SOURCES AND PATHWAYS OF PARTICULATE ORGANIC-CARBON IN A SUBMARINE CAVE WITH SULFUR WATER SPRINGS

The origin, quality and pathways of particulate organic carbon (POC) w ere studied from May 1991 to May 1995 in a submarine cave (Grotta Azzu rra, Tyrrhenian Sea, Italy) with warm sulphur springs that support den se mats of sulphur-oxidizing bacteria. Multifactorial sampling designs were used to specifically address: (1) differences in the quantity an d quality of suspended and sedimenting particles in two distinct regio ns of Grotta Azzurra, a weakly-illuminated outer region (Central Hall) and an innermost dark region (Snow Hall) characterized by the presenc e of sulphur waters; (2) the composition and fluxes of particulate mat erial above and below the sulphurous boundary. The water and sediment trap samples were analysed for total particles, POC, particulate organ ic nitrogen (PON), chlorophyll a (Chl a) and phaeopigments (Phaeo). Th e microbial mats were assayed for carbon fixation and RubisCo activity . Stable carbon isotope ratios of the bacteria, benthic fauna and sedi ments were measured. The overall amount and flux of POC did not differ between the two regions of the cave. By contrast photosynthetic pigme nts decreased significantly from the outer to the inner region. The av erage POC:PON ratios of suspended and sedimenting material were lower than 10.2, and there was no apparent ageing or degradation from the Ce ntral Hall to the Snow Hall. Fluxes of POC above the boundary were on average 23.8% of fluxes measured below the boundary, and no Chl a was detected above the boundary. The mats of sulphur-oxidizing bacteria ha d a C-13 depletion of -30 to -31 parts per thousand, providing a marke r for tracing POC from this source. The majority of the benthic animal s showed partial derivative(13)C from -20 to -24 parts per thousand. S ome polychaete worms and echinoderms were more depleted (-25 parts per thousand), indicating greater consumption of bacterial carbon, while sponges were not depleted compared with the benthos of the control cav e (partial derivative(13)C -18 to -19 parts per thousand). It is estim ated that bacteria supply about 31% of the suspended POC in the Snow H all. A larger input may be available to animals living close to the ba cterial mats and to those in the more sulphurous Grotta Sulfurea cave.