BENTHIC FORAMINIFERA OF COASTAL HYDROTHERMAL VENTS, AEOLIAN ARC, TYRRHENIAN SEA

A census of benthic Foraminifera (>63µm) collected by scuba diving from the sediment and seagrass (Posidonia) within the explosive crater of a hydrothermal vent off the coast of Panarea island (Aeolian Arc, Tyrrhenian Sea, Italy) shows a much greater species richness (s=6-31) on Posidonia leaves and rhizomes (water depths 12-15 m) than in the surrounding seafloor sediment (s=1-7; water depth 5-6 m). Published data show that CO₂ is the main component of the venting gas (generally above 95%), with 3-4% H₂S (Calanchi et al., 1995). The gas collected during an exceptional venting event (November 2002), however, contained abnormally high amounts of HCl (>6000 ppm), HF (25 ppm), and SO₂ (256 ppm), with low H₂S (1600 ppm); the temperature and pH had changed to 45-50^oC and 5 from earlier values of 34-35^oC and about 8, respectively. Although some foraminiferal shells collected at this time picked up the Rose Bengal stain, we surmise that the individuals were dead at the time of collection. The Posidonia leaves had a patina of colloidal sulfur, and many epiphytic calcareous Foraminifera (especially Cibicides sp. and Lobatula lobatula) were present as decalcified organic linings.

Judging by the extraordinarily high relative abundance of the agglutinated group (with dominant Lepidodeuterammina ochracea) and the very low species richness in the surface sediment samples, the effect of the hydrothermal gas emission on the foraminiferal community (even before the 2002 venting episode) seems more deleterious at and below the sediment-water interface than in the seagrass meadows; the low water depth of the sediment collection sites may also partially explain the low species richness. Overall, neither the seagrass community nor the sediment community of benthic Foraminifera contains species that may be regarded as endemic to hydrothermal venting sites; past studies of foraminiferal communities of hydrocarbon seeps have led to a parallel conclusion.