206
Craig R. SMITH and Amanda W.J. DEMOPOULOS
effects of manganese-nodule mining on abyssal Pacific
communities, a sled 8-m wide with plowshares (the
“plow-harrow”) was towed 78 times through a circular
study area 3.6 km in diameter in 4160 m of water in
the eastern tropical Pacific (~7ºS, 88ºW: Borowski and
Thiel, 1998). The plow-harrow disturbed roughly 20%
of the seafloor within the study area, digging furrows
to roughly 10–15 cm into the sediment. Samples were
collected from disturbed and undisturbed areas of the
seafloor using a box corer within days of plowing,
and then approximately six months and three years
later. Within plowed tracks, macrofaunal abundance
was initially reduced by 39%, the polychaetes being
most heavily disturbed (Borowski and Thiel, 1998).
After three years, the abundance of most higher-level
taxa had returned to the levels in the background
community, but species diversity remained significantly
depressed, indicating a sustained disturbance effect
(Borowski and Thiel, 1998). The vertical distribution
of macrofauna within the sediment also remained
anomalous, apparently because physical and chemical
characteristics had not returned to normal. The unexpectedly rapid recolonization of plow tracks apparently
occurred by lateral migration of benthic individuals
from adjacent unplowed sediments, rather than by
larval settlement. Lateral migration was facilitated by
the relatively small width of individual plow furrows
(~1 m: Borowski and Thiel, 1998). These results
indicate that recovery of the infaunal community from
moderate, relatively small-scale, physical disturbance
in the equatorial abyss requires more than three years
(Borowski and Thiel, 1998). Recovery of the sediment
community from actual nodule mining, which would
disturb much greater areas at higher intensities, is
virtually certain to require much longer time periods –
decades (Borowski and Thiel, 1998).
The oligotrophic abyss
More than 40% of the abyssal seafloor in the Pacific
underlies oligotrophic central gyres, which are the
vast nutrient-poor deserts of the ocean. In the North
Pacific, the central gyre stretches from roughly 15ºN
to 35ºN, and from 135ºE to 135ºW, covering an area
of approximately 2×10
7 km
−2 (Karl, 1999); a similar
gyre is present in the South Pacific. Because of deep
nutriclines and great distances from continental sources
of nutrients (e.g., river outflow and dust), the central
gyres sustain lower rates of primary production than
any other ice-free areas of the ocean (e.g., Berger,
1989). This low productivity, combined with great
water depths (typically 5000 to 6000 m), results in
extremely low flux rates of particulate organic carbon
to the underlying seafloor (typically ~0.3 g C m
−2 y
−1 :
K.L. Smith, 1992). Ecosystem characteristics in these
extraordinarily food-poor habitats differ markedly from
those in the eutrophic deep sea.
Habitat and community description
The benthic ecology of two abyssal sites in the North
Pacific Gyre have been investigated in some detail.
The first is the CLIMAX II region (named after the
CLIMAX II research expedition), which is ~50 km in
diameter and centered on 28º28
N, 155º20
W (Hessler
and Jumars, 1974). The second area, MPG-I (Fig. 6.1),
falls roughly within the box 30º to 32ºN, 157º to 159ºW
(K.L. Smith, 1992). Both areas have water depths
ranging from 5500 to 6100 m, and very sluggish bottom
currents with no evidence of sediment resuspension
(K.L. Smith, 1992) and appear to be representative of
the oligotrophic abyss. These sites were studied, in part,
to explore the feasibility of burying high-level nuclear
wastes within the clay sediments of the North Pacific
central gyre during the SubSeabed Disposal Program of
the United States in the late 1970s and early 1980s.
Sediments at these oligotrophic sites are red clays
(85% of mass consisting of particles <6 mm in diameter) of very low organic-carbon content (typically
~0.25% by weight) studded with manganese nodules
(Fig. 6.6). Net sedimentation rates are extremely low,
with sediments accumulating at ~1 mm yr
−1 . Bottom
waters are well oxygenated (3.7 ml O 2 °
−1 ) and sediment pore-waters typically contain oxygen to tens
of centimeters below the sediment–water interface
(Hessler and Jumars, 1974; personal observations).
Biogenic structures are much rarer at the sediment
surface than in more eutrophic settings, and include occasional holothurian trails and decimeter-scale
mounds formed by echiurans and other unidentified
infaunal megabenthos. The dynamics of these biogenic
structures have not been evaluated in the oligotrophic
Pacific, but by extrapolation from eutrophic and
mesotrophic habitats one may suppose that such
structures likely persist for years.
The known oligotrophic megafauna is characterized
by two components: (1) a very sparse epibenthos
composed mainly of holothurians, cnidarians and
xenophyophores, and (2) highly mobile scavengers.
