. (2009) for up to 28 days in estuarine water in equilibrium with air of four different CO2 concentrations (280, 380, 560, and 800 ppm), which were chosen to represent atmospheric conditions in the preindustrial era, the present day, and the years 2050 and 2100, respectively, as projected by the IS92a business-as-usual scenario of the IPCC, which were maintained by periodically aerating the different aquaria employed in the study with air containing 1 percent CO2. Larval growth was assessed via image analysis, and calcification was determined by means of chemical analyses of calcium in the shells of the oyster larvae.

When the larvae of both species were cultured continuously from 96 hours post fertilization for 26 to 28 days while exposed to elevated CO2 concentrations, the authors state they “appeared to grow, calcify and develop normally with no obvious morphological deformities, despite conditions of significant aragonite undersaturation.” They write these findings “run counter to expectations that aragonite shelled larvae should be especially prone to dissolution at high pCO2.” More specifically, the authors state “both oyster species generated larval shells that were of similar mean thickness, regardless of pCO2, Oarag [aragonite compensation point] or shell area,” remarking they “interpret the pattern of similar shell thickness as further evidence of normal larval shell development.” And because these two calcifying organisms appeared not to have suffered deleterious consequences, the four researchers concluded “biological responses to acidification, especially [in] calcifying biota, will be species specific and therefore much more variable and complex than reported previously.”

In another study examining “the very earliest, and critical, process of fertilization,” Havenhand and Schlegel (2009) collected specimens of the oyster Crassostrea gigas—which they obtained from a mixed mussel/oyster bed on the coast of western Sweden and kept within flow-through tanks of filtered sea water that they maintained at either (1) the normal ambient pH level or (2) a level reduced by about 0.35 units that was created by bubbling CO2 through the water—and observed and measured the species’ sperm-swimming behavior and fertilization kinetics. Their results indicated that in water of pH 8.15, mean sperm-swimming speeds were 92.1 ± 4.8µm/s, but in water of pH 7.8 they were slightly higher, at 94.3 ± 5.5µm/s, although the difference was not statistically significant. Mean fertilization success in water of pH 8.15 was 63.4 percent, whereas in water of pH 7.8 it was also slightly higher at 64.1 percent, although this difference, too, was not statistically significant.

Based on these findings, the Swedish scientists state “the absence of significant overall effects of pH on sperm swimming behavior and fertilization success is remarkable,” and they emphasize the power analyses they conducted “showed clearly that these results were not due to inadequate statistical power.” Moreover, they write, “the absence of significant effect is likely a true reflection of the responses of Crassostrea gigas gametes and zygotes from the Swedish west coast to levels of CO2-induced acidification expected by the end of this century,” a very encouraging finding.


In studying the common cuttlefish, Sepia officinalis, Gutowska et al. (2008) found it “is capable of not only maintaining calcification, but also growth rates and metabolism when exposed to elevated partial pressures of carbon dioxide.” Over a six-week test period, they found “juvenile S. officinalis maintained calcification under ~4000 and ~6000 ppm CO2, and grew at the same rate with the same gross growth efficiency as did control animals,” gaining approximately 4 percent body mass daily and increasing the mass of their calcified cuttlebone by more than 500 percent. These findings led them to conclude specifically that “active cephalopods possess a certain level of pre-adaptation to long-term increments in carbon dioxide levels,” and to conclude generally that our “understanding of the mechanistic processes that limit calcification must improve before we can begin to predict what effects future ocean acidification will have on calcifying marine invertebrates.”

In another study examining the common cuttlefish (Sepia officinalis) and published one year later, Lacoue-Labarthe et al. (2009) monitored fertilized eggs of this species throughout their full development time at controlled temperature (16 or 19°C) and pH (8.1, 7.85, or 7.6) conditions. The latter values were maintained within ± 0.05 of a pH unit by periodically bubbling pure CO2 into the bottles (which were continuously aerated with CO2-free air), resulting in mean CO2 concentrations of the air in contact with the surface of the water of either 400, 900, or 1,400 ppm.

This group of authors found “decreasing pH resulted in higher egg weight at the end of development at both temperatures (p < 0.05), with maximal values at pH 7.85 (1.60 ± 0.21 g and 1.83 ± 0.12 g at 16°C and 19°C, respectively).” In addition, they found “hatchlings were smaller when they developed at 16°C than at 19°C (p < 0.05).” They also observed zinc (Zn) accumulation “was higher at pH 7.85 during the full developmental period,” when “high embryonic requirements for Zn are not fully covered by the maternal pool,” so the higher accumulation of Zn “was associated with a greater rate of growth of both egg and embryo.” Concurrently, there was also a greater accumulation of potentially detrimental silver in the tissues of the hatchlings; but any deleterious effects of the extra silver were apparently more than overcome by the positive effects of lowered pH on beneficial zinc accumulation, while toxic cadmium accumulation was actually reduced in the lower pH (or higher CO2) treatments.

The seven scientists conclude their paper by noting “decreasing pH until 7.85,” such as could be expected to occur in air enriched with carbon dioxide to a concentration of 900 ppm, “should lead to some possibly beneficial effects, such as a larger egg and presumably hatchling size and a better incorporation of the essential element[s] such as Zn in the embryonic tissue.” These phenomena, in their words, “may improve the survival [of] the newly hatched juveniles.”

Given the findings of both papers presented above, it would appear the ongoing rise in the air’s CO2 content would benefit cuttlefish.


