THE DEEP ATLANTIC OCEAN
131
and sometimes covered as much as 25% of the bottom.
Megafaunal abundance at this site is several orders of
magnitude higher than that reported for other sedimentfree (non-reducing) sites of comparable depth. Genin
et al. (1992) attributed these high abundances to an
increased rate of food supply resulting from the high
particle flux generated by the current, as surface
productivity in the area is not known to be especially
large.
Northwest Atlantic Abyss and trenches
The abyssal and hadal environments of the Northwest
Atlantic have received minimal attention. By far the
best-studied are the Hatteras Abyssal Plain (Fig. 5.2)
and the Puerto Rico Trench, but even for these areas
there are only a handful of reports, with the emphasis
largely on meiofauna.
Hatteras and Nares abyssal plains
The relations between metazoan meiofaunal abundance and biomass, bacterial abundance and particulate
flux were examined at a depth of 5411 m on the
Hatteras Abyssal Plain (32ºN, 70ºW) (HAP), and
compared to values in the Puerto Rico Trench (PRT)
(Tietjen et al., 1989). Meiofaunal abundance on the
Hatteras Abyssal Plain was 11.4±2.6 individuals cm
−2 ,
and the biomass was 3.84 micrograms C cm
−2 .
Although nematodes comprised 75% of the individuals,
harpacticoid copepods made up 85% of the biomass.
Most of the meiofaunal biomass was present below
the 0–2 cm layer. Comparisons with the Puerto Rico
Trench indicate higher meiofaunal densities, biomass
and even bioturbation rates on the Hatteras Abyssal
Plain than in the Puerto Rico Trench (7460–8189 m).
Nematode faunas at the same two sites, and at a bathyal
silty-sand site in the Puerto Rico Trench (2217 m),
were largely dominated by species of Acantholaimus,
Halalaimus and Theristus, with some additional genera
(Camaicolaimus, Desmodora, Greeffiella, Leptolaimus,
Setoplectus and Tricoma) also important at the bathyal
silty-sand site (Tietjen, 1989). Many species were
endemic – 67% in the Hatteras Abyssal Plain area and
77% in the Puerto Rico Trench area.
Foraminiferal faunas on the Nares Abyssal Plain
have been described by Schr¨ oder (1986) and Schr¨ oder
et al. (1989). The assemblage at a 5800 m site located
below the calcium compensation depth is dominated
by fragile agglutinated taxa, komokiaceans being particularly abundant and diverse. More than 70% of the
54 species (komokiaceans excluded) recognized here
are also present in the central North Pacific, reflecting
the wide occurrence of many abyssal agglutinated
foraminiferal species (Schr¨ oder et al., 1988).
Puerto Rico Trench
Early, non-quantitative reports of the fauna of the
Puerto Rico Trench focused on taxonomic descriptions
and provided little general information about the
benthic community (Nybelin, 1951; Bruun, 1957;
P´ er` es, 1965; Staiger, 1972). Recent sampling has
demonstrated that, in contrast to the typical view that
trenches support abundant, low-diversity assemblages,
the Puerto Rico Trench has a depauperate benthic
assemblage (Richardson et al., 1995). Three 0.25 m
2
boxcores collecting macrofauna (>297 mm) along the
central axis of the trench (8371–8376 m) collected only
21 specimens (= 28 individuals m
−2 ) belonging to
13 species, with a biomass of only 0.079 g wet wt m
−2
(Richardson et al., 1995). Polychaetes and tanaids
accounted for 54% of the individuals and 96% of
the wet biomass. Slightly higher faunal biomass (0.3–
0.42 g m
−2 ) was estimated for the Puerto Rico Trench
by Pasternak et al. (1975) using dredge samples.
Metazoan meiofaunal abundance also was extremely
low: 69±10.8 individuals 10 cm
−2 . Nematodes made
up 95% of the total meiofauna; harpacticoid copepods were 2.3% and ostracods were 1.0%. George
and Higgins (1979) reported even lower meiofaunal
densities of 17.1–17.3 individuals 10 cm
−2 from 8560 m
in the Puerto Rico Trench, but used a 63 mm rather
than 42 mm mesh net. Their otter trawls contained
deposit-feeding holothurians, isopods, amphipods and
cumaceans. The presence of depositional laminae of
fine silt visible in x-radiographs is considered indicative
of low sediment mixing in the Puerto Rico Trench
(Richardson et al., 1995). This observation is in agreement with the very low biomass of bioturbating taxa.
The faunal values for the Puerto Rico Trench
are among the lowest reported for abyssal or hadal
environments. This is attributed to low food flux
and poor nutrient value (0.74% organic C) of the
food supply. Disturbance by turbidity flows may also
contribute to the depauperate nature of the benthic
assemblages (Richardson et al., 1995).
Northeast Atlantic
The deep-sea fauna along the northeast Atlantic
continental margin, first sampled during the 1860s and
131
and sometimes covered as much as 25% of the bottom.
