(1) Light is essential for photosynthesis so that the determination of the so-called Photosynthetically Available Radiation (PAR) with depth is required to understand and quantify (e.g. through bio-optical models) carbon fixation by phytoplankton. (2) In open ocean waters in particular, the exponential decrease of light with depth at specific wavelengths (e.g. 490nm) is essentially controlled by the amount of phytoplankton (Chlorophyll-a). Therefore, the measurement of this light decrease (the diffuse attenuation coefficient), can be used in return to derive Chlorophyll-a concentration. Other specific wavelength (e.g. 412nm) can also be used to derive additional variables, like the Colored Dissolved Organic Matter (CDOM). http://www.oao.obs-vlfr.fr/datasm/measured-variablessm/colored-dissolved-organic-matter-cdomsm
2) How is it measured ?
Irradiance is obtained by a radiometer which is a device for measuring the radiant flux at specific wavelengths or in a given spectral range (e.g. PAR corresponds to irradiance over the visible range, i.e. within 400-700nm).
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.
Xing et al. (2011). Combined processing and mutual interpretation of radiometry and fluorimetry from autonomous profiling Bio-Argo Floats. Cholorophyll-a retrieval. Journal of Geophysical Research, 116, C06020, doi:10.1029/2010JC006899. (pdf)
Xing et al. (2011). Combined processing and mutual interpretation of radiometry and fluorimetry from autonomous profiling Bio-Argo Floats. 2. The CDOM absorption retrieval. Journal of Geophysical Research, submitted.
Morel, A. & Maritorena, S. (2001). Bio-optical properties of oceanic waters: A reappraisal. Journal of Geophysical Research, 106, 7763-7780. (pdf)