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Fertilizing naturally placid waters with nutrients is essentially changing the food web structure. You're changing the nutrient concentration but not changing the turbulence in a corresponding manner. By adding nutrients to naturally placid waters you are not mimicking fertilization by natural turbulence. The point is that it's difficult to predict what would happen with nutrient additions to these waters. But there are ocean areas that are analogs to high-nutrient, low-turbulence situations, for example in regions of coastal eutrophication. Those situations can promote a different assemblage of phytoplankton, some of which are toxic species or are species that hog the nutrients, and are not readily eaten by fish or the grazers that lead to fish production.

imageIf you fertilize the ocean to sequester carbon, you definitely change the food web structure. It's not something that might happen. If you don't change food web structure, you don't sequester carbon. An important point to remember from Jon Cole's earlier talk on lakes is that changing the food web is very unpredictable, and strongly dependent on time-scale.

imageOther possible consequences of adding nutrients to ocean systems are reduced oxygen in the deep sea, which can increase methane production, and increased denitrification. Both processes can release greenhouse gases to the atmosphere (methane and nitrous oxide) that are more potent than CO2.

Another important consequence of nutrient additions to surface water is to decrease the water transparency. When phytoplankton bloom, the water becomes murky and less sunlight penetrates to depth. If nutrient enrichment influences coastal waters, or waters over coral reefs, the lack of sunlight would starve bottom habitats with some very important ecological consequences.

The bottom line is that ideas are being explored, but there are uncertainties about what might happen ecologically.

imageLet's take a step back and talk about calculating potential carbon credits from ocean sequestration. It's clear that the procedures for calculating carbon credits are anything but settled. First, simple calculations will not work.

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As Kenneth Coale and others have said here today, after the initial fertilization experiment there was a reduction in the concentration of CO2 in the fertilized patch. But that lower CO2 level cannot be simply equated to carbon that goes away to the deep ocean. First, it is likely that the CO2 that was consumed during the experiment came up from the deep water with the nutrients that were consumed, and not from the atmosphere. It was not determined if the CO2 was replaced by CO2 from the atmosphere or from depth. So, CO2 measurements alone may not be a good indicator of carbon sequestration.

A second means of calculating carbon credits is to measure particles sinking from a phytoplankton bloom.
One could go out in an ocean area that's productive, and find particles of carbon are sinking all day, every day. However, an equivalent amount of carbon dioxide is coming up from the depths to match that downward flux in the 'Redfield Ratio' ocean that Dave Karl talked about earlier.

Alteration of deepwater chemistry from intentional fertilization will also change productivity patterns elsewhere. The ocean is a big system and the deep water moves around and will eventually influence surface layers somewhere else apart from the site of the fertilization. Furthermore, the organic matter that goes down in the deep ocean doesn't just decompose and automatically turn into carbon dioxide or nutrients. Decomposition is a complicated process that affects different elements in different ways.

The interaction of the decomposition with the productivity of the ocean is very important. To describe the process requires a very clear understanding, and it requires computer modeling approaches.

imageSo, the short message here is that procedures for calculating carbon sequestration are anything but settled. Many schemes exist, but no scheme is both rigorous and understandable to the general public and legislators. Nobody that I know can say, "This is how the biological pump works," so that scientists would concur and a general audience or anyone who has to make policy decisions would understand it.

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