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An introduction to Mediterranean deep-sea biology* - page 16 / 32

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FIG. 3. – Regression of total meiofaunal abundances (ind. 10 cm-2) from 195 to 4617 m depth in the eastern Mediterranean (from Tselepides and Lampadariou, 2004).

tion to depth and a suite of sedimentary environ- mental parameters, which were used as indicators of food availability. The results indicate a strong dependence of meiofaunal abundance on depth (Fig. 3), distance from the coast and labile sources of organic carbon such as proteins and lipids. The Pliny and Hellenic trenches were also sampled and found to function as traps of organic matter supporting comparatively similar meiofaunal abundances to those occurring at shallower depths, and microbial activity potentials that exceeded those at the abyssal plain by an order of magnitude (Boetius et al., 1996). Although most of the meiofaunal compo- nents were found not to respond dramatically to the influx of fresh phytodetrital material, the fact that their values were relatively similar to those found at shallower depths plus the drastic increase in harpacticoid copepods together with the enhanced microbial activities partially justified the characteri- sation of the eastern Mediterranean trenches as “benthic hot spots”. The results of this study present a deviation from what is considered as normal. It seems that the ultra oligotrophic deep Mediter- ranean environment occasionally undergoes periods of increased food availability, which can be related to events occurring further up in the water column. Such an event was the eastern Mediterranean “Tran- sient”, which was triggered by a combination of atmospheric, meteorological and hydrological fac- tors (Roether et al., 1996).

It seems that this event had a pronounced effect on the entire ecosystem. The enhancement in benth- ic activity recorded in the trenches of the eastern Mediterranean may well have been the indirect result of a basin-scale uplifting of nutrient-rich deep waters into the euphotic zone. The latter phenome- non was followed by an increase in biological pro-

22 F. SARDÀ et al.

ductivity, vertical fluxes and POM propagation to the deep sea through cascading, down-slope sliding and entrapment of organic rich sediments into the Hellenic and Pliny trenches.

Recenty, Danovaro et al. (2004), attempt to relate climate change to nematode diversity in the deep eastern Mediterranean. By using a decadal data set (1989-1998) they provide evidence that deep-sea nematode diversity can be strongly and rapidly affected by temperature shifts.

Benthic response to particulate fluxes

In a thorough review regarding benthic response to particulate fluxes, Danovaro et al. (1999) gave a very comprehensive comparison of particle fluxes, sedimentary OM composition, and microbial and meiofaunal parameters. They used quantitative data from various stations in the Gulf of Lions and Cata- lan Sea (northwestern Mediterranean), as well as from the Cretan Sea (eastern Mediterranean). Ben- thic responses in relation to the different trophic conditions prevailing in the two areas were com- pared in terms of: (1) temporal and spatial variabili- ty of the mass fluxes; (2) pelagic-benthic coupling in organic matter composition (such as proteins, lipids, soluble carbohydrates and CPE); (3) microbial response to changes in organic matter composition and POC fluxes; (4) meiofaunal response to changes in organic matter composition and particle fluxes; and (5) relative significance of bacteria and meio- fauna. Mass fluxes at equal depths were found to be up to two orders of magnitude higher in the western than in the eastern Mediterranean. Clear seasonal changes were reported in both areas, although mass fluxes and variability were consistently higher in the northwestern Mediterranean. From primary produc- tion estimates in the western and eastern Mediter- ranean (140-160 vs. 19-60 mgC m-2 d-1 respective- ly), it was calculated that a carbon export from the euphotic layer to 1000 m depth is equivalent to about 10% in the Gulf of Lions and 2-3% in the Cre- tan Sea. Chlorophyll-a concentrations at similar depths were 2-3 times higher in the western basin. Carbohydrates were the dominant biochemical com- ponent in the Cretan Sea sediments, while total amino acids represented an important fraction of the biodegradable material in the Gulf of Lions-Catalan Sea. In the western Mediterranean, bacterial densi- ties (annual mean 7.9 x 108 cell g–1) were about 4 times higher than in the Cretan Sea (annual mean 2.1 x 108 cell g–1), indicating that in deep-sea sedi-

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