16
David THISTLE
studies of deep-sea community energetics, which have
implications for global carbon cycling. Free-vehicle
traps have been crucial to the study of food-parcelattending species in the deep sea (Hessler et al., 1978).
Costs and benefits
Good techniques are available with which to sample,
and reasonable techniques are available with which
to do experiments in the deep sea, but the expense
is substantial. Both sampling and experimentation
require the use of large, and therefore expensive, ships.
Research submarines and ROV’s add additional costs.
For soft bottoms, separating the animals from the
sediment and identifying the diverse fauna (Grassle
and Maciolek, 1992) are time-consuming, so sample
processing is costly. These expenses are among the
reasons why relatively few data have been collected
from this vast ecosystem and why few ecological
experiments have been performed.
Despite these costs, scientists persist in the study
of the deep sea, and their research provides a variety benefits for society. For example, research on
hydrothermal-vent animals led to the discovery of DNA
polymerases that work at high temperatures, which are
crucial tools in pure and applied molecular biology.
Safe repositories for human waste, such as dredge
spoils, sewage sludge, industrial waste, and radioactive
materials, are needed. Ongoing ecological work will
help determine whether wastes dumped in the deep sea
make their way back into contact with humans, and the
effects of these wastes on the functioning of natural
ecosystems in the ocean (Van Dover et al., 1992). The
deep-sea floor contains mineral resources; for example,
economically important amounts of cobalt and nickel
occur in manganese nodules. The work of deep-sea
ecologists is helping to determine the environmental
consequences of deep-ocean mining (Ozturgut et al.,
1981). More generally, the deep-sea benthos provides
critical ecological services (e.g., recycling of organic
matter to nutrients: Snelgrove et al., 1997).
THE SOFT-BOTTOM FAUNA OF THE DEEP-SEA
FLOOR
Taxonomic composition
At high taxonomic levels (i.e., phylum, class, and
order), the soft-bottom, deep-sea fauna is similar to that
of shallow-water soft bottoms (Hessler, 1974; Gage,
1978). For example, the megafauna consists primarily
of demersal fishes, sea cucumbers, star fishes, brittle
stars, and sea anemones. The macrofauna consists
primarily of polychaetes, bivalve mollusks, and isopod,
amphipod, and tanaid crustaceans. The meiofauna
consists of primarily of foraminifers, nematodes, and
harpacticoid copepods. At lower taxonomic levels
(family and below), however, the similarities disappear.
In particular, the species that live in the deep sea are
not, in general, found in shallow water. Gage and Tyler
(1991) have reviewed the natural history of deep-sea
taxa.
Many taxa that have large numbers of species in
shallow water have a few members that penetrate into
the deep sea. For example, of 300 stomatopod (mantis
shrimp) species, only 14 occur below 300 m (Manning
and Struhsaker, 1976). The decapod crustacean fauna
in shallow water (<200 m) consists of more than
200 species, predominantly brachyuran crabs. Below
a depth of 1500 m, there are fewer than 40 species
of decapods, and brachyurans make up ~10% of
this total. The proportion of the bivalve mollusks
that are eulamellibranchs decreases as well (Sanders
et al., 1965). In contrast, the proportion of isopod
species that are asellotes (Hessler and Wilson, 1983)
and of bivalve mollusk species that are protobranchs
increases with depth. Finally, some taxa inhabit the
deep sea exclusively; for instance, the protist group
of xenophyophores have not been found above ~500 m
(Tendal, 1996).
Variation of biomass and numbers with depth
The general pattern of the distribution of the biomass
of organisms on the deep-sea floor is known (Fig. 2.13)
and appears to be controlled by the rate at which
food is supplied to the seabed (Rowe, 1971). The
basic pattern is set by the productivity of the surface
waters. For example, primary productivity is highest
nearest the continents, and the deep-sea floor near
continents tends to have the highest biomass. The
depth of the overlying water modifies this pattern. As
food particles sink, a portion of each particle is lost
to decay, and some particles are consumed by midwater organisms. The deeper the water, the longer it
takes particles to reach the seabed, and the greater
the loss from these processes. Given two regions with
identical primary productivities in the overlying water,
the deeper location will have the lesser food input
David THISTLE
studies of deep-sea community energetics, which have
implications for global carbon cycling. Free-vehicle
traps have been crucial to the study of food-parcelattending species in the deep sea (Hessler et al., 1978).
