Abstract
Abstract. This paper provides an extensive vertical and longitudinal description of the biogeochemistry along an East-West transect of 3000 km across the Mediterranean Sea during summer 2008 (BOUM cruise). During this period of strong stratification, the distribution of nutrients, particulate and dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) were examined to produce a detailed spatial and vertically extended description of the elemental stoichiometry of the Mediterranean Sea. Surface waters were depleted in nutrients and the thickness of this depleted layer increased towards the East from about 10 m in the Gulf of Lion to more than 100 m in the Levantine basin, with the phosphacline deepening to a greater extent than that for corresponding nitracline and thermocline depths. We used the minimum oxygen concentration through the water column in combination with 2 fixed concentrations of dissolved oxygen to distinguish an intermediate layer (Mineralization Layer; ML) from surface (Biogenic Layer; BL), and deep layers (DL). Whilst each layer was represented by different water masses, this approach allowed us to propose a schematic box-plot representation of the biogeochemical functioning of the two Mediterranean basins. Despite the increasing oligotrophic nature and the degree of P-depletion along the West to East gradient strong similarities were encountered between eastern and western ecosystems. Within the BL, the C:N:P ratios in all pools largely exceeded the Redfield ratios, but surprisingly, the nitrate vs. phosphate ratios in the ML and DL tended towards the canonical Redfield values in both basins. A change in particulate matter composition has been identified by a C increase relative to N and P along the whole water column in the western basin and between BL and ML in the eastern one. Our data showed a noticeable stability of the DOC:DON ratio (12–13) throughout the Mediterranean Sea. This is in good agreement with a P-limitation of microbial activities but in contradiction of the accepted concept that N is recycled faster than C. The western and eastern basins had similar or close biological functioning. Differences come from variability in the allochtonous nutrient sources in terms of quantity and quality, and to the specific hydrodynamic features of the Mediterranean basins.
Highlights
The carbon (C), nitrogen (N), and phosphorus (P) elemental composition of mineral and organic matter in marine environments is a key factor for relating carbon and nutrient cycles
The Mediterranean Sea is divided in two main basins, the western and eastern basins which are separated by the Sicilian strait (Fig. 1a)
The deep waters temperature ranged from 12.9 to 13.0 ◦C in the western basin and from 13.4 to 13.6 ◦C in the eastern basin, with the exception of the extreme East of the Levantine part, where deep temperature approached 13.8 ◦C (Fig. 1b). These relatively warm deep waters are attributed to the memory of the recent change in deep water formation of the Eastern basin named the Eastern Mediterranean Transient, EMT (Robinson et al, 2001)
Summary
The carbon (C), nitrogen (N), and phosphorus (P) elemental composition of mineral and organic matter in marine environments is a key factor for relating carbon and nutrient cycles. The nutrient ratios in the sea have been recurrently used as water mass tracers and as effective markers of biotic and of the biogeochemical processes taking place in the ocean interior (Redfield et al, 1963; Sterner et al, 2008). Measurements of C:N:P ratios and anomalies relative to the Redfield values are generally useful to describe the functioning of complex food webs and to examine the processes that regulate marine biogeochemistry. These anomalies have been successfully used to trace temporal changes occurring in the global ocean on a wide range of scales These anomalies have been successfully used to trace temporal changes occurring in the global ocean on a wide range of scales (e.g. Falkowski, 1997; Pahlow and Riebesell, 2000; Bethoux et al, 2002; Sarmiento and Gruber, 2006), including estimation of the global distribution of anthropogenic carbon (Gruber and Sarmiento, 1997), rate of nitrogen fixation (Deutsch et al, 2007), denitrification (Tyrrell and Lucas, 2002), and ocean mixing (Schroeder et al, 2010)
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