Bio-optical properties of the subtropical North Atlantic. II. Relevance tomodels of primary production

Based on measurements of phytoplankton biomass, absorption coefficients and photosynthetic performance obtained from samples collected in the subtropi cal Atlantic Ocean, standard sets of bio-optical parameters were selected a nd applied to a spectral model of primary production. Computed profiles of instantaneous production, integrated over the day, and spectral irradiance were compared with measurements of 12 h in situ production and in situ spec tral irradiance to assess the model's performance. Although the model perfo rmed well for the stations located on the western side of the transect, at the eastern side of the basin the model overestimated both primary producti on and irradiance below the mixed layer. The modelled irradiance profile wa s then reconciled with the observations of irradiance by increasing the ass umed contribution to absorption by yellow substances. When the production m odel was re-implemented with the new irradiance values below the mixed laye r, the computed and measured production profiles of all the stations were i n good agreement. In the spectral model used in this study, the shape of th e absorption spectrum of phytoplankton was used as a proxy for the shape of the photosynthetic action spectrum. Because non-photosynthetic pigments (N PPs) were abundant in pigment samples collected in the study region, the in fluence of NPPs on the shape of the absorption spectrum was examined and it s effect on computations of primary production was quantified. When total a (ph)(z, lambda) and photosynthetic a(ps)(z, lambda) phytoplankton absorptio n at wavelength h and depth z were used to quantify the error resulting fro m failure to correct for the influence of NPPs on the shape of the absorpti on spectra, the results showed small errors in the computation of productio n at depth (up to 20 %) and integrated, water-column primary production (a maximum of 10 %).