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An introduction to Mediterranean deep-sea biology* - page 17 / 32

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ments different trophic conditions influence bacteri- al densities. Meiofaunal abundance and biomass were similar on the continental shelves of the two areas (940-2558 ind. 10 cm–2) but at bathyal depths densities in the Cretan Sea (range: 60-120 ind. 10 cm–2) were 4-25 times lower than those in the north- western Mediterranean (500-1500 ind. 10 cm–2). In contrast to what was observed in the northwestern Mediterranean, deep-sea meiofaunal assemblages of the Cretan Sea did not react (in terms of density or biomass) to the seasonal variations in food inputs. Here, the bacterial to meiofaunal biomass ratio dis- played much higher values (up to > 20), possibly causing competition for food sources with small metazoans. The efficiency with which the POC flux- es were being exploited was estimated. In the north- western Mediterranean about 0.14 mgC d–1 are available to each meiofaunal individual compared to the 0.07 mgC ind.–1 in the eastern Mediterranean. Similarly, 3.3 mgC were provided daily per 1 mgC of bacterial biomass in the western Mediterranean, compared to 0.07 in the eastern Mediterranean. These data indicate that the benthic components in the Cretan Sea, and in the eastern Mediterranean in general, are subject to more limiting trophic condi- tions, and therefore might have a higher efficiency at exploiting particulate organic fluxes.

Concluding remarks

The deep-sea floor has traditionally been consid- ered as a stable unchanging environment inhabited by sparsely distributed organisms that depend upon a constant supply of fine (mostly refractory) parti- cles settling from the euphotic zone. This contention was also considered to be true for the Mediterranean Sea, which at the turn of the 20th century was even considered to be “azoic”.

However, during the past three decades, new technological developments, sampling gear and environmental monitoring methods in concert with a proliferation of multidisciplinary research programs have gradually changed this notion. We now know that the deep Mediterranean Sea is a dynamic envi- ronment linked to upper water column processes through the varying (in both space and time) influx of organic matter. It is also driven and characterised by the dynamics of major geomorphological fea- tures (deep abyssal basins, the Mediterranean ridge system, anoxic hypersaline abyssal basins, mud vol- canoes, cold and hydrothermal seepage, seamounts, submarine canyons, the Nile deep-sea fan, etc.)

which are encountered in specific areas, and to a large extent in the eastern Mediterranean.

The benthic community has been shown to react very rapidly to seasonal or/and episodic sedimenta- tion of organic matter. Small size fractions of benth- ic organisms (bacteria and protozoa) are known to respond to such inputs, even when the quality and quantity of the sedimenting matter is low or poor. The main processes therefore controlling pelagic- benthic coupling are primarily the export flux of organic matter from the euphotic zone, the quantity and quality of the OM arriving on the seabed and of course its rate of oxidation and mineralisation.

In addition to the above, the deep eastern Mediter- ranean also seems to be governed by cataclysmic events that alter the entire biogeochemical balance of the seabed. These events are either the end result of a combination of climatic factors and/or local reactions to a suite of hydrological “perturbations” which cause uplifting of nutrient rich deep-waters into the euphot- ic zone and therefore enhance biological productivity and OM flux to the seabed. The large deep water tran- sient in the eastern Mediterranean is now considered a classic example of how a “deep-sea desert” can rel- atively abruptly be converted into an “oasis” or a monoculture of certain opportunistic species (e.g. the polychaete Myriochele fragilis). Species such as Chaceon mediterraneus, which were once only known from the western Mediterranean basin are now found everywhere and in high numbers even in the deep eastern basin. The questions thus arising are whether the deep-sea ecosystem responds to climate change, and whether this is also evident in the olig- otrophic eastern Mediterranean. The answer to these “ambitious” or “provocative” questions is yes. There is accumulating evidence that climate change has caused an immediate accumulation of organic matter on the deep-sea floor, altered the carbon and nitrogen cycles and affected benthic activity in general. Inves- tigations undertaken in the deep eastern Mediter- ranean during the past 15 years have revealed that it is occasionally a very dynamic environment respond- ing to major climatic or hydrological changes or forc- ings by altering its biogeochemistry and concomitant- ly its benthic community structure.

Recently, large changes in the physico-chemical characteristics of the eastern Mediterranean deep water, known as the “transient event”, have been reported (Roether et al., 1996). Long-term investi- gations of deep-sea biology have been carried out in the eastern Mediterranean, providing a unique opportunity to study the response of a deep-sea

MEDITERRANEAN DEEP-SEA BIOLOGY 23

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