From carbon dioxide sink to acid bath

Leading journals knocked back Bradley Opdyke in the early 1990s when he tried to publish research giving some of the first hints that ocean acidification due to high atmospheric carbon dioxide levels could wreak havoc on marine ecosystems.
The work by the young scientist, then based at the University of Michigan, was zeroing in on a problem that researchers now fear will hit marine life hard: aberrations in ocean chemistry caused by surges in dissolved carbon dioxide.

It is now known that the casualties could be organisms such as coral and clams, which extract calcium carbonate from seawater to form their exoskeletons and shells.

A World Without Whales?

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My grandfather was Jacques Cousteau, a pioneer of ocean exploration and the co-inventor of scuba diving. Back in the 1940′s when he tested out his invention which allowed humans to swim freely in the ocean with a portable air source for the first time in history, very little of the ocean had been explored let alone captured on film. I remember growing up with his stories, about when he took his first breath underwater off the coast of southern France and how stunned he was by the raw beauty that surrounded him. However, I was also told of how devastated he was by what has happened to those very same reefs which have crumbled and virtually disappeared. The work of my grandfather and then my father, Philippe Cousteau Sr., over the following 50 years laid the groundwork for most of what we know about the marine world. There is an irony that while we have seen the greatest amount of exploration of our planet in the last 50 years, we have also seen the greatest destruction of it. And the oceans are no exception. Now, we face yet another challenge: ocean acidification.

Ocean acidification is caused by the ocean absorbing excess carbon dioxide from the atmosphere, the same carbon dioxide that is the primary cause of global warming, hence the nickname “the other carbon problem.” As they do so, the oceans become more acidic with terrible consequences. Scientists have proven a direct link between the excessive carbon we have been spewing into the atmosphere since the Industrial Revolution and the rise in ocean acidity. Indeed, since that time, the pH of the surface of the ocean has dropped by 0.1 pH units (an approximate 30% increase in acidity in the ocean).

Climate change and coral reefs: Trojan horse or false prophecy?

Maynard et al. (Coral Reefs 27:745–749, 2008a) claim that much of the concern about the impacts of climate change on coral reefs has been “based on essentially untested assumptions regarding reefs and their capacity to cope with future climate change”. If correct, this claim has important implications for whether or not climate change represents the largest long-term threat to the sustainability of coral reefs, especially given their ad hominem argument that many coral reef scientists are guilty of “popularising worst-case scenarios” at the expense of truth. This article looks critically at the claims made by Maynard et al. (Coral Reefs 27:745–749, 2008a) and comes to a very different conclusion, with the thrust and veracity of their argument being called into question. Contrary to the fears of Grigg (Coral Reefs 11:183–186, 1992), who originally made reference to the Cassandra syndrome due to his concern about the sensationalisation of science, the proposition that coral reefs face enormous challenges from climate change and ocean acidification has and is being established through “careful experimentation, long-term monitoring and objective interpretation”. While this is reassuring, coral reef ecosystems continue to face major challenges from ocean warming and acidification. Given this, it is an imperative that scientists continue to maintain the rigour of their research and to communicate their conclusions as widely and clearly as possible. Given the shortage of time and the magnitude of the problem, there is little time to spare.

Oxygenation of the ocean and sediments: Consequences for the seafloor carbonate factory

Observed changes in the source of CaCO3 sediments since Archean time suggest a first order pattern of decreasing abundance of carbonate cements precipitated directly on the seafloor. We propose that the observed reduction in CaCO3 precipitation on the seafloor is caused by a decrease in CaCO3 saturation in sediments related to increased oxic cycling of organic carbon and a decline in the size of the marine DIC reservoir. Using a simple model of CaCO3 saturation in the ocean, we show that changes in ocean–atmosphere redox and the size of the marine carbon reservoir strongly influence the ability of sediments to dissolve or precipitate CaCO3. Oxic oceans like the modern are characterized by large gradients in CaCO3 saturation. Calcium carbonate precipitates where CaCO3 saturation is high (surface ocean) and dissolves where CaCO3 saturation is low (sediments). In contrast, anoxic respiration of organic carbon and/or a large ocean carbon reservoir leads to a more homogeneous distribution of CaCO3 saturation in the ocean and sediments. This effect suppresses CaCO3 dissolution and promotes CaCO3 precipitation on the seafloor. Our results suggest that the growth or contraction of gradients in CaCO3 saturation in the ocean and sediments may explain the observed trends in carbonate precipitation on the seafloor in the Precambrian and changes in the global CaCO3 cycle, such as the reappearance of seafloor precipitates and the drowning of carbonate platforms during episodes of widespread anoxia in the Phanerozoic marine basins. Our work provides novel insights into the consequences of the long-term geochemical evolution of the ocean and atmosphere for the global CaCO3 cycle.

Synergistic effects of climate change and local stressors: CO2 and nutrient-driven change in subtidal rocky habitats

Climate-driven change represents the cumulative effect of global through local-scale conditions, and understanding their manifestation at local scales can empower local management. Change in the dominance of habitats is often the product of local nutrient pollution that occurs at relatively local scales (i.e. catchment scale), a critical scale of management at which global impacts will manifest. We tested whether forecasted global-scale change [elevated carbon dioxide (CO2) and subsequent ocean acidification] and local stressors (elevated nutrients) can combine to accelerate the expansion of filamentous turfs at the expense of calcifying algae (kelp understorey). Our results not only support this model of future change, but also highlight the synergistic effects of future CO2 and nutrient concentrations on the abundance of turfs. These results suggest that global and local stressors need to be assessed in meaningful combinations so that the anticipated effects of climate change do not create the false impression that, however complex, climate change will produce smaller effects than reality. These findings empower local managers because they show that policies of reducing local stressors (e.g. nutrient pollution) can reduce the effects of global stressors not under their governance (e.g. ocean acidification). The connection between research and government policy provides an example whereby knowledge (and decision making) across local through global scales provides solutions to some of the most vexing challenges for attaining social goals of sustainability, biological conservation and economic development.

