Limited in scope, chemosynthesis-driven hydrothermal vents are one benthic system which is rich in life. Alternative sources of energy provide the grist for flourishing communities of bacteria worms and molluscs. Hydrothermal vents are deserving of much greater attention than can be devoted here.
Most other open-ocean benthic communities are food- rather than space-limited, unlike so many shallow water marine systems. The full functional significance of this difference has yet to be appreciated. In contrast to the pelagic ecosystems, the benthos is dominated by deposit feeders and scavengers which depend upon the down-fall of organic material from the epipelagic zone (see previous section), and consequently, overall productivity is considerably lower. Much of the variability in the deep sea is driven directly from changes in the epipelagic zone (Lampitt 1987; Smith 1987). Through their activities, deposit feeders modify the physical and chemical characteristics of the sediment, and could be considered "ecosystem engineers" or "habitat fabric interactors" (sensu Jones et al. 1994). Some deep-sea species may be important in the creation of biogenic structures in the benthos which provide microhabitats for many other species which inhabit this realm (Jumars 1975; Thistle 1979; Gooday 1984; Levin et al. 1986).
The possibility of losing specific taxa or strains from the bcnthic microbial loop as a result of direct human alteration of sections of sea bottom should be taken seriously. The diversity of microbial activity in the deep benthos is not only a key attribute of the system, but also of enormous potential value to humanity. The infauna (that life below the living in the sediments) lives in an environment where basic life conditions are directly determined by the dynamic flow of life-sustaining molecules (oxygen, sulfur, nitrogen). The point at which the environment becomes toxic to an organism strongly depends upon bioturbation, i.e. the physical mixing of sediments caused by animal movement. Life quickly becomes limited to groups of specialists who can tolerate these extreme habitats. Some species appear to play a key role in modulating this environment (Thistle 1979; Gooday 1984; Levin et al. 1986; Grassle 1989; Levinton 1994), presumably altering the microbial loop and the productivity/food-web structure of deep benthic environments. Whether many of the maerofauna species perform the same role, and could be compensated for if eliminated, is not clear, at least for coastal benthic communities (Giblin et al. 1994; Levinton 1994). The high species diversity found in deep-sea environments is thought to be the result of small-scale nutrient pulses from the pelagic zone, which act as small disturbances to the system (Grassle 1989). If this is true, then much of the deep-sea benthic diversity may be composed of rare species with greatly overlapping fundamental niches, and the elimination of much of this diversity will have little impact on either the regeneration of nutrients or benthic productivity. This result ignores any potential effect of species-specific biotic interactions which could percolate through the food web; however, we are unaware of evidence for such specific interactions. To recapitulate, it is possible that many of the deep sea spccies may act like "passengers"' rather than "rivets" in deep-sea processes, although there is only scant evidence to support this idea at the moment. Taxa which alter the physical structure of the sediments are most likely to be key functional species, taxa which impact the microbial loop taxa are another key group in the benthos.
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