SPECTRAL ABSORPTION-COEFFICIENT OF PHOTOSYNTHETICALLY ACTIVE PIGMENTSIN THE EQUATORIAL PACIFIC-OCEAN (165-DEGREES-E-150-DEGREES-W)

Spectral absorption coefficients of total particulate material and det ritus were measured throughout the euphotic zone along the equator bet ween 165 degrees E and 150 degrees W and during time-series for each o f these two longitudes in October 1994 (JGOFS-FLUPAC cruise). The sum of pigments obtained by spectrofluorometry (tChla = DV - chla + Chla) was used for normalization (and was also compared to fluorometric and HPLC measurements as an intercalibration study). In order to assess th e specific absorption coefficient of photosynthetically active pigment s (alpha(ps)) from the pigment-specific absorption coefficient for ph ytoplankton (alpha(ph)), we made a multiple regression analysis of me asured phytoplankton absorption spectra onto published in vivo spectra of pure pigments. This made it possible to calculate the concentratio ns of photoprotective carotenoids (tPPC) when HPLC measurements were n ot available and thus to subtract their contribution to absorption fro m the total phytoplanktonic absorption coefficient (a(ph)). Methodolog ical uncertainties in both coefficients used for calculating absorptio n coefficients and in pigment measurements are discussed. Pigments and absorption measurements made during the cruise enabled us to describe two typical trophic regimes in the equatorial Pacific ocean: oligotro phic waters of the ''warm pool'' west of 170 degrees W and high-nutrie nt, low-chlorophyll waters (HNLC) of the upwelling east of 170 degrees W. The vertical decreasing gradient of a(ph) from the surface to the deep chlorophyll maximum (DCM) was due to a high tPPC/tChla ratio at the surface and was higher in the oligotrophic (0.14-0.065 m(2)mg (tCh l a)(-1) biomass dominated by Prochlorococcus, rich in zeaxanthin) tha n in the mesotrophic area (0.07-0.06 m(2) mg (tChl a)(-1) biomass domi nated by picoeucaryotes). Below the DCM, a(ph) reached a similar mini mum value in both oligotrophic and mesotrophic areas. a(ps) varied le ss than a(ph) from the surface layer to the DCM in both oligotrophic and mesotrophic areas. The difference in a(ph) and a(ps)* from west t o east of the transect could be interpreted as a shift in the phytopla nkton composition, with a dominance of procaryotes in the west and a d ominance of eucaryotes in the upwelling area. Higher a(ps) in well-li t typical oligotrophic waters indicated that phytoplankton communities dominated by Prochlorococcus might be more efficient for capturing li ght usable for photosynthesis than those present in the HNLC situation . (C) 1998 Elsevier Science Ltd. All rights reserved.