Researchers have proposed that ocean iron fertilization, which stimulates phytoplankton growth, could be a probable method to mitigate ocean acidification. Long Cao and Ken Caldeira, from Stanford University, examined this idea by creating a simulation that diminished phosphate concentrations in near-surface waters to zero. This placed an upper bound on the maximum possible effect of ocean iron fertilization. This extreme situation, which is unlike real-world conditions, was used to show the basic effects of fertilization on ocean acidification.

When implementing iron fertilization into the simulation, it was determined that fertilization may mitigate ocean acidification in surface waters but if this fertilization also generated carbon credits within the ocean, it would allow greater CO2 emissions to enter the sea and there would be no benefit to the fertilization. However, in all cases it was found that fertilization created greater acidification in deep-sea waters. Because fertilization could lead to worsened deep-sea conditions without aiding surface conditions it is an ineffective mitigating technique.

Can ocean iron fertilization mitigate ocean acidification? Climatic Change, Springer Netherlands. Volume 99, 1-2. DOI: 10.1007/s10584-010-9799-4 (2010)

With the increasing amount of CO2 being absorbed into the oceans, one can only imagine the impact it is and will have, and the breadth and depth of the effects of ocean acidification. Effects on surface water chemistry, calcification, various biological processes, and coral reef growth provide an idea of the breadth of impacts, but observing something specific, like phytoplankton, can be used to show the depth of negative effects.

The basis behind ocean acidification is that the ocean is becoming more acidic as a result of increased CO2 levels, which are being absorbed from the emissions in the atmosphere. Acids have higher concentrations of H+ ions, and this is exactly what is happening as CO2 is absorbed into the ocean: there are more H+ ions. Essentially, the CO2 combines with water to form carbonic acid (H2CO3). This carbonic acid then breaks down, forming bicarbonate (HCO3) and a remaining H+ ion, the ion found in acids.

Of course a more acidic environment could prove detrimental to ecosystems and organisms alike simply because of a change in the environment. But as we see, a decreased pH will even have effects on nutrient acquisition, and it starts with the nutrients themselves. The nutrients taken in by organisms are essentially chemical compounds. And because we have changed the chemistry of the ocean, it makes sense that the compounds themselves, which are taken in by organisms, are now also changed. Iron, among many trace metals, is an essential element for different biological processes. Unfortunately, a decreased pH causes dissolved iron to less bioavailable, and as low iron levels limit primary production in Phytoplankton, they now experience a limiting growth factor.

Other nutrients, such as phosphate, exhibit similar behavior in regards to their chemical form as result of a change in pH. These elements can bind to organic compounds, and the rate and type of binding occur relative to the pH. So Phytoplankton will have to use enzymes to biochemically remove the phosphate from the organic compound. It so happens that the enzymatic activity is also affected by the pH, and the enzymes which cleave phosphate or other nutrients from the organic compounds are located externally; they are directly affected with the pH change.

Additional to the nutrients being altered, increased CO2 levels increase Phytoplankton synthesis and primary production, and has been shown in various studies. Further studies have shown that the composition of what Phytoplankton produce in higher CO2 levels have greater carbon to nitrogen rations (C:N). A higher organic carbon ratio can mean the Phytoplankton have higher nutritional values, and since our Phytoplankton are at the bottom of the sea trophic levels, consumers can potentially expect an increased growth and reproduction rate.  Looking at a study on bottom-up affects on trophic levels, we see that if trophic level 1 is increased, above trophic levels experience increases relative to the ratio of change in trophic level 1. If Phytoplankton encounter increased nutritional values, above trophic levels will reach a new population density equilibrium, as reproductions can also be affected at higher trophic levels.

However, other studies argue that even after comparing eight different species of phytoplankton, a decrease in pH, even to the predicted pH for the end of the century (7.8), showed neither an increase or decrease in growth or production rates. This is not to dismiss our theoretical situation where consumers reach a new population density equilibrium, but to note that sufficient studies are not yet available and to show that predications of what ocean acidification can do are fairly difficult, as the potential impacts cover a large scope.

Bibliography

Berge e al. “Effect of Lowered PH on Marine Phytoplankton Growth Rates.” MARINE ECOLOGY PROGRESS SERIES 416 (2010): 79-91. 14 Oct. 2010. Web. 4 Sept. 2011. <http://www.int-res.com/articles/meps2010/416/m416p079.pdf>.

Devaraj, Maurice S. THREE TANK MODEL: A Top down or Bottom up Dominance Analysis. Journal of Theoretics. Web. 4 Sept. 2011. www.journaloftheoretics.com/Link/Papers/ TDBU.pdf