Or is there a reliable way to convert global irradiance to PAR. I am investigating the relationship between light and heterotrophic protists grazing in the Northern North Atlantic.
Yes. You can calculate the diffuse attenuation coefficient of PAR (KdPAR = 1/PAR*dPAR/dz), say from 10-30 m (assuming vertically homogeneous waters), then use this to extrapolate your 10 m PAR reading to the surface. Then, you'll need to account Fresnel reflectance at the surface waters. Let me know if you need more detail, but check out the following Ocean Optics Web Book chapters first:
K Functions: http://goo.gl/I37u3
Reflectances: http://goo.gl/dZYya
/eric
Thanks! I will check out the references you provided and get back to you if I have more questions
F.
If your profiles data are reliable, you can effectively calculate KD-PAR as suggested by Eric. Another way, if you do not have such profiles, is to use the formulation developped by Andre Morel (1988) between KD-PAR and the chlorophyll concentration. PAR SeawiFS (ocean color sensor) products are also available at global scale from the algorithm developped by Robert Frouin. This produc is very reliable.
Morel, A. (1988). Optical modeling of the upper ocean in relation to its biogenous matter content (case 1 water), Journal of Geophysical Research, 93, 10,749-10,768.
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