Photosynthesis and light regime in the Azores Front region during summer: Are light-saturated computations of primary production sufficient?

Abstract

Bio-optical variables for primary production (PP) computations were determined at the Azores Front (AF) (30–38°N to 20–23°W) between 30 July and 21 August 1998. The area was characterized by the presence of a deep chlorophyll maximum (DCM) at ∼100 m and by high mesoscale variability in surface waters caused by the meandering eastern flow of the Azores Current associated with the AF. The vertical distribution of the photosynthetic parameters was typical of oligotrophic waters with high values of light-limited slope (αB) and maximum quantum yield (φm) at the DCM and high values of the chlorophyll-specific maximum photosynthetic rates (PmB) at the surface. Spatial variability in photosynthetic parameters at the surface was high and comparable to that over larger spatial scales. Photosynthetic parameters at the surface associated with contrasting hydrographic structures were significantly different but at the DCM no significant differences were apparent. Although comparisons between the light absorbed by phytoplankton (EzPUR) and the corresponding spectral light saturation parameters (EkPUR) indicted that photosynthesis was light-limited during sunrise and sunset, integrated PP could be estimated using a simple light-saturated model (DLS model) that considers the day length equivalent for saturating irradiance of photosynthesis and the vertical distribution of PmB and chlorophyll concentration. This means that PP computations for modeling could be greatly simplified by ignoring the spectral resolution and suggest that efforts in further research should be directed to characterize time and spatial variations of PmB in these oligotrophic waters.