THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
63
0
20
40
60
80
1 3
44°N
0
4
8
1
10
20
30
28°N
Rank
Percentage contribution
Fig. 3.9. Relative abundances of the 30 most abundant copepod
species in multiple samples collected from the Canary Islands (Roe,
1972) and 44ºN, 13ºW (Roe, 1984). Note that at this scale only four
of the species at 44ºN, 13ºW provided >1% of the total number
sampled and so the other data points do not show up.
(6) The patchiness of pelagic distributions may have
led by chance to the species being under-represented
in the samples.
(7) These may be expatriate species carried in by currents from far beyond their normal distributional
ranges.
(8) Finally, the species may indeed be rare.
Scotto di Carlo’s Mediterranean data also illustrate
how the composition of the assemblages can change
substantially with season. Seasonality is usually more
pronounced at higher latitudes, but it also occurs
in some subtropical and tropical areas, notably in
the Northwest Indian Ocean where the reversal of
the monsoon winds results in major readjustments
to the large-scale circulation and seasonal upwelling
(Tables 3.4 A,B,C).
In benthic ecosystems, species richness is often
estimated from rarefaction curves and generally attains
a maximum at depths of 2–3 km (Rex, 1983; see
also Chapter 10), although both the actual numbers
of species and of specimens observed decline at
depths below 1–2 km. For example, data for benthic polychaetes in the Rockall Trough reported by
Paterson and Lambshead (1995) showed that there
were 80 species present at a mean total population
density of 1828 individuals m
−2 at 1000 m, compared with 44 species at a mean total density of
960 individuals m
−2 at 2875 m depth. Paterson and
Lambshead concluded, based on the rarefaction curves,
that maximum polychaete diversity occurred at depths
Table 3.4A
Copepod species and their relative abundances at two stations in the
north-east Atlantic and in the Mediterranean. Numbers of species
contributing given percentages of the total sample in a combined
data set from four sampling depths 1 at 44ºN 13ºW
Range of %
contribution of
individual species to
total sample
Number of
species
% contribution of
category to the total
population
>1%
3
95.0%
<1 to >0.1%
15
4.1%
<0.1 to >0.01%
26
0.79%
<0.01 to >0.001%
35
0.12%
<0.001%
27
0.01%
1 100, 250, 450 and 600 m; the total number of specimens identified
was 520 115 and the total number of species was 106 (Roe, 1984).
of >2000 m (see Chapter 10). Analysis of repeated
samples of macroplankton at 1000 m at 44ºN 13ºW
showed, for several of the groups analysed, that there
was a steady exponential increase in the numbers
of species caught as the sampling effort increased
(Angel et al., 1982). However, in several of the groups
the total number of species known from the region
had been collected and no further additions to the
species lists were to be expected. For the planktonic
ostracods, all the known species from the region had
been collected once 10
5 specimens had been collected,
and no further species were found despite further
sampling effort increasing the numbers of specimens
to over 15×10
5 (Angel, 1984). This casts considerable
doubt on the effectiveness of the rarefaction method
for estimating the size of the total inventory of
species in an assemblage. In the pelagic environment
there must be a minimum abundance that must be
exceeded if a species is to be able to survive, otherwise
sexual reproduction would not be possible. Even for
phytoplankton different assumptions about the scaling
of species ranges and distributions have been used to
estimate global diversity of phytoplankton species to be
about 5000 (Tett and Barton, 1995). The application
of rarefaction to estimations of total species richness
assumes that spatial and temporal scaling of species
ranges and distributions is consistent; this is certainly
not the case in pelagic environments.
Another factor that may be influencing the biogeography of deeper-living species, and hence the
composition of the communities, is that some of
the deeper-living benthic species have longevities
63
0
20
40
60
80
1 3
44°N
0
4
8
1
10
20
30
28°N
Rank
Percentage contribution
Fig. 3.9. Relative abundances of the 30 most abundant copepod
species in multiple samples collected from the Canary Islands (Roe,
1972) and 44ºN, 13ºW (Roe, 1984). Note that at this scale only four
of the species at 44ºN, 13ºW provided >1% of the total number
sampled and so the other data points do not show up.
(6) The patchiness of pelagic distributions may have
led by chance to the species being under-represented
in the samples.
(7) These may be expatriate species carried in by currents from far beyond their normal distributional
ranges.
(8) Finally, the species may indeed be rare.
Scotto di Carlo’s Mediterranean data also illustrate
how the composition of the assemblages can change
substantially with season. Seasonality is usually more
pronounced at higher latitudes, but it also occurs
in some subtropical and tropical areas, notably in
the Northwest Indian Ocean where the reversal of
the monsoon winds results in major readjustments
to the large-scale circulation and seasonal upwelling
(Tables 3.4 A,B,C).
In benthic ecosystems, species richness is often
estimated from rarefaction curves and generally attains
a maximum at depths of 2–3 km (Rex, 1983; see
also Chapter 10), although both the actual numbers
of species and of specimens observed decline at
depths below 1–2 km. For example, data for benthic polychaetes in the Rockall Trough reported by
Paterson and Lambshead (1995) showed that there
were 80 species present at a mean total population
density of 1828 individuals m
−2 at 1000 m, compared with 44 species at a mean total density of
960 individuals m
−2 at 2875 m depth. Paterson and
Lambshead concluded, based on the rarefaction curves,
that maximum polychaete diversity occurred at depths
Table 3.4A
Copepod species and their relative abundances at two stations in the
north-east Atlantic and in the Mediterranean. Numbers of species
contributing given percentages of the total sample in a combined
data set from four sampling depths 1 at 44ºN 13ºW
Range of %
contribution of
individual species to
total sample
Number of
species
% contribution of
category to the total
population
>1%
3
95.0%
<1 to >0.1%
15
4.1%
<0.1 to >0.01%
26
0.79%
<0.01 to >0.001%
35
0.12%
<0.001%
27
0.01%
1 100, 250, 450 and 600 m; the total number of specimens identified
was 520 115 and the total number of species was 106 (Roe, 1984).
of >2000 m (see Chapter 10). Analysis of repeated
samples of macroplankton at 1000 m at 44ºN 13ºW
showed, for several of the groups analysed, that there
was a steady exponential increase in the numbers
of species caught as the sampling effort increased
(Angel et al., 1982). However, in several of the groups
the total number of species known from the region
had been collected and no further additions to the
species lists were to be expected. For the planktonic
ostracods, all the known species from the region had
been collected once 10
5 specimens had been collected,
and no further species were found despite further
sampling effort increasing the numbers of specimens
to over 15×10
5 (Angel, 1984). This casts considerable
doubt on the effectiveness of the rarefaction method
for estimating the size of the total inventory of
species in an assemblage. In the pelagic environment
there must be a minimum abundance that must be
exceeded if a species is to be able to survive, otherwise
sexual reproduction would not be possible. Even for
phytoplankton different assumptions about the scaling
of species ranges and distributions have been used to
estimate global diversity of phytoplankton species to be
about 5000 (Tett and Barton, 1995). The application
of rarefaction to estimations of total species richness
assumes that spatial and temporal scaling of species
ranges and distributions is consistent; this is certainly
not the case in pelagic environments.
Another factor that may be influencing the biogeography of deeper-living species, and hence the
composition of the communities, is that some of
the deeper-living benthic species have longevities