THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
53
Shelf
Slope
Rise
Abyssal Plain
0
10
100
1000
3000
6000
Neritic
Bathyal
Abyssal
Hadal
Shallow
Deep
Epipelagic
Mesopelagic
Bathypelagic
Abyssopelagic
Benthopelagic
Depth log m
Fig. 3.4. Schematic representation of the pelagic and benthic zonation in the oceans. Note that the depth scale is plotted logarithmically, and
that the interfaces between the zones are clinal rather than clearly defined boundaries. The depths of interfaces also show local and seasonal
variations, and may also be obscured by dominant hydrographic features such as the edge of the Gulf Stream on the eastern seaboard of
North America.
Biological profiles
The gradients of both chemical and physical characteristics have a strong influence on the distributions of
pelagic organisms. This leads to the development and
maintenance of strong vertical structuring or zonation
in the pelagic assemblages (Fig. 3.4). However, even
where the physical and chemical structuring is welldefined and contains sharp discontinuities, the biological discontinuities are usually quite fuzzy, so the
biological interfaces tend to take the form of zones of
rapid change (or clines) extending over several metres,
rather than sharp boundaries. Moreover, these zones
of change oscillate vertically in space and time in
response to changes in light, physical features like
eddies and fronts, and seasonality. Even so, the vertical
distributions of pelagic communities have a generally
consistent overall pattern of zonation that is common
to most oceans (Fig. 3.4).
The distribution of biomass
By far the greatest part of primary production, the
production of organic matter from carbon dioxide, is
restricted to the upper sun-lit layers of the ocean which
are sufficiently brightly illuminated for photosynthesis
to occur (chemosynthesis at hydrothermal vents is
thought to account for c. 0.3%). Thus, virtually all
life in the ocean is based on primary production in
the upper 100 m or so, some of which is subsequently
exported into deep water. As discussed above, the
sinking organic matter is progressively broken down
by microbial activity and chemical oxidation, and
consumed by deep-living pelagic organisms. Thus, as
the depth increases, the supply of organic material
dwindles. Between the euphotic zone and a depth
of 1000 m the total pelagic standing crop (biomass)
declines by about an order of magnitude (Fig. 3.5);
and between 1000 m and 4000 m it declines by a
further order of magnitude (i.e., to c. 1% of that in
the euphotic zone) (Angel and Baker, 1982). These
decreases reflect the proportions of primary production
reaching the deep ocean as a result of sedimentation of
particles and downward transport by migrating animals
(see pp. 59–60). There are also shifts in average size
of the populations. For example, profiles of ratios of
the standing crops of macroplankton to micronekton
was >10:1 in the euphotic zone in the vicinity of
the Azores Front, but 1:1 at 1000 m depth (e.g.,
Angel, 1989b). Biomasses of benthic communities
show similar exponential decreases with increasing
depth (Rowe, 1983; Lampitt et al., 1986). However,
the exponential decline with depth in the standing crop
of pelagic communities is often obscured by day in
53
Shelf
Slope
Rise
Abyssal Plain
0
10
100
1000
3000
6000
Neritic
Bathyal
Abyssal
Hadal
Shallow
Deep
Epipelagic
Mesopelagic
Bathypelagic
Abyssopelagic
Benthopelagic
Depth log m
Fig. 3.4. Schematic representation of the pelagic and benthic zonation in the oceans. Note that the depth scale is plotted logarithmically, and
that the interfaces between the zones are clinal rather than clearly defined boundaries. The depths of interfaces also show local and seasonal
variations, and may also be obscured by dominant hydrographic features such as the edge of the Gulf Stream on the eastern seaboard of
North America.
Biological profiles
The gradients of both chemical and physical characteristics have a strong influence on the distributions of
pelagic organisms. This leads to the development and
maintenance of strong vertical structuring or zonation
in the pelagic assemblages (Fig. 3.4). However, even
where the physical and chemical structuring is welldefined and contains sharp discontinuities, the biological discontinuities are usually quite fuzzy, so the
biological interfaces tend to take the form of zones of
rapid change (or clines) extending over several metres,
rather than sharp boundaries. Moreover, these zones
of change oscillate vertically in space and time in
response to changes in light, physical features like
eddies and fronts, and seasonality. Even so, the vertical
distributions of pelagic communities have a generally
consistent overall pattern of zonation that is common
to most oceans (Fig. 3.4).
The distribution of biomass
By far the greatest part of primary production, the
production of organic matter from carbon dioxide, is
restricted to the upper sun-lit layers of the ocean which
are sufficiently brightly illuminated for photosynthesis
to occur (chemosynthesis at hydrothermal vents is
thought to account for c. 0.3%). Thus, virtually all
life in the ocean is based on primary production in
the upper 100 m or so, some of which is subsequently
exported into deep water. As discussed above, the
sinking organic matter is progressively broken down
by microbial activity and chemical oxidation, and
consumed by deep-living pelagic organisms. Thus, as
the depth increases, the supply of organic material
dwindles. Between the euphotic zone and a depth
of 1000 m the total pelagic standing crop (biomass)
declines by about an order of magnitude (Fig. 3.5);
and between 1000 m and 4000 m it declines by a
further order of magnitude (i.e., to c. 1% of that in
the euphotic zone) (Angel and Baker, 1982). These
decreases reflect the proportions of primary production
reaching the deep ocean as a result of sedimentation of
particles and downward transport by migrating animals
(see pp. 59–60). There are also shifts in average size
of the populations. For example, profiles of ratios of
the standing crops of macroplankton to micronekton
was >10:1 in the euphotic zone in the vicinity of
the Azores Front, but 1:1 at 1000 m depth (e.g.,
Angel, 1989b). Biomasses of benthic communities
show similar exponential decreases with increasing
depth (Rowe, 1983; Lampitt et al., 1986). However,
the exponential decline with depth in the standing crop
of pelagic communities is often obscured by day in