At the station MPG-I, the epibenthic megafauna is
dominated by the holothurian Amperima sp. feeding
Craig R. SMITH and Amanda W.J. DEMOPOULOS
effects of manganese-nodule mining on abyssal Pacific
communities, a sled 8-m wide with plowshares (the
“plow-harrow”) was towed 78 times through a circular
study area 3.6 km in diameter in 4160 m of water in
the eastern tropical Pacific (~7ºS, 88ºW: Borowski and
Thiel, 1998). The plow-harrow disturbed roughly 20%
of the seafloor within the study area, digging furrows
to roughly 10–15 cm into the sediment. Samples were
collected from disturbed and undisturbed areas of the
seafloor using a box corer within days of plowing,
and then approximately six months and three years
later. Within plowed tracks, macrofaunal abundance
was initially reduced by 39%, the polychaetes being
most heavily disturbed (Borowski and Thiel, 1998).
After three years, the abundance of most higher-level
taxa had returned to the levels in the background
community, but species diversity remained significantly
depressed, indicating a sustained disturbance effect
(Borowski and Thiel, 1998). The vertical distribution
of macrofauna within the sediment also remained
anomalous, apparently because physical and chemical
characteristics had not returned to normal. The unexpectedly rapid recolonization of plow tracks apparently
occurred by lateral migration of benthic individuals
from adjacent unplowed sediments, rather than by
larval settlement. Lateral migration was facilitated by
the relatively small width of individual plow furrows
(~1 m: Borowski and Thiel, 1998). These results
indicate that recovery of the infaunal community from
moderate, relatively small-scale, physical disturbance
in the equatorial abyss requires more than three years
(Borowski and Thiel, 1998). Recovery of the sediment
community from actual nodule mining, which would
disturb much greater areas at higher intensities, is
virtually certain to require much longer time periods –
decades (Borowski and Thiel, 1998).
The oligotrophic abyss
More than 40% of the abyssal seafloor in the Pacific
underlies oligotrophic central gyres, which are the
vast nutrient-poor deserts of the ocean. In the North
Pacific, the central gyre stretches from roughly 15ºN
to 35ºN, and from 135ºE to 135ºW, covering an area
of approximately 2×10
7 km
−2 (Karl, 1999); a similar
gyre is present in the South Pacific. Because of deep
nutriclines and great distances from continental sources
of nutrients (e.g., river outflow and dust), the central
gyres sustain lower rates of primary production than
any other ice-free areas of the ocean (e.g., Berger,
1989). This low productivity, combined with great
water depths (typically 5000 to 6000 m), results in
extremely low flux rates of particulate organic carbon
to the underlying seafloor (typically ~0.3 g C m
−2 y
−1 :
K.L. Smith, 1992). Ecosystem characteristics in these
extraordinarily food-poor habitats differ markedly from
those in the eutrophic deep sea.
Habitat and community description
The benthic ecology of two abyssal sites in the North
Pacific Gyre have been investigated in some detail.
The first is the CLIMAX II region (named after the
CLIMAX II research expedition), which is ~50 km in
diameter and centered on 28º28
N, 155º20
W (Hessler
and Jumars, 1974). The second area, MPG-I (Fig. 6.1),
falls roughly within the box 30º to 32ºN, 157º to 159ºW
(K.L. Smith, 1992). Both areas have water depths
ranging from 5500 to 6100 m, and very sluggish bottom
currents with no evidence of sediment resuspension
(K.L. Smith, 1992) and appear to be representative of
the oligotrophic abyss. These sites were studied, in part,
to explore the feasibility of burying high-level nuclear
wastes within the clay sediments of the North Pacific
central gyre during the SubSeabed Disposal Program of
the United States in the late 1970s and early 1980s.
Sediments at these oligotrophic sites are red clays
(85% of mass consisting of particles <6 mm in diameter) of very low organic-carbon content (typically
~0.25% by weight) studded with manganese nodules
(Fig. 6.6). Net sedimentation rates are extremely low,
with sediments accumulating at ~1 mm yr
−1 . Bottom
waters are well oxygenated (3.7 ml O 2 °
−1 ) and sediment pore-waters typically contain oxygen to tens
of centimeters below the sediment–water interface
(Hessler and Jumars, 1974; personal observations).
Biogenic structures are much rarer at the sediment
surface than in more eutrophic settings, and include occasional holothurian trails and decimeter-scale
mounds formed by echiurans and other unidentified
infaunal megabenthos. The dynamics of these biogenic
structures have not been evaluated in the oligotrophic
Pacific, but by extrapolation from eutrophic and
mesotrophic habitats one may suppose that such
structures likely persist for years.
The known oligotrophic megafauna is characterized
by two components: (1) a very sparse epibenthos
composed mainly of holothurians, cnidarians and
xenophyophores, and (2) highly mobile scavengers.
At the station MPG-I, the epibenthic megafauna is
dominated by the holothurian Amperima sp. feeding