Costs and benefits
Good techniques are available with which to sample,
and reasonable techniques are available with which
to do experiments in the deep sea, but the expense
is substantial. Both sampling and experimentation
require the use of large, and therefore expensive, ships.
Research submarines and ROV’s add additional costs.
For soft bottoms, separating the animals from the
sediment and identifying the diverse fauna (Grassle
and Maciolek, 1992) are time-consuming, so sample
processing is costly. These expenses are among the
reasons why relatively few data have been collected
from this vast ecosystem and why few ecological
experiments have been performed.
Despite these costs, scientists persist in the study
of the deep sea, and their research provides a variety benefits for society. For example, research on
hydrothermal-vent animals led to the discovery of DNA
polymerases that work at high temperatures, which are
crucial tools in pure and applied molecular biology.
Safe repositories for human waste, such as dredge
spoils, sewage sludge, industrial waste, and radioactive
materials, are needed. Ongoing ecological work will
help determine whether wastes dumped in the deep sea
make their way back into contact with humans, and the
effects of these wastes on the functioning of natural
ecosystems in the ocean (Van Dover et al., 1992). The
deep-sea floor contains mineral resources; for example,
economically important amounts of cobalt and nickel
occur in manganese nodules. The work of deep-sea
ecologists is helping to determine the environmental
consequences of deep-ocean mining (Ozturgut et al.,
1981). More generally, the deep-sea benthos provides
critical ecological services (e.g., recycling of organic
matter to nutrients: Snelgrove et al., 1997).
THE SOFT-BOTTOM FAUNA OF THE DEEP-SEA
FLOOR
Taxonomic composition
At high taxonomic levels (i.e., phylum, class, and
order), the soft-bottom, deep-sea fauna is similar to that
of shallow-water soft bottoms (Hessler, 1974; Gage,
1978). For example, the megafauna consists primarily
of demersal fishes, sea cucumbers, star fishes, brittle
stars, and sea anemones. The macrofauna consists
primarily of polychaetes, bivalve mollusks, and isopod,
amphipod, and tanaid crustaceans. The meiofauna
consists of primarily of foraminifers, nematodes, and
harpacticoid copepods. At lower taxonomic levels
(family and below), however, the similarities disappear.
In particular, the species that live in the deep sea are
not, in general, found in shallow water. Gage and Tyler
(1991) have reviewed the natural history of deep-sea
taxa.
Many taxa that have large numbers of species in
shallow water have a few members that penetrate into
the deep sea. For example, of 300 stomatopod (mantis
shrimp) species, only 14 occur below 300 m (Manning
and Struhsaker, 1976). The decapod crustacean fauna
in shallow water (<200 m) consists of more than
200 species, predominantly brachyuran crabs. Below
a depth of 1500 m, there are fewer than 40 species
of decapods, and brachyurans make up ~10% of
this total. The proportion of the bivalve mollusks
that are eulamellibranchs decreases as well (Sanders
et al., 1965). In contrast, the proportion of isopod
species that are asellotes (Hessler and Wilson, 1983)
and of bivalve mollusk species that are protobranchs
increases with depth. Finally, some taxa inhabit the
deep sea exclusively; for instance, the protist group
of xenophyophores have not been found above ~500 m
(Tendal, 1996).
Variation of biomass and numbers with depth
The general pattern of the distribution of the biomass
of organisms on the deep-sea floor is known (Fig. 2.13)
and appears to be controlled by the rate at which
food is supplied to the seabed (Rowe, 1971). The
basic pattern is set by the productivity of the surface
waters. For example, primary productivity is highest
nearest the continents, and the deep-sea floor near
continents tends to have the highest biomass. The
depth of the overlying water modifies this pattern. As
food particles sink, a portion of each particle is lost
to decay, and some particles are consumed by midwater organisms. The deeper the water, the longer it
takes particles to reach the seabed, and the greater
the loss from these processes. Given two regions with
identical primary productivities in the overlying water,
the deeper location will have the lesser food input