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CO2 Uptake by Phytoplankton - September 27, 2015

The ocean has the ability to absorb and store large amounts of carbon dioxide. A conservative estimate is that the ocean currently stores about 26% of the CO2 produced by the burning of fossil fuels. Without the ocean’s ability to soak up excess gases in our atmosphere, the levels would be significantly higher. 

 A large contributor to carbon dioxide uptake is a diverse group of organisms called phytoplankton; they are the true unsung heroes of our ocean. Plankton is a general name for a large group of plants and animals that are suspended in the water column and cannot swim against the current, but instead move with it. Phytoplankton are the suspended plants, while zooplankton are the suspended animals. 

As with all plants, phytoplankton photosynthesize and take up CO, thereby removing it from the environment. Phytoplankton depend on light for photosynthesis, so they are typically found in the top 50 meters of the water column. Although phytoplankton remove CO2 from the environment, the way these organisms go about this process is very different from trees and other terrestrial plants. They act similar to a pump, transferring nutrients and gases from the surface down to the ocean depths. CO2 is absorbed during photosynthesis, and while not all the gases are retained, those that are go down into the ocean depths when the plants die, or when the organism that ate the phytoplankton die, and through their feces. As the organic material decays, the CO2 is released and becomes incorporated into the seawater. Centuries to millennia in the future, the CO2 will be brought to the surface again and that carbon dioxide will be released into the atmosphere once more. A small percentage of the COis incorporated into ocean sediments, but the proportion is unknown. 

Studies have been proposed to use phytoplankton CO2 absorption to our advantage and alleviate gas levels in the atmosphere. One example would be to add iron, which is a limiting nutrient for phytoplankton. It has been proposed that iron stores be emptied into particular places in the ocean where phytoplankton growth and population potential are high but not realized due to nutrient deficiency. With the increase in growth, more CO2 would be absorbed. While this sounds very promising, there are many factors that need to be considered. When there are more phytoplankton there is also more food for their predators whose populations will increase and lessen the number of phytoplankton. So far attempts to increase CO2 uptake by phytoplankton have been unsuccessful because there are many variables to consider and, if done hastily, the results could be disastrous for the plankton population and in turn the food web, in which plankton play a vital role. When manipulating biological systems that are not completely understood, the results typically have negative consequences that were never considered. While this may be an option to lessen COlevels in the future, many more studies need to be done at every level of the food web and the effects on each considered.

Although this uptake of CO2 by phytoplankton is substantial, it is much lower than the levels being absorbed at the beginning of the industrial revolution, when human-produced CO2 was first being released into the atmosphere in large quantities. This can be attributed to several factors, but the simplest is that the upper level of seawater has developed more resistance to additional CO2 absorption. As more carbon dioxide is absorbed in to the same volume of seawater, the resistance increases until it reaches a point, called saturation, where is will no longer absorb additional CO2. Another contributing factor is that climate change slows the mixing of ocean water, which also slows the transfer of CO2 from the surface to the ocean depths. As a result, waters that are 500 meters and deeper have a large absorption capacity that has not yet been utilized. 

Having the ocean absorb large quantities of carbon dioxide may not be a good alternative regardless; in the last ten years scientist realized that the combination of warming waters and huge absorptions of CO2 has altered water chemistry significantly. As the ocean dissolves carbon dioxide the water’s pH drops, becoming more acidic; an issue called ocean acidification. Ocean water has become 30% more acidic in the last 200 years; this is the quickest alteration in ocean chemistry in the last 50 million years. Many types of animals that produce and live in shells are already showing effects, with their shells dissolving in these increased acidic conditions. Ocean acidification is expected to change many aspects of marine ecosystems and many of those impacts will be negative. The extent of ocean acidification’s biological impacts is unknown. Most species have adapted to a fairly constant pH over several million years and the quick change in water chemistry is not allowing them time to adjust. 

One silver lining is that hardy estuarine species, like those in the Maryland Coastal Bays, are finding ways to adapt to these changing conditions and thrive. In addition to the loss of biodiversity and changes in density, acidification will negatively affect fisheries and aquaculture, threatening food security as well as tourism in ocean dependent economies. The need to lessen CO2 and find alternatives to burning fossil fuels goes beyond impacts on out atmosphere and climate change, but it also threatens our ocean’s health. 

 

Emma Rice is the Chesapeake Conservation Core Volunteer for the Maryland Coastal Bays Program. 

 


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