In a Brevia item published in Science, Checkley et al. (2009) “grew eggs and pre-feeding larvae of white sea bass (Atractoscion nobilis) under a range of CO2 concentrations [380, 993, and 2,558 ppm] and measured the size of their sagittal otoliths,” which, in the words of the authors, “are bony structures used by fish to sense orientation and acceleration and consist of aragonite-protein bilayers,” or as the dictionary states, they are “small vibrating calcareous particles in ... the ears of some animals, especially of fishes.” Noting atmospheric CO2 enrichment has been calculated, on a purely chemical basis, to decrease the saturation state of carbonate minerals such as aragonite in the world’s oceans, the six scientists “hypothesized that otoliths in eggs and larvae reared in seawater with elevated CO2 would grow more slowly than they do in seawater with normal CO2.” To test this hypothesis they conducted their experiment.

“Contrary to expectations,” in the words of Checkley et al., “the otoliths of fish grown in seawater with high CO2, and hence lower pH and aragonite saturation, were significantly larger than those of fish grown under simulations of present-day conditions.” More specifically, the researchers found “for 7- to 8-day-old fish grown under 993 and 2558 ppm CO2, the areas of the otoliths were 7 to 9% and 15 to17% larger, respectively, than those of control fish grown under 380 ppm CO2.”

As for why the otoliths were larger at a lower pH, the marine researchers went on to state young fish are “able to control the concentration of ions (H+ and Ca2+) ... in the endolymph surrounding the otolith,” where “with constant pH, elevated CO2 increases CO32- concentration and thus the aragonite saturation state, accelerating formation of otolith aragonite.”

Dupont et al. (2010) state, “echinoderms are among the most abundant and ecologically successful groups of marine animals (Micael et al., 2009), and are one of the key marine groups most likely to be impacted by predicted climate change events,” presumably because “the larvae and/or adults of many species from this phylum form skeletal rods, plates, test, teeth, and spines from an amorphous calcite crystal precursor, magnesium calcite, which is 30 times more soluble than normal calcite (Politi et al., 2004).” This fact would normally be thought to make it much more difficult for them (relative to most other calcifying organisms) to produce calcification-dependent body parts.

Working with naturally fertilized eggs of the common sea star Crossaster papposus, which they collected and transferred to five-liter culture aquariums filled with filtered seawater (a third of which was replaced every four days), Dupont et al. tested this hypothesis by regulating the pH of the tanks to values of either 8.1 or 7.7 by adjusting environmental CO2 levels to either 372 ppm or 930 ppm. During the testing period they documented (1) settlement success as the percentage of initially free-swimming larvae that affixed themselves to the aquarium walls, (2) larval length at various time intervals, and (3) degree of calcification.

The three researchers report just the opposite of what is often predicted actually happened, as the echinoderm larvae and juveniles were “positively impacted by ocean acidification.” More specifically, they found “larvae and juveniles raised at low pH grow and develop faster, with no negative effect on survival or skeletogenesis within the time frame of the experiment (38 days).” In fact, they state the sea stars’ growth rates were “two times higher” in the acidified seawater; and they remark, “C. papposus seem to be not only more than simply resistant to ocean acidification, but are also performing better.”

Given these findings, the Swedish scientists concluded, “in the future ocean, the direct impact of ocean acidification on growth and development potentially will produce an increase in C. papposus reproductive success” and “a decrease in developmental time will be associated with a shorter pelagic period with a higher proportion of eggs reaching settlement,” causing the sea stars to become “better competitors in an unpredictable environment.” Not bad for a creature that makes its skeletal rods, plates, test, teeth, and spines from a substance that is 30 times more soluble than normal calcite.

Lastly, Rodolfo-Metalpa et al. (2010) worked with bryozoans or “moss animals”—a geologically important group of small animals that resemble corals and are major calcifiers, found on rocky shores in cool-water areas of the planet, where they comprise a significant component of the carbonate sediments in shallow sublittoral habitats, and where they form long-lived, three-dimensional structures that provide attachment sites for numerous epifauna and trap sediment and food for a variety of infauna—in what they describe as “the first coastal transplant experiment designed to investigate the effects of naturally acidified seawater on the rates of net calcification and dissolution of the branched calcitic bryozoan Myriapora truncata.” They did this by transplanting colonies of the species to normal (pH 8.1), high (pH 7.66), and extremely high (pH 7.43) CO2 conditions at gas vents located just off Italy’s Ischia Island in the Tyrrhenian Sea, where they calculated the net calcification rates of live colonies and the dissolution rates of dead colonies by weighing them before and after 45 days of in situ residence in May–June (when seawater temperatures ranged from 19 to 24°C) and after 128 days of in situ residence in July–October (when seawater temperatures ranged from 25–28°C).

Throughout the first and cooler observation period, “dead M. truncata colonies dissolved at high CO2 levels (pH 7.66), whereas live specimens maintained the same net calcification rate as those growing at normal pH,” the researchers write. At the extremely high CO2 level, however, the net calcification rate of the live specimens was reduced to only about 20 percent of what it was at normal pH, but life continued. Throughout the second and warmer observation period, on the other hand, calcification ceased in both the normal and high CO2 treatments,and in the extremely high CO2 treatment, the transplants died.

Based on these findings the five scientists concluded, “at moderate temperatures,” such as those to which they are currently adapted, “adult M. truncata are able to up-regulate their calcification rates and survive in areas with higher levels of pCO2 than are predicted to occur due to anthropogenic ocean acidification, although this ability broke down below mean pH 7.4.” This latter level, however, is below what even the IPCC predicts will occur in response to continued burning of fossil fuels, and far below what the more realistic analysis of Tans (2009) suggests.


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