Carbon Dioxide and the Sea, Part 1

Introduction

Is carbon dioxide the enemy of a thriving reef? Though some debate remains, the short answer is yes. And also, no. Like so many other issues related to global climate change, the reason behind the decline of the worlds coral reefs is not fully agreed upon. However, it is clear that acidification due to escalating atmospheric carbon dioxide levels can be harmful to reef-building organisms. At the same time, carbon dioxide is a necessary component of global ecology. Without it, plants and algae would not be able to perform photosynthesis, which is the basis of nearly all ecological systems on Earth. In short, we could not be alive without CO2. Yet in spite of how essential carbon dioxide is to life on this planet, we must be aware of its dangerous dark side.

The problems of CO2 build-up are also important in marine husbandry, and lessons learned at the global level are applicable to aquariums as well. Acidification can occur in an aquarium when preventative measures are not in place. Corals may then succumb to ailments similar to those occurring in the ocean. Acidification can also lead to an undetected build-up of ammonia, followed by a deadly spike. Not surprisingly, understanding the carbon cycle is another notch in successful marine aquarists belt.

In this article, I discuss the positive and negative roles of CO2 in global and coral ecology. I look at the physical and chemical changes carbon dioxide causes in the environment, and what those changes mean for coral reefs. Then, in part 2, I detail the impact of CO2 on the marine aquarium, and how to mitigate against damaging build-up.

The Carbon Cycle

Carbon, as we all know, is the essential building block upon which all living organisms are constructed. Due to a unique combination of size and bonding tendencies, the carbon atom can create an enormous number of chemical compounds. In fact, carbon-based molecules are often referred to as organic because such a proliferation of them exists in biological systems. One such molecule, carbon dioxide, is of paramount importance to biological systems.

As a gas in the atmosphere, CO2 exists as a byproduct of cellular respiration in animals. The atmosphere provides an immense reservoir for carbon dioxide storage. In proper concentrations, the presence of the gas helps to maintain stable weather for the earth. CO2 is removed from the atmosphere by photosynthesis, the process by which solar energy is converted into food. The two most significant byproducts of photosynthesis are sugars and oxygenboth equally important ingredients for keeping animals alive. The oxygen created replenishes atmospheric reserves, while the sugars go on to form the basis of the food web.

Ordinarily, much of the carbon on earth remains sequestered within living organisms, coal, and fossil fuels. However, since human beings have removed great swaths of carbon-consuming forests, along with releasing carbon previously locked in coal and oil, the result is escalating atmospheric CO2 concentrations.

The Physical Effects of CO2 at Sea

When CO2 levels grow unchecked, as they have for the past two centuries, atmospheric build-up of the gaseous chemical causes dramatic physical changes around the globe. What was once known as the greenhouse effect and global warming is now called global climate change. Name changes not withstanding, CO2 is still implicated as a driving force behind the process. As solar rays enter the atmosphere, the earths surface reflects some it back in the form of infrared radiation (IR), or heat. Some the reflected IR is then trapped by greenhouse gases, including CO2, creating a warming effect.

As the planet warms, ice in the polar regions begins to melt, with a number of damaging results. The classic example is sea-level rise. While floating ice does not contribute to sea-level rise, melting glacial ice on land does. In fact, Earths glaciers contain enough water to elevate sea-level by 200 feet!

The increased amount of freshwater run-off has other negative effects on the planet as well. Persistent dilution of seawater will eventually cause the oceans circulatory currents to break down. Those currents, responsible for distributing warm and cold waters around the oceans great basins, are also weather stabilizers. Since the atmosphere and the ocean are really just the two sides of a larger whole, one cannot be destabilized without the effect transmitting to the other. Such a wholesale imbalance creates a cycle whereby the problem is perpetuated rather than diminished over time, especially when coupled with indiscriminate warming.

Interestingly, some soft corals may actually benefit from warming seas. There is evidence that softies grow faster and larger in warmer waters. However, the same conditions have been linked to bleaching events in hard corals, because the warmer temperatures lead them to expel their algal symbionts.

Put simply, the physical effects of elevated CO2 levels is bad! Imagine the effect on shallow water corals if the ocean depth increased by even 20 feeta tenth of what is possible. Lagoon based corals, such as many LPS species, would very likely go extinct. Moreover, those lagoon corals are some of hardiest, most adaptable species. Corals from even marginally deeper waters exist in a very stable environment. If that stability is undermined, there may be few survivors. Coral ecology is complex, but it is clear that CO2 levels are strongly linked to the physical parameters of their environment.

