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An introduction to Mediterranean deep-sea biology* - page 15 / 32

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pelagic-benthic coupling, organic carbon minerali- sation rates and environmental chemistry of deep- sea sediments (Tahey et al., 1994; Danovaro et al., 1999; Duineveld et al., 2000; Guidi-Guilvard, 2002; Giordani et al., 2002). Cartes et al. (2001) gave valuable quantitative information regarding the community structure of the bathyal suprabenthos in the Catalan Sea, which showed relatively high diversity at mid-bathyal depths. Nevertheless, the deep Mediterranean holds interesting surprises. Although it is generally considered to be a “biolog- ical desert”, times do come when certain areas dis- play such high benthic activity as to be characterised as “benthic hotspots” (Boetius et al., 1996; Fiege et al., 2000; Danovaro et al., 2001; Hausmann et al., 2002; Lykousis et al., 2002; Tselepides and Lam- padariou, 2004).

It is now well established that the basic factors responsible for the general impoverishment observed in the Mediterranean are: (i) its biogeo- graphic and geological history, (ii) the high prevail- ing temperatures (13-14ºC) below 200 m depth, and (iii) the scarcity of food supply. Indeed studies of the eastern Mediterranean bathyal and abyssal fauna have revealed significant correlations between fau- nal (both meio- and macrofaunal) abundances and biomass, and factors indicating food availability, all being at very low levels compared to temperate regions (Pérès, 1982; Sarà, 1985; Soyer, 1985; Thiel, 1983; De Bovée et al., 1990; Tselepides, 1992; Tselepides and Eleftheriou, 1992; Danovaro et al., 1999, 2000; Tselepides et al., 2000; Tselepi- des and Lampadariou, 2004).

Meiofauna

Compared to other areas of the world, the Mediterranean Sea, and in particular the eastern basin, is considered to be one of the most olig- otrophic areas in the world (Berman et al., 1984; Azov, 1986; Psarra et al., 2000). Several authors have related the oligotrophy of the eastern Mediter- ranean Sea to different characteristics, such as nutri- ent depletion and low productivity towards the east (Salihoglu et al., 1990), the westward transport and export of organic matter by the underlying deep Mediterranean compensation current (Dugdale and Wilkerson, 1988), and strong grazing of phyto- plankton by the high zooplankton standing stock in the upper layers (Weikert and Koppelmann, 1993). The continental slope and bathyal sediments of the eastern Mediterranean have been considered as a

separate subsystem (Tselepides et al., 2000a) which is characterised by: a) negligible inputs of primary organic matter, b) organic matter composed mostly of refractory compounds, and c) extremely low con- centrations of potentially limiting organic nutrients (e.g. proteins and lipids) that sharply decline with increasing distance from the coast and sediment depth.

Danovaro et al. (1995, 2000) found that in the deep eastern Mediterranean, the horizontal and ver- tical distribution of meiofauna is controlled by food quality (expressed as the percentage of labile versus total sedimentary organic matter), whereas other studies contend that the distribution of meiofaunal communities is directly related to the amount of chloroplastic pigment equivalent (CPE), which is an indicator of the presence of sedimentary phyto- plankton detritus, the main food source to deep-sea benthic communities (Pfannkuche et al., 1983; Pfannkuche, 1985; Soetaert et al., 1991a). Boetius et al. (1996) found that in the eastern Mediterranean concentrations of CPE decrease from 4-6 µg cm-2 on the continental shelf to 0.1-1 µg cm-2 at abyssal depths. However, strikingly high concentrations ( > 10 µg cm-2) were measured in the Hellenic and Pliny Trenches (where depths exceed 3750 m), supporting the hypothesis that deep-sea depressions tend to accumulate organic material and are therefore char- acterised as “hot spots”.

Quantitative and qualitative information on the ecology of meiofauna from trench areas worldwide is limited to only a few published papers on the Puerto Rico Trench (George and Higgins, 1979; Tietjen et al., 1989), the Philippine Trench (Thiel, 1979a), the Ogasawara Trench (Shirayama, 1984), the Japan Trench (Shirayama and Kojima, 1994) and the Ataca- ma Trench (Danovaro et al., 2002). Most of these studies were undertaken along bathymetric transects and attempted to relate meiofaunal abundances and biomasses to surface primary production or to surface derived organic matter. Overall, a general decrease in abundance and biomass of metazoan meiofauna with food availability is now well established (De Bovée et al., 1990; Tietjen, 1992; Vincx et al., 1994), thus indi- cating a clear relationship between meiofauna distrib- ution and food supply (Sibuet et al., 1989; Vincx et al., 1994; Danovaro et al., 2000).

Tselepides and Lampadariou (2004) conducted an extensive investigation of the bathyal and abyssal meiofauna of the deep Ionian and Levantine Seas in the eastern Mediterranean. They present a compre- hensive data set on meiofaunal distribution in rela-

MEDITERRANEAN DEEP-SEA BIOLOGY 21

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