The seasonal cycles of stratification and circulation in the Thermaikos Gulf Region Of Freshwater Influence (ROFI), north-west Aegean

The Thermaikos Gulf is a shallow basin in the north-west Aegean. Communication with the open Aegean is restricted by the long (170 km), narrow (∼50 km) nature of the Gulf and the weak tidal flows. In consequence, the northern section of the Gulf experiences severe water quality problems arising from the untreated sewage from the city of Thessaloniki (population 1.1 million), pollutant discharges from industry around the Gulf, and nutrient input from three rivers, which flow in near the head of the Gulf. New observations over a 16-month period during 1994–95 indicate distinct winter and summer circulation regimes. ter (December–April), strong freshwater input (∼350 m3/s) generates a thin (5 m), low salinity, surface layer which flows southward over much of the Gulf, above relatively homogeneous high-salinity waters that flow to the north. In the low-salinity layer, close to the river deltas, short pulses of extremely low-salinity water occur daily, principally as the result of releases from the Aliakmon hydro-electric power dam. Between October and February, a cold, dense water mass is observed in the deeper water of the eastern Gulf, which appears to originate in the shallow waters of central Saloniki Bay as the result of cooling during northerly gales. During winter it appears that buoyancy forcing from high run-off, in combination with persistent southward wind stress results in wind-enhanced estuarine exchange. mer (July–September), the surface low-salinity layer is not well defined and is confined to the western Gulf in the vicinity of the river sources. Throughout the Gulf, a thicker (10–20 m) mixed layer with low salinity, warm waters overlies a strong pycnocline. A weak barotropic gyre is observed in the Gulf at this time. y estimates of the total freshwater content of the northern Gulf indicate that this layer results in an accumulation of freshwater in the Gulf over the summer, when the local river input is at a minimum. It appears that this freshwater accumulation results from an influx of freshwater from low-salinity surface waters extending across the Northern Aegean at this time as a result of freshwater input from the Dardanelles Strait. A dramatic feature of the summer regime is the occurrence of abrupt changes of the depth of the warm, low-salinity layer or level of the Gulfʹs pycnocline. During these events, which result in relatively strong currents, the pycnocline rises sharply (by up to 10 m), and remains elevated for 3–14 days before dropping more gradually to its previous level. These displacements appear to represent the Gulfʹs adjustment to wind-forced pycnocline slopes across the Northern Aegean.