Cold Seep Communities on Continental Margins: Structure and Quantitative Distribution Relative to Geological and Fluid Venting Patterns

Cold seep ecosystems occur on active and passive continental margins. Chemosynthesisbased communities depend on autochtonous and local chemical energy and produce organic carbon in large quantities through microbial chemosynthesis. The high organic carbon production leads to the large size of the fauna and the high biomass of the communities. The remarkable abundance of giant tubeworms (vestimentiferans) and large bivalves (i.e. Vesicomyidae, Mytilidae and others) is one of the most striking features of such communities and one of the best indicators or tracers of fluid emissions at the seafloor. Cold seep communities are known since about 15 years and have shown that the chemoautotrophy and many symbiont containing organisms are not unique to hydrothermal vents. Ecosystem characteristics and functioning in continental margin habitats are incompletely understood and we do not know how detritus and chemosynthesis-based ecosystems interact. There is a clear need of more field investigations. But with progress in deep-sea submersible technology, our understanding continues to grow. Following a recent review that focused on biogeographical trends and comparisons with hydrothermal vent communities (Sibuet and Olu 1998), we review here the ecology of chemosynthesis-based communities from several cold seep areas. Our synthesis addresses biodiversity and abundance fluctuations and distribution patterns linked to geological and fluid venting features. The diversity of the “symbiotic” fauna expressed as species richness decreases with ocean depth. Species composition is an indicator of the biotope variability. The spatial extension of active seep areas is highly variable from hundreds of square meters to several hectares. Three distinct categories of cold seep sites are recognised. The shape, density and biomass of aggregations reflect the intensity of fluid flow, and characterise fluid circulation and different expulsion pathways through geological structures.