Abstract
Numerical modelling is a key tool to complement the current physical and biogeochemical observational datasets. It is essential for understanding the role of oceanographic processes on marine food webs and producing climate change projections of variables affecting key ecosystem functions. In this work we evaluate the horizontal and vertical patterns of four state-of-the-art coupled physical-biogeochemical models, three of them already published. Two of the models include data assimilation, physical and/or biological, and two do not. Simulations are compared to the most exhaustive dataset of in situ observations in the North Western Mediterranean, built ad hoc for this work, comprising gliders, conventional CTD surveys and complemented with satellite observations. The analyses are performed both in the whole domain and in four sub-regions (Catalan Shelf, Ebro Delta, Mallorca Channel and Ibiza Channel), characterized by a priori divergent primary production dynamics and driving mechanisms. Overall, existing models offer a reasonable representation of physical processes including stratification, surface temperature and surface currents, but it is shown that relatively small differences among them can lead to large differences in the response of biogeochemical variables. Our results show that all models are able to reproduce the main seasonal patterns of primary production both at the upper layer and at the Deep Chlorophyll Maximum (DCM), as well as the differential behaviour of the four sub-regions. However, there are significant discrepancies in the local variability of the intensity of the winter mixing, phytoplankton blooms or the intensity and depth of the DCM. All model runs show markedly contrasting patterns of interannual phytoplankton biomass in all four sub-regions. This lack of robustness should dissuade end-users from using them to fill gaps in time series observations without assessing their appropriateness. Finally, we discuss the usability of these models for different applications in marine ecology, including fisheries oceanography.
Highlights
Understanding ecological processes, biogeochemical fluxes, and potential changes in marine ecosystems requires a profound knowledge of the dynamics of primary production
The outputs from the NEMOBFM system (1/16◦ horizontal resolution, 72 vertical levels) (Teruzzi et al, 2016) and the NEMO-PISCES (1/12◦ horizontal resolution, 75 vertical levels) (Sotillo et al, 2015) system were obtained from the Copernicus Marine Environmental Monitoring Service (CMEMS) data center and cover the period 2000–2016
GETMMedERGOM (1/12◦ horizontal resolution, 25 vertical levels) is the oceanic component of the Marine Modelling Framework developed at the Joint Research Centre of the EU Commission (Stips et al, 2015)
Summary
Understanding ecological processes, biogeochemical fluxes, and potential changes in marine ecosystems requires a profound knowledge of the dynamics of primary production. The “Mediterranean paradox” has emerged from the unexpectedly high fishery yield in this basin, compared to levels expected only based on the chlorophyll-a and nutrient levels (Sournia, 1975; Estrada, 1996). This paradox was reinforced by ocean color images from remote sensing. Other works related this paradox to the dominance of small phytoplankton and the channeling of an important proportion of the carbon fluxes through the microbial food web, which may contribute to efficient energy transfer to the upper trophic levels (Siokou-Frangou et al, 2010)
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