Seasonal and inter-annual variability of plankton chlorophyll and primary production in the Mediterranean Sea: a modelling approach

Abstract

Abstract. This study presents a model of chlorophyll and primary production in the pelagic Mediterranean Sea. A 3-D-biogeochemical model (OPATM-BFM) was adopted to explore specific system characteristics and quantify dynamics of key biogeochemical variables over a 6 yr period, from 1999 to 2004. We show that, on a basin scale, the Mediterranean Sea is characterised by a high degree of spatial and temporal variability in terms of primary production and chlorophyll concentrations. On a spatial scale, important horizontal and vertical gradients have been observed. According to the simulations over a 6 yr period, the developed model correctly simulated the climatological features of deep chlorophyll maxima and chlorophyll west-east gradients, as well as the seasonal variability in the main offshore regions that were studied. The integrated net primary production highlights north-south gradients that differ from surface net primary production gradients and illustrates the importance of resolving spatial and temporal variations to calculate basin-wide budgets and their variability. According to the model, the western Mediterranean, in particular the Alboran Sea, can be considered mesotrophic, whereas the eastern Mediterranean is oligotrophic. During summer stratified period, notable differences between surface net primary production variability and the corresponding vertically integrated production rates have been identified, suggesting that care must be taken when inferring productivity in such systems from satellite observations alone. Finally, specific simulations that were designed to explore the role of external fluxes and light penetration were performed. The subsequent results show that the effects of atmospheric and terrestrial nutrient loads on the total integrated net primary production account for less than 5 % of the its annual value, whereas an increase of 30 % in the light extinction factor impacts primary production by approximately 10 %.

Spatiotemporal variability of alkalinity in the Mediterranean Sea

“…The physical and biogeochemical results of the present simulation have been extensively presented and validated by Béranger et al (2005) and Lazzari et al (2012), respectively. Here, we focus on the alkalinity time and space variability in the Mediterranean Basin.…”

“…However, in all these publications, alkalinity was derived from a salinity regression, in agreement with Schneider et al (2007). In contrast, in this manuscript, we dynamically simulated alkalinity by introducing a new full state variable into an existing state-of-the-art biogeochemical model of the Mediterranean Sea (Lazzari et al, 2012). We also use the extended model to integrate, in a coherent framework, the available measures of alkalinity within the Mediterranean Sea and along its boundaries with the aim of deriving a spatiotemporal climatology of alkalinity.…”

“…The present run covers the 1998-2004 period; the physical and biogeochemical setups are extensively detailed by Béranger et al (2005) and Lazzari et al (2012) and are briefly reported here. The atmospheric forcings are from the ERA40 reanalysis (Uppala et al, 2005), and from the ECMWF analysis.…”

“…It is capable of resolving the large-scale variability and seasonal cycle of nutrients and primary producers (Lazzari et al, 2012). The transport of dissolved and particulate matter is resolved using the OPATM transport model (Lazzari et al, 2010) offline forced by the dynamical fields that are produced by the MED16 Ocean General Circulation Model (OGCM) (Béranger et al, 2005).…”

“…The dynamics of the biogeochemical properties are described by the BFM model (http://bfm-community.eu), a biogeochemical model that describes the cycles of nitrogen, phosphorus, silica and carbon through biotic (four phytoplankton, two microzooplankton and two mesozooplankton pools, heterotrophic bacteria) and abiotic (detritus, labile, semi-labile and refractory dissolved organic matter, dissolved inorganic compounds) components as a function of temperature and light (Lazzari et al, 2012).…”