IMIS

In this study, a comparison is made between the vertical distribution (0-10 cm) of living (stained) foraminifera in sediments from the shallow (19 & 20 m) eutrophic north-western Adriatic Sea and the deep (1288 & 1300 m) oligotrophic Aegean Sea. Vertical distribution was examined in relationship to oxygen and food gradients in the sediment. Sediments in the Adriatic Sea supported a significantly large standing stock both in surface and subsurface layers. Although the oxygen penetration depth in the Adriatic does not exceed 2 mm, the habitat depth (defined as the depth above which 95 % of the total assemblage or population is found) range from 6 to 7 cm. Maximum oxygen penetration depth was found at the station in the southern Aegean Sea (E1, 10 cm) where the habitat depth was the shallowest (2 cm). The habitat depth was the deepest at the second Aegean Sea station (E2, 9 cm) that had an oxic zone of 27 mm. No direct correlation was found between the vertical distribution of the total assemblage and food concentration. Dominant genera in the Adriatic Sea (Reophax, Nonionella, Stainforthia, Bolivina & Hopkinsina) had significantly higher densities in the upper 2-5 centimetres of the sediment but foraminifera were found down to 10 cm. Although both Adriatic stations (A1 & A2) have comparable sediment characteristics and oxygen penetration depth, a relatively larger proportion of the assemblages was found in deeper layers of the sediment at A2. On average, 70 % of the foraminifera were found below the main oxic zone at A2 and approximately 34 % at station A1. Vertical distribution patterns of the different genera exhibited no direct correlation with oxygen, grain size or food content. Epistominella was the most dominant genus at E1 in the Aegean Sea and was found only in the upper 2 cm although the oxygen penetration depth was much deeper (10 cm). At the other Aegean station (E2) where oxygen penetrated to 27 mm, Epistominella was encountered down to 10 cm with no clear peak densities. The other dominant genus was Melonis that was irregularly distributed throughout the sediment. Brizalina was not dominant but exhibited a total different pattern; this genus was encountered only below the 0-1 cm layer. On average, 42 % of the foraminifera were found below the main oxic zone at E2. Three major patterns in the vertical distribution was evident: some genera have peak densities in the upper 2 cm, some genera that have peak densities in the upper 5 cm and, although not dominant, some genera were absent in the upper cm but occurred irregularly in deeper layers of the sediment. These trends could not be directly correlated either with oxygen or food gradients in the sediment. This suggests that site-specific factors (e.g. bioturbation, competition , species interactions) may be involved. This is supported by the fact that, in spite of similar abiotic conditions, different vertical distribution patterns are seen for common genera at different stations. On average, the > 63 µm fraction represented 40 - 60 % of the total assemblage in the Adriatic Sea and 55-74 % of the total assemblage in the Aegean Sea. Although the major trends remain evident in the 63 µm fraction, finer details are obscured; this was more so the case in the shallow Adriatic Sea sediments.