Water masses of the Saronikos gulf in winter
Abstract
Source water to Saronikos Gulf is provided from the Aegean Sea, and water of appropriate characteristics (temperature 14.75°C, salinity 38.8‰) was always found at 200 m depth in the Outer Gulf. In December, above the Aegean water, an intrusive layer with a small temperature maximum (> 16°C at 100 m) was observed but was not present in January when the upper layer water (~ 80 m) graded regularly into Aegean water. Waters inside the Gulf were not directly affected by water below approximately 100 m depth from outside.
During winter at least, the upper 50 m of the water column is essentially isothermal and isohaline. Under such homogeneous conditions, energy inputs primarily from the wind readily excite vertical convection, as evidenced by the frequent neutral or unstable surface density gradients. The depth of the convective layer had deepened by January, as the deeper (70 to 90 m) water columns of the Gulf which were stratified below 50 m in the December were mixed to the bottom.
In December, horizontal ranges were 15.5 to 17.5°C and 38.0 to 38.5‰, with the exception of the relatively isolated Elefsis Bay which was always colder by 3°C and denser by 0.5 σt units. In January the horizontal salinity range had narrowed to between 38.1 and 38.3‰, no net change in salt content had occurred, and the temperature range had narrowed to between 13.6 and 14.2°C, a cooling of 2.5°C.
Five water masses in the upper layer based on subtle but distinct differences in T-S characteristics were distinguished:
Outer Gulf water provides the source water to the upper layer and was the least saline in both December and January. In December it was also the coolest (with the exception of Elefsis water) but in January Inner Gulf water was slightly cooler.
Inner Gulf water occupied the head of Saronikos Gulf, and was, therefore, both connected with Elefsis Bay via Keratsini Bay, the major port of Athens, and was the recipient of pollutants from the sewage and power plant cooling water outfalls. In December, Inner Gulf water was an approximately 50 to 50 mixture of Outer Gulf and Central waters. In January, upwelling in the area made Inner Gulf water slightly more saline and slightly (0.1 σt unit) denser.
Central water, the warmest water in Saronikos Gulf, occupied the area between Salamis and Aigina. These islands affect separation of the Gulf into an eastern and western portion and Central water probably extended throughout the western portion to the west of Salamis though no observations were made there. This water is residual from the previous summer. It was the most saline in December, but Inner Gulf water was slightly more saline in January.
Elefsis Bay water was much colder than the other water masses, being 14.3°C in December and 10.7°C in January, while the salinity was reduced from 38.28‰ to 38.14‰.
Western basin water, above the 80 m sill depth between Aigina and the Methanon Peninsula, was essentially the same as Outer Gulf water. Deeper than 80 m temperature and salinities approach unique values of 14°C and 38.6‰, demonstrating the isolation of this deep basin.
Circulation in the Saronikos Gulf was interpreted from distribution of PO4 and NH4, both added at point sources in Keratsini, and the T-S characteristics. The western portion of the Gulf appeared to be less active than the eastern portion. In December circulation in the eastern portion was clockwise, with Outer Gulf water entering toward the western (Aigina) side, mixing with Central water to form the water of the Inner Gulf, and outflow was along the Attikis Peninsula. In January, circulation was counterclockwise with Outer Gulf water entering along Attikis. There was evidence for some upwelling in the northern part of Saronikos proper and towards Salamis. The Inner Gulf water was exciting around the east side of Aigina together with but underneath Central water. The winds observed during both cruises were northerly with only small differences in average directions and speeds, and the cause for the basically different circulation modes awaits further research.
Elefsis Bay contained enormously high concentrations of PO4, and NH4, and was slightly undersaturated in O2 (90 to 100 percent) in December and slightly supersaturated (100 to 110 percent) in January. There was a net circulation from west to east judging from a mean transport of 240 m3/sec estimated from the longitudinal gradients and curvatures of temperatures in Keratsini and Elefsis. The transport would provide an approximate renewal time for Elefsis of two months, and was sufficient to reduce the nutrient concentrations by one-half over the 41 day period between cruises.
A heat budget calculation for Elefsis gave a mean atmospheric heat loss over the 41 days of about 180 cal/cm2/day. The atmospheric heat loss of Outer Gulf water was about 250 cal/cm2/day, but that from Inner Gulf and Central waters was significantly greater. The difference was removed from the Gulf by advection, with Outer Gulf water serving as a heat sink. Calculations suggested the advected heat loss could have been accomplished by an exchange transport of 2500 m3/sec between the Outer Gulf water and either the Central or Inner Gulf water.
The observed salinity reduction in Elefsis showed an admixture of freshwater equivalent to 2 cm of direct rainfall. The discrepancy between this and the actual rainfall of 6 cm suggests the importance of evaporation.
