Deep-sea and shallow-water hydrothermal vent communities: Two different phenomena?

Abstract

Hydrothermal vent communities from 76 areas lying at depths from 0 to 4100 m were split into two groups — “deep” and “shallow-water”, based on the occurrence of vent obligate (restricted to hydrothermal vents) taxa of a high rank (genus and family). The two groups were then compared based on a large set of environmental, faunistic and community parameters. The degree of obligacy of fauna sharply changes at the depth of approximately 200 m. In 21 shallow-water areas, located at depths < 200 m, vent obligate taxa are absent or their taxonomic rank does not exceed a species. The total number of vent obligate species in shallow water is 8, and their obligacy is uncertain. Data from 55 areas lying at depths from 200 to 4100 m indicate there are ca. 400 obligate species. In 47 of these areas, obligate vent taxa of the genus or the family rank are present. Obligate taxa of the genus rank appear at 200 m depth, and of the family rank — at 386 m. The depth ca. 200 m corresponds well with differences in community structure and composition and environmental parameters. Deep-sea (> 200 m) hydrothermal communities differ from shallow-water ones (< 200 m) in a much higher ratio of vent obligate taxa. Deep-sea vent communities are also distinguished by the lack of diatom and algae-bacterial mats, phytoplankton, lower species richness, the development of large sulphide structures, higher role of active vents in structuring communities in space, replication of a set of dominant taxa in communities within large regions reproduced through the succession at newly formed vent fields, more pronounced concentric and vertical zonality, higher biomass of macrofauna, the important role of symbiotrophic forms dominating the biomass. The ratio of species with a “point” species range (endemic to small areas) is higher in the deep sea. There are no significant differences between the vent and non-vent biogeography both in shallow-waters and the deep ocean. Modern deep-sea hydrothermal vent communities have a longer evolutionary history than their shallow-water counterparts. To explain major differences between deep and shallow-water hot vent communities, evolutionary processes need to be analysed over long geological periods and within global vertical zones. It is suggested that the proportion of organic matter derived from chemosynthesis and photosynthesis plays a crucial role in evolution. When calculated for global zones this proportion increases with depth to a certain critical level that separates deep and shallow-water hot vent communities and fauna. Vertical zonation in the distribution of obligate taxa in hydrothermal vent communities indicates that this level is reached at approximately 200 m depth. Differences between deep and shallow-water hydrothermal vent communities are significant enough to distinguish the two different phenomena. The principle differences are a higher ratio of vent obligate taxa and a dominance of symbiotrophic forms in deep-sea hydrothermal vent communities.