Enhanced biological carbon consumption in a high CO2 ocean: a revised estimate of the atmospheric uptake efficiency

A recent mesocosm study under high CO2 conditions has found phytoplankton carbon consumption is elevated beyond typical Redfield ratios (Riebesell et al., 2007). We investigate the efficacy of this elevated biological carbon consumption to increase global oceanic CO2 uptake from the atmosphere in an ocean general circulation model (OGCM). In the OGCM, elevated biological carbon consumption throughout the ocean increased oceanic CO2 uptake by 46 Pg C during 1800 to 2100 period, which is less than half the value estimated by (Riebesell et al., 2007). Our study’s lower ratio of oceanic CO2 uptake from the atmosphere caused by enhanced biological carbon consumption (export production) is due to a more realistic 3-D circulation and the resulting spatial patterns in the re-supply of carbon from the interior ocean to the surface. In our OGCM simulations, despite increased biological carbon export to the ocean interior, some regions like the eastern equatorial Pacific and Southern Ocean actually take up less CO2 from the atmosphere. This is due to the pooling of exported carbon at intermediate depths within these regions (analogous to nutrient trapping) and its subsequent re-supply back to the surface that exceeds the enhanced biological carbon export in the high CO2 world. Thus large-scale increases in biological carbon export can lead to some areas where surface ocean pCO2 increases more rapidly than atmospheric CO2. Furthermore, our results demonstrate that enhancing biological carbon export via other means such as iron fertilization is inefficient in regions like the Southern Ocean because of the rapid vertical re-supply of carbon-rich waters. This vertical resupply of carbon-rich waters in the Southern Ocean dampens the oceanic CO2 uptake efficiency due to enhanced biological carbon consumption to be only 16% and suggests a very low efficacy of biological fertilization in the region.

University of Hawaii at Manoa researchers reveal ocean acidification at Station ALOHA

The burning of fossil fuels has released tremendous amounts of the greenhouse gas carbon dioxide (CO2) into the atmosphere, significantly impacting global climate. Were it not for the absorption of CO2 by the oceans, the alarming growth of atmospheric CO2 concentration would be substantially greater than it is. However, this beneficial role of the oceans as a CO2 “scrubber” does not come without undesired consequences. When dissolved, CO2 acts as an acid, and lowers seawater pH. Since the beginning of the industrial age, CO2-driven acidification of the surface oceans has already caused a 0.1 unit lowering of pH, and models suggest that another 0.3 pH unit drop by the year 2050 is likely. Continued acidification of the sea may have a host of negative impacts on marine biota, and has the potential to alter the rates of ocean biogeochemical processes.

The subtle effects of sea water acidification on the amphipod Gammarus locusta

We report an investigation of the effects of increases in pCO2 on the survival, growth and molecular physiology of the neritic amphipod Gammarus locusta which has a cosmopolitan distribution in estuaries. Amphipods were reared from juvenile to mature adult in laboratory microcosms at three different levels of pH in nominal range 8.1–7.6. Growth rate was estimated from weekly measures of body length. At sexual maturity the amphipods were sacrificed and assayed for changes in the expression of genes coding for a heat shock protein (hsp70 gene) and the metabolic enzyme glyceraldehyde-3-phosphate dehydrogenase (gapdh gene). The data show that the growth and survival of this species is not significantly impacted by a decrease in sea water pH of up to 0.5 units. Quantitative real-time PCR analysis indicated that there was no significant effect of growth in acidified sea water on the sustained expression of the hsp70 gene. There was a consistent and significant increase in the expression of the gapdh gene at a pH of ~7.5 which, when combined with observations from other workers, suggests that metabolic changes may occur in response to acidification. It is concluded that sensitive assays of tissue physiology and molecular biology should be routinely employed in future studies of the impacts of sea water acidification as subtle effects on the physiology and metabolism of coastal marine species may be overlooked in conventional gross “end-point” studies of organism growth or mortality.

University of Hawaii at Manoa researchers reveal ocean acidification at Station ALOHA

The burning of fossil fuels has released tremendous amounts of the greenhouse gas carbon dioxide (CO2) into the atmosphere, significantly impacting global climate. Were it not for the absorption of CO2 by the oceans, the alarming growth of atmospheric CO2 concentration would be substantially greater than it is. However, this beneficial role of the oceans as a CO2 “scrubber” does not come without undesired consequences. When dissolved, CO2 acts as an acid, and lowers seawater pH. Since the beginning of the industrial age, CO2-driven acidification of the surface oceans has already caused a 0.1 unit lowering of pH, and models suggest that another 0.3 pH unit drop by the year 2050 is likely. Continued acidification of the sea may have a host of negative impacts on marine biota, and has the potential to alter the rates of ocean biogeochemical processes.

Dying reefs

Increasingly acidic oceans and warming water temperatures due to carbon dioxide emissions could kill off the world’s ocean reefs by the end of this century.

Scientists predicted that the pace of emissions means a level of 450 parts per million (ppm) of carbon dioxide (CO2) in the atmosphere will be reached by 2050, putting corals on a path to extinction in the following decades.