Atlantic forcing of the Mediterranean oligotrophy

Abstract

The Mediterranean Sea shows a peculiar anomaly in its nutrient pattern compared to the global ocean, as there is decrease in nutrient concentration from west to east. This feature has been attributed to the antiestuarine circulation at the Strait of Gibraltar, where an eastward flow of Atlantic nutrient‐poor surface waters is compensated by a westward countercurrent of Mediterranean nutrient‐rich deep waters. This water exchange has been suggested as the ultimate cause for the oligotrophy of the Mediterranean basin, even though only a few studies have accurately examined the magnitude of the nutrient flux through the Strait of Gibraltar. In this work, data from the Gibraltar Fixed Time series (GIFT) between 2005 and 2008 were used to assess nutrient distributions. Applying a two‐layer model of water mass exchange and using the Mediterranean outflow recorded in situ, the net export of nutrients from the Mediterranean to the Atlantic was calculated as 139 and 4.8 Gmol yr−1 of nitrate and phosphate, respectively. The results also demonstrated that the Atlantic inflow is not nutrient depleted and in particular contains significant levels of phosphate, which is the limiting factor for biological productivity in the eastern Mediterranean. The distribution of the quasi‐conservative parameter N* in the western and eastern basins indicated that nitrate‐deficient surface waters are transformed into phosphate‐deficient bottom waters by internal cycling processes. Therefore, phosphate depletion in the Mediterranean does not have its origin in the entry of a phosphorus‐impoverished Atlantic inflow through the Strait of Gibraltar.

Inter-comparison of the potentially active prokaryotic communities in the halocline sediments of Mediterranean deep-sea hypersaline basins

“…The 30 Gammaproteobacterial OTUs recovered from sediments at the control site were related to known bacterial genera in deep--sea sediments such as Pseudomonas, Pseudolateromonas, Psychrobacter and Aliivibrio. We also detected Nitrosococcus--related OTUs in our Urania control and in one of the sediment samples from under the Discovery upper halocline, indicative of ammonia--oxidizing taxa (Ward & O'Mullan 2002 Table S2) and contrast with the rather uniform and homogeneous overlying oligotrophic water column (Huertas et al 2012). These differences are most likely shaping prokaryotic community structure in the waters of DHABs (Kerhervé et al 1999).…”

Physical forcing and physical/biochemical variability of the Mediterranean Sea: a review of unresolved issues and directions for future research

“…Despite its small section the strait is still responsible for the largest flux of water and elements into the basin. In the last decade observations have become more systematic and data are, therefore, increasing (e.g., Huertas et al, 2012). However, what is crucial is the net flux, which, as mentioned above, is significantly negative.…”

The seasonal cycle of the Atlantic Jet dynamics in the Alboran Sea: direct atmospheric forcing versus Mediterranean thermohaline circulation

“…Water and substance interchanges take place through the narrow Strait of Gibraltar where a typical anti-estuarine circulation is present; a surface inflow of relatively fresh and nutrient poor Atlantic waters and a deep outflow of salty and nutrient-rich Mediterranean waters (Armi and Farmer 1988). Such interchanges are fundamental for the matter budget of the semi-enclosed Mediterranean Sea (Macías et al 2007a;Huertas et al 2012) and also for the hydrodynamics and biogeochemistry of the Alboran Sea itself (e.g., Ruiz et al 2001;Sarhan et al 2000;Macías et al 2008).…”