VIDEOSCOPIC STUDY OF DEEP-SEA HYDROTHERMAL VENT ALVINELLID POLYCHAETEPOPULATIONS - BIOMASS ESTIMATION AND BEHAVIOR

Biomass is a poorly documented parameter of the hydrothermal ecosystem , partly due to the difficulties in estimating it. Because the hydroth ermal alvinellid polychaete worms mostly live in tubes, and on very ir regular surfaces difficult to sample, an original biomass estimation m ethod based on video analysis was proposed and tested on populations o f the East Pacific Rise at 13-degrees-N. The area covered by the 3 mai n alvinellid species was accurately estimated from geometrical calcula tions in oblique-angle conditions, requiring only 3 object measurement s on the monitor screen. The total density of alvinellids was estimate d on video by modelling the pattern of worm movements in and out of th eir tubes. The Richards function was found appropriate to model the re sulting asymptotic curves. Biomass was calculated from mean alvinellid weights for populations from 3 white smokers and 1 black smoker. Valu es ranged from 10.7 to 70.1 g ash-free dry weight m-2, which is a much lower estimate than in previous studies, perhaps because of differenc es in sample size, sampling methods and the heterogeneity of studied a ssemblages. It was possible to discriminate Alvinella spp. from Paralv inella grasslei on the video because of differences in behaviour, allo wing separate biomass calculations. Spatial variability of the alvinel lid biomass was assessed f or one chimney. Biomass was maximal at the top and the base, and minimal in the middle. Coverage of the chimney b y the alvinellid tubes was spatially heterogeneous, and the genera dis played different distribution patterns. P grasslei seems to move a lot and settle on chimneys in an opportunistic way, while Alvinella spp., more dependent on the environmental conditions, settle at the base an d gradually move upwards as the chimney grows.