Pigment absorption and quantum yields in the Arabian Sea

Carbon assimilation and optical properties were measured at several stations in the Arabian Sea, during the Spring Intermonsoon (March–April 1995) and the Northeast Monsoon (December 1995). Biological measurements, as a function of depth (z), included: 14C uptake (P), total daily (photosynthetically active) irradiance (EPAR), spectral irradiance, spectral absorption by phytoplankton, and a variety of HPLC-determined phytoplankton pigments. Phytoplankton absorption (aph) was optically weighted to the spectral quality of submarine irradiance. Absorption spectra based on particulates collected on filters were compared with spectra reconstructed from phytoplankton pigments. These two methods generally agreed, except in the blue region of the spectrum at intermediate and shallow depths. Quantum yield (φ, on a daily basis) was estimated by non-linear regression from the relationship, P(z)/āph(z)=φ(EPAR)EPAR(z). Absorption by non-photosynthetic pigments in most cases causes a decrease in maximum realized quantum yield ranging from 30% to a factor of four. Generally, stations, with higher non-photosynthetic pigment absorption had lower maximum quantum yields but a greater ability to utilize high irradiances. The data suggest the hypothesis that adaptation, through absorption properties, is the primary determinant of the magnitude of photosynthetic quantum yield in phytoplankton.