Dear @Abhijit, "he first method (atmosphere-ocean flux) is based on the measurement of CO2 partial-pressure differences between the ocean surface and the atmosphere. Partial pressure is the amount of pressure that a particular gas such as CO2 within a gas mixture (the atmosphere) contributes to the total pressure. Partial pressure is thus also one possibility for quantitatively describing the composition of the atmosphere." The article Measuring exchange between the atmosphere and ocean follows!

http://worldoceanreview.com/en/wor-1/ocean-chemistry/co2-reservoir/2/

Thank you dear @Ljubomir for the answer; I learn from it how stored carbon can be estimated in phytoplankton.

yes sir i am also thankful to sir for this useful information as i don't know much about this technique . 

Fundamental cycle on photosynthesis resulting in conversion of atmospheric CO2 into particulate carbon is a consequence of  diel cycles of biogeochemical and optical properties that can be observed in the sunlit ocean. The particulate beam-attenuation coefficient,cp, is an in-herent optical property that exhibits diurnal variations cp depends, on all the particles present in the water column (i.e., autotrophic and heterotrophic micro-organisms, as well as detritus and mineral particles). In practice however, cp is influenced by particles with equivalent spherical diameters (ESD) between 0.5 and 20 μm. Within this size range, each particle pool contribute a variable fraction of cp, depending on its concentration, size, refractive index and shape . In the open ocean, cp is with particulate organic carbon concentration (POC), but it is difficult to ascribe variations of cp to any specific particle pool

http://www.biogeosciences.net/8/3423/2011/bg-8-3423-2011.pdf

The first method (atmosphere-ocean flux) is based on the measurement of CO2 partial-pressure differences between the ocean surface and the atmosphere. Partial pressure is the amount of pressure that a particular gas such as CO2 within a gas mixture (the atmosphere) contributes to the total pressure. Partial pressure is thus also one possibility for quantitatively describing the composition of the atmosphere. If more of this gas is present, its partial pressure is higher. If two bodies, such as the atmosphere and the near-surface layers of the ocean, are in contact with each other, then a gas exchange between them can occur. In the case of a partial-pressure difference between the two media, there is a net exchange of CO2. The gas flows from the body with the higher partial pressure into that of lower pressure. This net gas exchange can be calculated when the global distribution of the CO2 partial-pressure difference is known.

The answer depends on what do you want to estimate. Instantaneous phyto-carbon stock? Short-term CO2 fixed? Long-term CO2 storage?

Phytoplankton fix atmospheric CO2 into biomass (measurable for example as particulate organic carbon, POC). A major (but variable) fraction of this biomass is respired back to CO2 by the microbial food web in a timescale of days to weeks. The remainder can sink to the sediment, where some of it will be respired and some will be buried and thus preserved for longer periods. The partition between these processes depends strongly on the phytoplankton community size structure and taxonomy: big cells sink faster than small ones, especially if they have silicon walls (like diatoms). The presence of predators is important too, because they can pack phytoplankton into faecal pellets that sink much faster. Finally, the partition between these pathways depends also on the time scale you look at.

If you specify what measurement you are interested in i can try to provide you with some suggested readings.

Phytoplankton does *uptake* C (thereby lowering pCO2, increasing particulate C14, etc.. for various assay methods) using their photosynthetic pigments.  The uptake rate depends on the pigment concentration and activity (this can also be determined using samples or in situ fluorescence).

Phytoplankton *also releases* C as part of their metabolic functioning (e.g., exsudates), or when cells die / are grazed.  At steady state, C storage by plankton corresponds roughly to the C harvested and pulled out of the system under consideration.  That's also the sense of Marti's answer above.

Could you describe a bit more what you are after?