Seasonal variability of hydrographical properties of the Syrian marine water

The hydrographical properties of the Syrian marine water are described on the basis of three cruises performed during December 2006, March 2009 and October 2009. In all cruises a Conductivity-Temperature-Depth (CTD) instrument equipped with a fluorometer and oxygen sensor was used for casts that extended to a maximum depth of 480 m.The hydrographic data reveal the presence of Levantine Surface Water (LSW) and Atlantic Water (AW) within the upper 90 m layer, Levantine Intermediate Water (LIW) between 90 and 250 m, and Deep Water (DW) further below. Stratification was clearer in October and December compared to March. Cyclonic and anticyclonic eddies were observed during the three cruises at different locations situated along the Syrian coast.The flow structure along the Syrian coast is controlled by the shape of the coastline and the bottom topography of the continental shelf. From March 2009 to October 2009 a dynamic height rise (within 6 months) of about 3.7–9.8 cm reflected the seasonal cycle of sea level due mainly to thermosteric expansion of the water column. This gave a rise rate in the range of 0.6–1.7 cm month− 1.Dissolved oxygen was higher in March 2009 (214 ± 4.8 μM) than in December 2006 (202 ± 11.5 μM) or in October 2009 (188 ± 18.9 μM). During March 2009 the water column oxygen distribution was homogeneous. In December 2006 the oxygen distribution was homogeneous in the upper 125 m where LSW was present and subsequently decreased in concentration due to bacterial oxidation of detritus. However, a shallow oxygen maximum (oversaturated) was present at 50–80 m depth during October 2009. Oversaturation was attributed mainly to the biological and physical processes of rapid capping and trapping of oxygen in the AW mass.Chlorophyll-a concentration varied substantially depending on depth and season, having values of 0.05 ± 0.01 mg m− 3 during December 2006, 0.08 ± 0.01 mg m− 3 during March 2009 and 0.06 ± 0.01 mg m− 3 during October 2009. The deep Chl-a maxima, of 0.08 mg m− 3 during December 2006, 0.1 mg m− 3 during March 2009 and 0.12 mg m− 3 during October 2009, were observed at 50–100 m. These Chl-a maxima were associated with cyclonic and anticyclonic eddies at 40–75 m and at 80–105 m, during December 2006 and March 2009, respectively, and along all the stations sampled in October 2009. Comparisons of Chl-a estimates during December 2006 with the Sea-viewing Wide Field of view Sensor (SeaWiFS) and in situ measurements revealed a significant correlation of r2 = 0.82 and showed that the study region is the most oligotrophic in the Mediterranean sea.

Research Highlights
► We examine the hydrographical properties of the Syrian marine water which reveal the presence of Levantine Surface Water and Atlantic Water within the upper 90 m layer, Levantine Intermediate Water between 90 and 250 m, and Deep Water further below.
► The flow structure along the Syrian coast is controlled by the shape of the coastline and the bottom topography of the continental shelf.
► We find that the Syrian margin is the most oligotrophic region in the Mediterranean sea.