* Corresponding author. Tel.: #1-202-358-0310; fax: #1-202-358-2770. E-mail address: jmarra@mail.hq.nasa.gov (J. Marra). Deep-Sea Research II 47 (2000) 1279}1299 Pigment absorption and quantum yields in the Arabian Sea John Marra*, Charles C. Trees, R.R. Bidigare, R.T. Barber NASA Headquarters, Oceanography Program Ozce of Earth Science/Code YS, Room 5P33, 300 E Street SW, Washington, D.C. 20546, USA Center for Hydro-Optics and Remote Sensing, San Diego State University San Diego, CA 92120, USA Department of Oceanography, University of Hawaii, Honolulu, HI 96822, USA Duke University Marine Laboratory, Beaufort, NC 28516, USA Received 1 September 1998; received in revised form 4 May 1999; accepted 20 May 1999 Abstract 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: C uptake (P), total daily (photosynthetically active) irradiance (E  ), spectral irradiance, spectral absorption by phytoplankton, and a variety of HPLC-determined phytoplankton pigments. Phytoplan- kton absorption (a  ) was optically weighted to the spectral quality of submarine irradiance. Absorption spectra based on particulates collected on "lters 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 (, on a daily basis) was estimated by non-linear regression from the relationship, P(z)/a   (z)"(E  )E  (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.  2000 Elsevier Science Ltd. All rights reserved. 0967-0645/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 9 ) 0 0 1 4 4 - 7