The Chemical Effects of CO2 at Sea

Just as CO2 has physical impacts on the marine environment, it has equally important chemical properties that influence coral ecology. As it dissolves into seawater, CO2 is converted into carbonic acid. In this manner seawater is acidified and pH is lowered.

Several problems stem from from acidification. For instance, reef-building Scleractinians and coralline algae would find it increasingly difficult to deposit their carbon-based skeletons. Calcium carbonate, aragonite, is the primary component of these skeleton and is dissolved under acidic conditions. Clearly, coral reefs cannot grow if they cannot continue to deposit the substance upon which they build themselves. Acidification can also lead to an undetected build-up of ammonia. Actually, under the right conditions, dissolved ammonia gas develops as ammonium ions instead. Ammonium is relatively harmless and can exist in much higher concentrations without deleterious biological effects. If, for some reason, the pH of the water returns to normal, the ammonium will be instantly converted back into ammonia gas. The resultant spike can be deadly to marine animals. In the ocean, acidification is more likely to lead to skeletal deficiencies, while ammonia spikes are more of an aquarium issue.

Ecological Considerations

Another serious issue caused by oceanic acidification is more directly linked to algae. While there are some species which may thrive under these conditions, many corals may expel their symbiotic algae, thus causing bleaching events. For many corals, bleaching quickly leads to irreversible and deadly damage. Not only does the coral starve, it has also lost its form of natural sunblock. The pigments that give corals their radiant colors are actually a means of providing protection against harmful ultraviolet radiation. The pigments, created by the algal symbionts, reflect UV radiation away from the corals tissues. Without these, the corals suffer tissue damage from solar rays. Less typically, some symbiotic algae may instead respond to acidification by reproducing too rapidly. Where this occurs, a proliferation of oxygen can build up in the corals tissues, leading to harmful oxidation.

In other cases, increased CO2 levels can lead to harmful algal blooms in the water column. These blooms may inhibit hermatypic processes by decreasing the sunlight available for photosynthesis. In others words, the algae gets in the way and absorbs too much light before it reaches the reef. Even more significantly, algal blooms cause enormous damage when the algae dies off. The algal cells die and get consumed by aerobic bacteria, which also consumes oxygen from the water column. Since seawater is already a poor medium for holding dissolved oxygen, it does not take much to strip the water of this necessary chemical. The result is an anoxic dead zone. This process already occurs in large areas of the Gulf of Mexico, where run-off from the Mississippi River feeds the anoxic cycle.

Recently, other serious ecological problems have been linked to ocean acidification as well. Evidence has been presented which indicates that coral bleaching is very harmful to fish species associated with reefs. As many are colorfully camouflaged, it is not surprising that they suffer increased predation as the background colors disappear. In other cases, reef fish have been known to demonstrate out-of-character aggression, perhaps as a way to secure the remaining unbleached habitats.

Upcoming: Part 2

In part two of this article, I discuss the role of CO2 in the marine aquarium. I cover the sources of carbon dioxide build-up, the indications, and the results. I also talk about ways to prevent harmful increases in this toxic chemical. Stay tuned for Part 2 of Carbon Dioxide and the Sea.

Works Cited:

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Castro, Peter and Michael E. Huber. Marine Biology, Fifth Edition. McGraw Hill: Boston. 2005.

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McDonald, Timothy. Coral bleaching gives rise to reef bullies. Nov. 04, 2009. URL: http://www.abc.net.au/news/stories/2009/11/04/2732742.htm

Physorg.com. USF Study Shows First Direct Evidence of Ocean Acidification. Jan. 20, 2010. URL: http://www.physorg.com/news183231158.html

Sunday Observer. Fish with attitude: Some like it hot. Jan. 24, 2010. URL: http://www.sundayobserver.lk/2010/01/24/jun03.asp

EDF. Threats: Coral Reef Bleaching. Aug. 16, 2007. URL: http://www.edf.org/article.cfm?contentID=4709

Thurman, Harold V. and Alan P. Trujilo. Introductory Oceanography, Tenth Edition. Pearson Prentice Hall: Upper Saddle River. 2004.