Measured and modeled primary production in the northeast Atlantic (EUMELI JGOFS program): the impact of natural variations in photosynthetic parameters on model predictive skill

Use of ocean color satellite data in global biogeochemical studies requires models to predict primary production from the satellite-derived chlorophyll fields. In this paper, measured bio-optical and photo-physiological data are used in place of standard (constant) parameters to adjust a previously published primary production model. In the JGOFS-France program, systematic studies were carried out at three locations in the tropical northeast Atlantic, selected to represent typical EUtrophic, MEsotrophic and oLIgotrophic regimes (EUMELI cruises). During cruise no. 4, these studies included the spectral measurements of the photosynthetically available radiation at sea level and within the water column, the determination of the algal absorption spectra and the determination of the physiological parameters derivable from P versus E experiments (photosynthesis-irradiance responses). The model predictions are compared with in situ determinations made by the 14C technique (JGOFS core parameter). At the three sites, the physical structure (mixed layer and euphotic depths), the algal abundance and community structure, as well as their bio-optical and physiological properties, are very different, so that the predictive performance of the model was tested in trophic conditions that span most of those expected in the global open ocean. The model, when adjusted by entering the actual physiological parameters (chlorophyll-specific absorption of algae, maximum quantum yield, and light saturated carbon fixation rate), provides satisfying results compared to those observed in situ. The relative roles of the physiological parameters are analyzed and sensitivity studies are performed. For global applications, and in the absence of specific information when all seasons and provinces of the world ocean are considered, it will remain necessary for a while to rely on generic models and a selected standard set of physiological properties. The sensitivity studies here presented help in this choice, and a modified set of parameters is proposed and tested. With this set, reconstructed production profiles are close to those determined in the field, and the integrated values are retrieved with no bias and a reduced scatter (18\% at one SD) for 17 stations (cruises 3 and 4) and daily production ranging from 0.3 to 2.3 gC m−2.