"Production within dense mats of the filamentous macroalga Chaetomorpha linum in relation to light and nutrient availability"
http://algaescrubber.net/ChaetoStudy.pdf
or
https://web.archive.org/web/20190501...4/m134p207.pdf
This is a good study, because chaeto is familiar, and because some people consider putting it in a refugium instead of using a scrubber.
Fig 5B shows how, under bright light, chaeto productivity (filtering) drops 72 percent with just 2 cm of chaeto thickness.
Also, speaking of how far light will travel through algae, there is this: "in phytoplankton communities, chlorophyll densities may range from 0.2 to 0.4 in productive lakes to 0.00015 g chl A m3 in oceanic waters". This means that lakes can have 1000 times the density of phyto.