In vascular plant, the portable chlorophyll meter (such as CCM-200 or SPAD 502PDL) is widely used to estimated Chlorophyll content. In moss, Varela (2013) used the chlorophyll meter to measure Chlorophyll content. But I seem to not see this method used in Sphagnum.
Varela Z, Roiloa S R, Fernández J A, Retuerto R, Carballeira A, Aboal J R. Physiological and Growth Responses of Transplants of the Moss Pseudoscleropodium purum to Atmospheric Pollutants [J]. Water Air Soil Pollution, 2013. 224: 1753. DOI: 10.1007/s11270-013-1753-4