Article révisé par les pairs
Résumé : Variability of several bio-optical and photosynthetic properties of phytoplankton in the highly productive coast off Concepción, central Chile (∼36°S) was quantified during contrasting seasons at offshore, shelf, and nearshore sites. During October 1998 (upwelling) and July 1999 (non-upwelling), 21 stations were sampled for chlorophyll-a, pigment spectral light absorption, primary production, and attenuation coefficients (kλ) of downwelling irradiance (Ed(λ)); kλ was used to estimate the depth of the euphotic zone (Zeu). Primary production was measured through 14C-fixation experiments using simulated in situ and in situ incubations with water from six depths within the Zeu. The photosynthetic parameters (αB, PBmax) were obtained through photosynthesis-irradiance experiments with samples from two Zeu depths. Bio-optical properties showed large spatial variability during both seasons, including spectral in vivo phytoplankton absorption (aph(λ)), pigment specific phytoplankton absorption (a*(λ)) within the upper 25 m, and water-column transmittance (T(λ)), calculated for the upper 10 m using Ed(λ) at four wavelengths. Under non-upwelling conditions, a*(λ), T(λ), and αB presented higher values whereas a ph(λ) values decreased accordingly. Under upwelling conditions, primary productivity ranged from 0.7 to 7.5 g C m-2 d-1 at the coastal stations and was <0.6 g C m-2 d -1 at the oceanic stations; in the non-upwelling season, values over the entire area ranged from 0.2 to 1.9 g C m-2 d-1. The significant correlations found among bio-optical properties and physiological parameters highlight both the short- and long-term changes in the area's environmental conditions. The combination of three independent biological parameters - αB, PBmax, and a ph(442) - accounted for most of the horizontal and vertical variability among and within stations for each cruise. These results support a distinction of different functional zones for the two contrasting upwelling conditions. © 2004 Elsevier Ltd. All rights reserved.