Carbon Budgets and Fluxes

The large carbon reservoirs on earth and the flux between reservoirs are not precisely known. This applies particularly to the carbon content in vegetation, soil and sediments, and in the deep ocean, and also to the net land-to-atmosphere and ocean-to-atmosphere flux. Best known is the emission of fossil fuel and the atmospheric increase. Budgets and fluxes are shown below in two graphic illustrations with different complexity. The fossil fuel emission of 6.40.4 GtC per year minus fluxes caused by sinks on land and in the oceans result in a net increase of atmospheric carbon by 3.20.1 GtC per year.
 
Carbon budgets in GtC in vegetation and soil, in the atmosphere and oceans, and carbon fluxes in GtC per year (shown by arrows) between these reservoirs. (GtC: Gigatons or 109 tons of carbon) 

Detailed view of budgets and fluxes in the global carbon cycle in the 1990ies. Pre-industrial or ‘natural’ values are given in black, anthropogenic values in red
Scenario
Obviously, the oceans are a relevant sink for carbon dioxide which is emitted to the atmosphere as a result of combustion of oil and gas. Although the biomass in the oceans is much smaller than on land, biological productivity is almost the same. Hence, carbon fixation is in the same order! No wonder that attempts are underway to increase biological productivity in the oceans, hoping that an essential part of the carbon fixed to phytoplankton particles will sink to the deep ocean. If so, then the carbon will be buried there! Since currents are very slow in the ocean, this part of the greenhouse gas will be removed from the climate system for thousands of years.
We become a geo-engineer! 
This sounds like a good idea: fertilizing the oceans with nutrients shall increase the storage of carbon in the deep sea, thus reducing greenhouse gas concentrations in the atmosphere!
Questions
1.	Which factors (e.g., light, temperature, salinity, transparency of the water, nutrients,  …) determine the biological productivity of phytoplankton in the oceans? Which quantities are most important, which ones are less relevant? Are there essential differences when compared with vegetation on land?
2.	Which factors (e.g., grazing by zooplankton and fish, bacterial attack, lack of nutrients, …) determine the loss of phytoplankton?
3.	Assume that a phytoplankton bloom degrades because of circumstances mentioned in 2). Then the plankton particles start sinking slowly into the deep. Which processes can lead to a decay of particles during sinking, so that they will not reach the sea floor? 
4.	Please find arguments for the following hypotheses:
•	Biological productivity in the ocean is limited by nutrients. Therefore, fertilization should lead to an increase of productivity and hence, to a higher uptake of carbon by phytoplankton which will be stored in the deep ocean.
•	Increasing biological productivity will not result to an increase of carbon in the deep ocean due to degradation processes which take place in the upper water column. Instead, biomass will be re-mineralised. Organic carbon will be oxidised to carbon dioxide and subsequently released to the atmosphere.
Materials
•	Linear regression analysis, i.e. the calculation of best-fit lines and correlation coefficients, is presented in the SEOS tutorial Time Series Analysis, chapter 2, pages 4-6. 
•	A detailed discussion of the Mauna Loa carbon dioxide curve is found in the SEOS tutorial on Time Series Analysis, chapter 2, pages 7.
•	Satellite data of the biosphere are presented in the SEOS tutorial on Ocean Currents, chapter 4, page 1.
Background information
•	Lohafex provides new insights on plankton ecology - Only small amounts of atmospheric carbon dioxide fixed: http://www.awi.de/en/news/press_releases/detail/item/lohafex_provides_new_insights_on_plankton_ecology_only_small_amounts_of_atmospheric_carbon_dioxide/?cHash=1c5720d7a1 
•	Fertilizing the Ocean with Iron: http://www.whoi.edu/oceanus/viewArticle.do?id=34167 
•	Smetacek V & S W A Naqvi: The next generation of iron fertilization experiments in the Southern Ocean. Philosophical Transactions of the Royal Society A, 2008.  http://www.see.ed.ac.uk/~shs/Climate%20change/Phil%20Trans%20November%202008/Smetacek%20iron%20fertization.pdf
•	Intergovernmental Panel on Climate Change (IPCC, http://www.ipcc.ch/), 4th Assessment Report: 
Biological Uptake in Oceans and Freshwater Reservoirs, and Geo-engineering, http://www.ipcc.ch/ipccreports/tar/wg3/index.php?idp=176 
•	Intergovernmental Panel on Climate Change (IPCC, http://www.ipcc.ch/), 4th Assessment Report: 
Couplings Between Changes in the Climate System and Biogeochemistry, http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter7.pdf