Megafaunal abundance at this site is several orders of
magnitude higher than that reported for other sedimentfree (non-reducing) sites of comparable depth. Genin
et al. (1992) attributed these high abundances to an
increased rate of food supply resulting from the high
particle flux generated by the current, as surface
productivity in the area is not known to be especially
large.
Northwest Atlantic Abyss and trenches
The abyssal and hadal environments of the Northwest
Atlantic have received minimal attention. By far the
best-studied are the Hatteras Abyssal Plain (Fig. 5.2)
and the Puerto Rico Trench, but even for these areas
there are only a handful of reports, with the emphasis
largely on meiofauna.
Hatteras and Nares abyssal plains
The relations between metazoan meiofaunal abundance and biomass, bacterial abundance and particulate
flux were examined at a depth of 5411 m on the
Hatteras Abyssal Plain (32ºN, 70ºW) (HAP), and
compared to values in the Puerto Rico Trench (PRT)
(Tietjen et al., 1989). Meiofaunal abundance on the
Hatteras Abyssal Plain was 11.4±2.6 individuals cm
−2 ,
and the biomass was 3.84 micrograms C cm
−2 .
Although nematodes comprised 75% of the individuals,
harpacticoid copepods made up 85% of the biomass.
Most of the meiofaunal biomass was present below
the 0–2 cm layer. Comparisons with the Puerto Rico
Trench indicate higher meiofaunal densities, biomass
and even bioturbation rates on the Hatteras Abyssal
Plain than in the Puerto Rico Trench (7460–8189 m).
Nematode faunas at the same two sites, and at a bathyal
silty-sand site in the Puerto Rico Trench (2217 m),
were largely dominated by species of Acantholaimus,
Halalaimus and Theristus, with some additional genera
(Camaicolaimus, Desmodora, Greeffiella, Leptolaimus,
Setoplectus and Tricoma) also important at the bathyal
silty-sand site (Tietjen, 1989). Many species were
endemic – 67% in the Hatteras Abyssal Plain area and
77% in the Puerto Rico Trench area.
Foraminiferal faunas on the Nares Abyssal Plain
have been described by Schr¨ oder (1986) and Schr¨ oder
et al. (1989). The assemblage at a 5800 m site located
below the calcium compensation depth is dominated
by fragile agglutinated taxa, komokiaceans being particularly abundant and diverse. More than 70% of the
54 species (komokiaceans excluded) recognized here
are also present in the central North Pacific, reflecting
the wide occurrence of many abyssal agglutinated
foraminiferal species (Schr¨ oder et al., 1988).
Puerto Rico Trench
Early, non-quantitative reports of the fauna of the
Puerto Rico Trench focused on taxonomic descriptions
and provided little general information about the
benthic community (Nybelin, 1951; Bruun, 1957;
P´ er` es, 1965; Staiger, 1972). Recent sampling has
demonstrated that, in contrast to the typical view that
trenches support abundant, low-diversity assemblages,
the Puerto Rico Trench has a depauperate benthic
assemblage (Richardson et al., 1995). Three 0.25 m
2
boxcores collecting macrofauna (>297 mm) along the
central axis of the trench (8371–8376 m) collected only
21 specimens (= 28 individuals m
−2 ) belonging to
13 species, with a biomass of only 0.079 g wet wt m
−2
(Richardson et al., 1995). Polychaetes and tanaids
accounted for 54% of the individuals and 96% of
the wet biomass. Slightly higher faunal biomass (0.3–
0.42 g m
−2 ) was estimated for the Puerto Rico Trench
by Pasternak et al. (1975) using dredge samples.
Metazoan meiofaunal abundance also was extremely
low: 69±10.8 individuals 10 cm
−2 . Nematodes made
up 95% of the total meiofauna; harpacticoid copepods were 2.3% and ostracods were 1.0%. George
and Higgins (1979) reported even lower meiofaunal
densities of 17.1–17.3 individuals 10 cm
−2 from 8560 m
in the Puerto Rico Trench, but used a 63 mm rather
than 42 mm mesh net. Their otter trawls contained
deposit-feeding holothurians, isopods, amphipods and
cumaceans. The presence of depositional laminae of
fine silt visible in x-radiographs is considered indicative
of low sediment mixing in the Puerto Rico Trench
(Richardson et al., 1995). This observation is in agreement with the very low biomass of bioturbating taxa.
The faunal values for the Puerto Rico Trench
are among the lowest reported for abyssal or hadal
environments. This is attributed to low food flux
and poor nutrient value (0.74% organic C) of the
food supply. Disturbance by turbidity flows may also
contribute to the depauperate nature of the benthic
assemblages (Richardson et al., 1995).
Northeast Atlantic
The deep-sea fauna along the northeast Atlantic
continental margin, first sampled during the 1860s and