| Abstract |
In order to study the ecology of meiobenthos with special reference to foraminifera at the bathyal and abyssal zone of the Eastern Mediterranean Sea, sediment samples were collected from a wide range of sampling stations, from 582m to 4617m depth. Sampling stations were indicative of the abiotic parameters and the specific characteristics of the study area (variety in depths, oligotrophic environment, extreme environmental conditions). At the bathyal zone, the analysed samples were collected at the Sporades Basin (1221-1250m) and the Cretan Sea (1840m), as well as at the West and the East Ionian Sea (582-1735m depth). At the abyssal zone, sampling stations were established along a transect from the Central to the Eastern Basin (2714-4000m) as well as at the Peloponnese (4617m) and Ierapetra Basin (4157-4261m). In addition, abyssal ssamples were also collected from both the anoxic zone and the area outside the influence of the brines at the Deep Hypersaline Anoxic Basins Discovery, L' Atralante, Urania and Bannock (3080-3522m). Three more stations were studied at the eastern basin indicative of the sapropel S1 layer, proto-sapropel and post sapropel layers included. Samples were collected using a multicorer or a box corer and were further subsampled for physicochemical and sedimentological analyses as well as for biological analysis (of both metazoa and foraminifera). Foraminifera were identified into species or genus level. Benthic foramininifera were the second most abundant group after nematodes, followed by harpacticoid copepods and copepods nauplii. The contribution of foraminifera to the total meiobenthos was more important at the abyssal zone (up to 54% of the total meiobenthos). The foraminiferal abundance as well as the abundance of major metazoan groups were significantly decreased at the abyssal zone. The species number and diversity of the foraminiferal community were also decreased at the abyssal stations. The quantity and quality of the food available in the sediments was found to be the most important factor controlling the structure of the community at the bathyal zone, at all the stations at the abyssal zone, and outside the influence of the brines of the anoxic basins. In the South Aegean Sea (Cretan Sea), low levels of abundance were recorded (compared to the northern stations) thus indicating the oligotrophic nature of the south, while the North Aegean is strongly influenced by the rich in nutrients waters coming from the Black Sea. In the East Ionian Sea and at 600m depth in the West Ionian Sea, low abundances of meiobenthic groups are also indicative of the oligotrophy that characterises the Ionian Sea. The Peloponnese Basin, characterised by surprisingly high abundances compared to the other abyssal stations and coupled with high concentrations of chloroplastic pigments, verifies the role of such deep basins as natural traps for organic matter accumulated from the euphotic zone. Moreover, the high concentration of organic matter together with the increased numbers of planktonic individuals found in the anoxic zone of the Deep Hypersaline Anoxic Basins, further verifies the contention that some of these basins behave as natural sediment traps by accumulating sedimenting organic matter and decaying planktonic organisms. The results of the present study also indicate that all four Deep Hypersaline Anoxic Basins support, to a different extent, meiobenthic organisms such as bivalves, gastropods, foraminifera, nematodes. However, it is not so far known the specific adaptations they need in order to be able to survive at this kind of extreme environments, are not yet known. From the analysis of the dead assemblages of benthic foraminifera (benthic foraminiferal number and the diversity) preserved before, during and after the deposition of the sapropel S1, it is evident that foraminiferal community showed a clear response before, during and after the anoxia event during the Holocene. The typical of the deep-sea foraminiferal community is changed to a community represented by species tolerant to low concentrations of oxygen, followed by the typical normal community characterising the deep sea of the Eastern Meditrranean Sea, after the anoxic event.
|
Οικολογική μελέτη των τρηματοφόρων ως βασικής συνιστώσας του μειοβένθους, στη βαθύαλη και αβυσσική ζώνη της Α. Μεσογείου
