Imants G. Priede
University of Aberdeen
Oceanlab, Newburgh
Aberdeen. AB41 6AA
Scotland UK
i.g.priede@abdn.ac.uk

Extended Abstract
The Eastern Mediterranean features some deep basins very close to the coast so that in some areas such as south of the Peleponnese, Crete and Rhodes the 4000m depth contour is less than 25 km offshore. Whilst similar steep slopes from coast to the abyss are known in the vicinity of oceanic islands the close juxtaposition of a populated coastline and a deep enclosed basin is unusual. This suggests that these basins may be peculiarly vulnerable to anthropogenic influences.

The basins of the eastern Mediterranean are also unusual because of the prevailing physical conditions with a temperature of over 140C in the abyss compared with <40C in most of the world’s oceans. The marine fauna of the Mediterranean is thought to have originated largely from the Atlantic Ocean within the last 5 million years following the Pleiocene normalisation event and ingress of waters from the west. However the deep basins did not form until about 2 million years ago and seem throughout most of their history to have been stagnant, only becoming oxygenated and hence colonisable within the last 6000 years. These deep basins are therefore populated by very recent, presumably opportunistic, immigrants.

Autonomous lander vehicles provide a new means of investigating this fauna. The lander descends to the sea floor by free fall and then collects data autonomously under control of an on board computer. In the meantime the research ship can move to other operations and indeed it is possible to operate up to a dozen landers simultaneously from a single ship if it is big enough. The ship returns after a period of 24h to one year depending on duration of the subsea mission, sends an acoustic command whereupon ballast is shed and the lander rises to the sea surface. The vehicle is retrieved and data are downloaded. Work at depths down to 4500m have been achieved using the small IMBC RV Philia which otherwise owing to the short length of cable can only work to maximum depth of a few hundred metres. The lander vehicles can also be supplemented by free-fall baited traps to capture animals at depths.

Jones (1999) and Jones et al (2003) deployed the Aberdeen University Deep Ocean Submersible (AUDOS) Priede & Bagley (2000) equipped with a camera and bait placed on the sea floor to attract active scavenging fauna at depths from 1500m to 4260 in the Cretan Sea, Ierapetra basin and Rhodos basin. On the slopes down to a depth of 2500m several species of shark were present, the six-gilled shark, Hexanchus griseus that can grow to a length of 5m and the smaller Blackmouth catshark, Galeus melastomus and velvet belly Etmopterus spinax. These fish are all known in deep waters from the Atlantic Ocean where H.griseus is described as living usually in cooler waters from 100 to 2000m. In the Mediterranean these sharks are living in very warm waters which would double their metabolism and hence food requirement compared with the Atlantic Ocean. Our observations indicate that the H.griseus in the eastern Mediterranean is a voracious feeder. It is thought not to be aggressive to man (Boeseman, 1989) and probably cannot tolerate high surface temperatures prevailing in the Mediterranean.

Whereas in the Atlantic Ocean there is a diverse fauna of teleost (bony) fishes living at depth including eels, macrourids and ophidiids in the Eastern Mediterranean, at depths greater than 2500m there is only one species of fish attracted to bait, the macrourid Chalinura mediterranea. At 4264m in the Ierapetra basin Jones et al. (2003) observed up to 49 individuals swarming over bait placed on the sea floor.

Similarly amongst the crustaceans, the crab, Geryon longipes occurs down to 2000m depth, and Chaceon mediterraneus down to 3800m. The shrimp Acanthephyra eximia however is abundant at all depths and is the dominant scavenger together with Chalinura mediterranea in the bottom of the deep basins. These two species are not abundant in the Atlantic but through tolerance of high temperature, unusual for deep sea animals, they have successfully colonised the deep Mediterranean. Interestingly Acanthephyra has been observed in warm water around hydrothermal vents on the Mid-Atlantic ridge; possibly this habit has pre-adapted the species for warm deep sea conditions in the Mediterranean.

The ecosystem of the abyssal floor of the Eastern Mediterranean basins is therefore very simple with one mobile crustacean, the shrimp A. eximia , one fish C. mediterranea and within a sediment one species of polychaete worm Myriochele fragilis (Fiege et al. 2000). Remarkably this species of worm is most abundant in the Arctic Ocean.

The overlying waters of the Rhodos basin for example are amongst the most oligotrophic in the world (Psarra et al 2000), implying that the food supply to the deep sea is very low. However the animals there live at a temperature 100C higher than would be considered normal for deep sea fauna and therefore metabolism is likely to be approximately double that of a typical oceanic abyssal situation. The question arises as to whether these animals have adapted to warm conditions by down regulating their metabolism. Using a new lander respirometer vehicle Bailey et al. (submitted) have measured the metabolic rates of A. eximia at 4200m depth in the Rhodos basin. They discovered that this species shows no metabolic compensation but that the metabolic rate and swimming speeds are high exactly as predicted by the Arhennius equation.

It is concluded therefore that despite the simple ecosystem in the Eastern Mediterranean basins, there is life even at the greatest depths and per unit of biomass the metabolic rate is high, capable of rapidly processing organic matter reaching the deep sea floor. Carrion falls are rapidly intercepted and consumed within hours. The deep basins, if protected from toxins and inorganic pollution, are capable of playing a major role in the carbon cycle of the eastern Mediterranean. Despite low diversity, aided by high temperature and oxygenated bottom water, these areas are remarkably active. These basins are dynamic, conditions are changeable on annual, decadal and millennial time scales. Long-term times series studies are necessary to understand their role in sustainability of the Eastern Mediterranean.