Phytoplankton absorption, photosynthetic parameters, and primary production off Baja California: summer and autumn 1998

To estimate ocean primary production at large space and time scales, it is necessary to use models combined with ocean-color satellite data. Detailed estimates of primary production are typically done at only a few representative stations. To get survey-scale estimates of primary production, one must introduce routinely measured Chlorophyll-a (Chl-a) into models. For best precision, models should be based on accurate parameterizations developed from optical and photosynthesis data collected in the region of interest. To develop regional model parameterizations 14C-bicarbonate was used to estimate in situ primary production and photosynthetic parameters ( α * , P m * , and Ek) derived from photosynthesis–irradiance (P–E) experiments from IMECOCAL cruises to the southern California Current during July and October 1998. The P–E experiments were done for samples collected from the 50% surface light depth for which we also determined particle and phytoplankton absorption coefficients (ap, aφ, and a φ * ) . al data collected during both surveys indicated that the 1997–1998 El Niño was abating during the summer of 1998, with a subsequent transition to the typical California Current circulation and coastal upwelling conditions. Phytoplankton chl-a and in situ primary production were elevated at coastal stations for both surveys, with the highest values during summer. Phytoplankton specific absorption coefficients in the blue peak ( a φ * (440)) ranged from 0.02 to 0.11 m2 (mg Chl-a)−1 with largest values in offshore surface waters. In general a φ * was lower at depth compared to the surface. P–E samples were collected at the 50% light level that was usually in the surface mixed layer. Using α * and spectral absorption, we estimated maximum photosynthetic quantum yields (φmax; mol C/mol quanta). φmax values were lowest in offshore surface waters, with a total range of 0.01–0.07. Mean values of φmax for July and October were 0.011 and 0.022, respectively. In July P m * was approximately double and α * was about 1.4 times the values for October. Since the P–E samples were generally within the upper mixed layer, these tendencies in the photosynthetic parameters are attributed to deeper mixing of this layer during October when the mean mixed layer for the photosynthesis stations was 35 m compared to a mean of 10 m in July. Application of a semi-analytical model using mean values of P–E parameters determined at the 50% light depth provided good agreement with 14C in situ estimates at the discrete 50% light depth and for the water-column integrated primary production.