It is composed by living (phytoplankton, zooplankton and bacteria) as well as non-living materials (aggregates, pellets and detritus) and is an important parameter of the global carbon cycle. Indeed POC is the main form by which organic carbon synthesized through photosynthesis in the upper layer is transferred to the deep ocean. There it can be sequestered over geological time-scales.
2) How is it measured ?
> In situ (sensor): attenuation (cp) coefficient, particulate backscattering (bbp) coefficient;
> In vitro (in the lab): Chemical analysis of Carbon (particles retained on a filter) on an autoanalyser.
> Space-based (satellite sensor): measurement of the backscattering coefficient.
The optical coefficients (e.g. cp, bbp) are converted into POC thanks to established relationships.
3) Where is more information available ?
IOCCG (2011). Bio-Optical Sensors on Argo Floats. Claustre, H. (ed.), Reports of the International Ocean-Colour Coordinating Group, No. 11, IOCCG, Darthmouth, Canada.
Stramski et al. (2008). Relationships between the surface concentration of particulate organic carbon and optical properties in the eastern South Pacific and eastern Atlantic Oceans. Biogeosciences, 4, 1041-1058. (access to paper online) http://www.biogeosciences.net/4/1041/2008/bg-4-1041-2008.pdf
Claustre et al. (1999). Variability in particle attenuation and stimulated fluorescence in the tropical and equatorial Pacific : scales, patterns and some biogeochemical implications. Journal of Geophysical Research, 104, 3401-3422. (pdf)
Loisel, H. & Morel, A. (1998). Light scattering and chlorophyll concentration in case 1 waters: A reexamination. Limnology and Oceanography, 43, 847-858. (pdf)