Biological characteristics of a hydrothermal edifice mosaic community

This study defines the composition and biomass characteristics of 5 of 6 pr eviously described faunal assemblages that form a mosaic community on hydro thermal sulfide edifices of the Juan de Fuca Ridge (northeast Pacific). Qua ntitative samples of each assemblage were acquired during 'ROPOS' Remote Op erated Vehicle (ROV) dive programs in 1994 and 1996. Total abundance, and s pecies richness, as well as wet and dry weights, were calculated for each a ssemblage, and sampled surface area was measured directly or from scaled im ages of sample scars. These data were used to compare species composition, richness and biomass of the distinct assemblages and to estimate total biom ass of a sulfide edifice. In addition to major compositional differences, w e observed an increase in density, biomass and species richness along a pro posed successional sequence from the Paralvinella sulfincola (annelid polyc haete) assemblage (Assemblage I) to the low-now Ridgeia piscesae (vestiment ifera) community (Assemblage V-LF). Biomass (dry weight without vestimentif eran tubes) of the different sampled assemblages ranged from 0.011 kg m(-2) for Assemblage I to 4.68 kg m(-2) for Assemblage V-LF and 2.33 kg m(-2) fo r the rarer high-flow Ridgeia piscesae community (V-HF). Resulting quantita tive information was used to refine a previous model of community successio n. Comparisons with other marine ecosystems showed that the biomass of thes e and other hydrothermal assemblages dominated by symbiont-bearing organism s (vestimentifera, bivalvia) is similar to those found in the most producti ve photosynthetically based assemblages. Tubeworm growth and sulfide accret ion greatly increase total surface area available to vent organisms, and ma y attenuate competition for space. The 3-dimensional habitat formed by Ridg eia piscesae tubes may influence species distribution and enhance species r ichness. The tube worm assemblages comprise the major and probably the most stable component of total edifice biomass. At one site, over a 4 yr period , there was substantial environmental change and major shifts in coverage b y other assemblages but relatively little change in total coverage by R. pi scesae. As a result total edifice biomass (219 to 251 kg dry weight) varied by only 16% over 1 to 3 yr intervals. Considerable quantitative ecological information can be derived from analyses of submersible-collected imagery, with sampling serving primarily as a ground truthing tool. Limitations of sampling and surface area determinations are considered.