Poseidon E1-M3A station

Location of Poseidon E1-M3A station
Location of Poseidon E1-M3A station

Why is it interesting for scientists to observe the ocean here?

  • Characteristics of the oceanic region:

The Mediterranean basin displays a great variety of climatic, physical, ecological, social, economic and cultural traits. In spite of its apparent diversity the Mediterranean region has been recognised for a long time as a single functional system.

Mediterranean is a semi-enclosed sea, which represents only 0.69% of the global ocean surface and 0.27% of the global ocean volume with several deep basins and a large number of vast and relatively shallow bays. Some of the largest rivers of Europe and Africa discharge nutrient and sediment-rich waters into the Mediterranean basin (for example the Nile (Egypt) or the Rhone (France)).

Illustration : Rhone plume.jpg
Illustration : Rhone plume.jpg
Credits : NASA, Visible Earth (http://visibleearth.nasa.gov/ )

Early studies of the Mediterranean system concluded that the basin is oligotrophic (nutrient-poor) but it was later shown that there is an eastward decreasing trend in primary productivity (production of organic matter from atmospheric or oceanic carbon dioxide trough photosynthesis) . This phenomenon is linked to the surface circulation of Atlantic Water (low in salinity and nutrients) going along the north coast of Africa and reaching the central Levantine basin. During winter the cold winds cool down the surface waters which then become colder and denser and sink to 300 meters forming the Levantine Intermediate Water (water saltier and richer in dissolved nutrients). The Levantine Intermediate Water follows a parallel course with Atlantic Water but in the opposite direction, and eventually outflows into the Atlantic contributing to the impoverishment of the basin.

BOUM Project
The schematic of the thermohaline circulation in the Mediterranean Sea with the major conveyor belt systems indicated by dashed lines with different colour. The yellow indicates the AW stream which is the surface manifestation of the zonal conveyor belt of the Mediterranean. The red indicates the mid-depth LIW recirculation branch of the zonal thermohaline circulation. The blue lines indicate the meridional cells induced by deep waters. LIW branching from the zonal conveyor belt connects meridional and zonal conveyor belts. (Pinardi & Masetti, 2000).
Credits : BOUM Project (http://www.obs-vlfr.fr/proof/boum/)

  • Particular characteristics of the site:

The Cretan Sea is the largest and deepest basin (2500 m) in the south Aegean Sea with an average depth of 1000 meters and two deeper troughs in the eastern part (2561m and 2295 m).

cretansea
Illustration : cretansea.jpg
Credits : NASA, Visible Earth ( http://visibleearth.nasa.gov/ )

The Cretan Sea is linked to the Levantine basin and the Ionian Sea through the eastern and western straits of the Cretan Arc via sills that are no deeper than 700 meters. Outside the straits the seabed plunges towards the deep basins of the Hellenic Trench (depth ~ 3000 - 4000 m). The north of the Cretan sea is bounded by the Cyclades Plateau at a depth of 600m.

The hydrological structure of the Cretan Sea is dominated by multiple scale circulation patterns and is an area of deep-water formation. It acts as a reservoir for heat and salt for the Eastern Mediterranean. The circulation in the Cretan Sea is dictated by the combined effect of two gyral features, an anticyclonic eddy (a circular swirling of water ) in the west and a cyclonic eddy in the east.

current
Image : Current.jpg

During the spring, summer and autumn months the Cretan Sea surface waters are stratified, which limits the physical mixing between the surface waters and the deeper, nutrient-rich layers. The result is an oligotrophic (nutrient-poor) surface environment, characterized by very small phytoplankton cells, and small heterotrophs, like bacteria, which feed on the scarce organic matter (VIDEO on how climate and stratification can impact ocean productivity). This oligotrophic surface ecosystem is not a good source of organic matter particles on which deep ecosystems rely on for food and this has a negative effect on the energy transfer to the benthos (all sea floor organisms ; VIDEO A Year in the life of the deep sea (in one minute!)). The already low flux of organic matter to the sea floor is even further depleted by the high temperatures (>14 °C) and high oxygen concentrations (>4 ml l-1) of the Cretan Sea deep waters, that act as a filter and enhance the decomposition rates of the nutritious particles sinking out of the oligotrophic surface waters.


How climate and stratification can impact ocean productivity by Dr. Jeffrey Krause, Oregon State University (USA)


A Year in the life of the deep sea (in one minute!) With the courtesy of the NOCS and the DEEPSEAS Group (http://www.noc.soton.ac.uk/obe/PROJECTS/DEEPSEAS/index.html)

Scientists or technicians in charge of the observatory

6
Dr. Kostas Nittis
7
Dr. George Petihakis

Name of their institution

Hellenic Centre for Marine Research, Athens, Greece

Distance from shore

The station is 24 nautical miles from the North coast of Crete (~ 2h30 steaming depending on the ship speed).

Max depth

1440 m