Pigment absorption and quantum yields in the Arabian Sea

Carbon assimilation and optical properties were measured at several station s in the Arabian Sea, during the Spring Intermonsoon (March-April 1995) and the Northeast Monsoon (December 1995), Biological measurements, as a funct ion of depth (z), included: C-14 uptake (P), total daily (photosyntheticall y active) irradiance (E-PAR), spectral irradiance, spectral absorption by p hytoplankton, and a variety of HPLC-determined phytoplankton pigments. Phyt oplankton absorption (a(ph)) was optically weighted to the spectral quality of submarine irradiance. Absorption spectra based on particulates collecte d on filters were compared with spectra reconstructed from phytoplankton pi gments. These two methods generally agreed, except in the blue region of th e spectrum at intermediate and shallow depths. Quantum yield (phi, on a dai ly basis) was estimated by non-linear regression from the relationship, P(z )/(a) over bar(ph)(z) = phi(E-PAR)E-PAR(z). Absorption by non-photosyntheti c pigments in most cases causes a decrease in maximum realized quantum yiel d ranging from 30% to a factor of four. Generally, stations, with higher no n-photosynthetic pigment absorption had lower maximum quantum yields but a greater ability to utilize high irradiances. The data suggest the hypothesi s that adaptation, through absorption properties, is the primary determinan t of the magnitude of photosynthetic quantum yield in phytoplankton. (C) 20 00 Elsevier Science Ltd, All rights reserved.