Turbidity/clarity vs optical depth
A secchi disk is basically a measurement of optical depth, and is very similar in principle to the measurements of AOD made in meteorology.  Those measurements are quantified at a particular wavelength of light (or a few discrete wavelengths), related to the Beer-Lambert law of light extinction.  Light extinction occurs due to scattering and absorption and the total attenuation is integrated.  A nephelometer, in contrast, has it's own light source which is shining on the particles, and a detector measures the reflected light (absorption is not being measured).  The nephelometer is calibrated to measure particle density by volume of a particular type of particle, based on the reflected light signature.

http://en.wikipedia.org/wiki/Nephelometer

http://en.wikipedia.org/wiki/Secchi_disk

http://en.wikipedia.org/wiki/Optical_depth