THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
69
Offshore, where it is too deep for enough sunlight to
reach the seabed to fuel photosynthesis, phytoplankton
suspended in the near-surface waters is solely responsible for primary production [apart from the estimated
0.03% produced by chemosynthesis at hydrothermal
vents (Chapter 4)]. Phytoplankton cells are small and
their turnover is rapid, so that the standing crop of
plant biomass is small and dilute. Moreover, it has
been estimated that the total global flora of oceanic
pelagic primary producers consists of only about
5000 species (Tett and Barton, 1995) in the oceans, a
very low species richness compared with the estimate
of 250 000 species of green plant on land. Bearing in
mind that a single tree, the sessile oak Quercus robur,
has about 600 species of insects associated with it in the
United Kingdom, the low diversity of phytoplankton
is likely to be reflected in an equally low diversity
of herbivores. Moreover, oceanic herbivores are either
suspension-feeders or individual particle feeders; hence
they too are functionally constrained to being much
smaller than most terrestrial herbivores. Most are small
zooplankton, the only exceptions being found in those
regions (or seasons) where large diatoms, which can
be sieved mechanically out of suspension, are the
dominant primary producers. Thus, in the upwelling
region off Peru/Chile, the dominant fish, the anchovetta
(Engraulis ringens) is able to sieve large diatoms
directly on to its gill rakers. Even so, in the North
Pacific the abundant populations of large copepods,
formerly thought to be herbivores, have now been
shown to be detritivores and do not feed directly on
phytoplankton (Dagg, 1993). In oligotrophic regions,
50 to 80% of primary production is produced by
picoplankton – cells <2mm in diameter. The majority
of the animals that compose the microbial loop and are
able to consume these tiny cells (mostly ciliates) need
to be just as small – too small to generate particles
that will sediment under the influence of gravity. Hence
very little of the organic production that flows through
the microbial food web is exported to deep water, most
being recycled within the euphotic zone.
Another trend is for food webs to be more complex,
particularly at low latitudes, and for the component
chains to be longer. Thus, a greater proportion of
carbon is recycled before it reaches the end-consumers.
Also, although the aqueous medium gives much greater
physical support, which makes it possible for marine
animals to reach much larger body sizes – one may
compare the size of the blue whale with that of an
elephant – the mean individual biomass of metazoa
in the ocean is considerably smaller. Smaller animals
have much shorter generation times, so that the ocean
ecosystems cycle energy and material at a faster rate
than most terrestrial ecosystems.
In addition, most of the organic matter that is
synthesized by the autotrophs is removed very rapidly
by grazing and microbial degradation. This not only
keeps the standing stocks of grazers and detritivores
at much lower levels, but it also means that seldom is
there any build-up of large accumulations of organic
matter, as occurs in freshwater wetlands and forests.
Elton (1935), a terrestrial ecologist, argued that
there are global principles governing the functioning of
ecosystems. He claimed that body size is a fundamental
characteristic – “Animals form food chains in which
the species become progressively larger in size or,
in the case of parasites, smaller in size. A little
consideration will show that size is the main reason
underlying the existence of these food chains . . . . We
have very little information as to the exact relative
sizes of enemies and prey, but future work will
no doubt show that the relation is fairly regular
throughout all animal communities”. The size spectra
and spatial distribution of primary producers in openocean ecosystems are strikingly different from those
in terrestrial and even most shallow-water ecosystems.
Standing crops of plants are quite often much smaller
than the annual primary production. Turnover rates are
high and residence times of organic carbon in oceanic
biomass are much shorter, being about 0.08 years
compared with 11.2 years in terrestrial ecosystems
(Harte, 1988) – a 140-fold difference, which is large
enough to be real, even if the data are imprecise.
The small size and unpredictable occurrence of
phytoplankton in the oceans appears to have inhibited
the evolution of the sorts of specific associations
between animal and plant species that are such a
notable feature of terrestrial ecosystems. Away from
shallow coastal waters, plants are seldom large enough
to provide a physical substratum for the herbivores,
Sargassum weed being the obvious exception. In
addition, autotrophs are almost entirely restricted to
the upper sunlit depths, which constitute a very small
fraction (c. 2.5%) of the total living space within
the oceans. This not only limits the distributional
ranges of herbivorous grazers to the upper waters but
also, because their food is so tiny, most grazers are
constrained physiologically to being quite small in size.
Animals of small size have limited ability to regulate
their vertical ranges. Even those which are large enough
69
Offshore, where it is too deep for enough sunlight to
reach the seabed to fuel photosynthesis, phytoplankton
suspended in the near-surface waters is solely responsible for primary production [apart from the estimated
0.03% produced by chemosynthesis at hydrothermal
vents (Chapter 4)]. Phytoplankton cells are small and
their turnover is rapid, so that the standing crop of
plant biomass is small and dilute. Moreover, it has
been estimated that the total global flora of oceanic
pelagic primary producers consists of only about
5000 species (Tett and Barton, 1995) in the oceans, a
very low species richness compared with the estimate
of 250 000 species of green plant on land. Bearing in
mind that a single tree, the sessile oak Quercus robur,
has about 600 species of insects associated with it in the
United Kingdom, the low diversity of phytoplankton
is likely to be reflected in an equally low diversity
of herbivores. Moreover, oceanic herbivores are either
suspension-feeders or individual particle feeders; hence
they too are functionally constrained to being much
smaller than most terrestrial herbivores. Most are small
zooplankton, the only exceptions being found in those
regions (or seasons) where large diatoms, which can
be sieved mechanically out of suspension, are the
dominant primary producers. Thus, in the upwelling
region off Peru/Chile, the dominant fish, the anchovetta
(Engraulis ringens) is able to sieve large diatoms
directly on to its gill rakers. Even so, in the North
Pacific the abundant populations of large copepods,
formerly thought to be herbivores, have now been
shown to be detritivores and do not feed directly on
phytoplankton (Dagg, 1993). In oligotrophic regions,
50 to 80% of primary production is produced by
picoplankton – cells <2mm in diameter. The majority
of the animals that compose the microbial loop and are
able to consume these tiny cells (mostly ciliates) need
to be just as small – too small to generate particles
that will sediment under the influence of gravity. Hence
very little of the organic production that flows through
the microbial food web is exported to deep water, most
being recycled within the euphotic zone.
Another trend is for food webs to be more complex,
particularly at low latitudes, and for the component
chains to be longer. Thus, a greater proportion of
carbon is recycled before it reaches the end-consumers.
Also, although the aqueous medium gives much greater
physical support, which makes it possible for marine
animals to reach much larger body sizes – one may
compare the size of the blue whale with that of an
elephant – the mean individual biomass of metazoa
in the ocean is considerably smaller. Smaller animals
have much shorter generation times, so that the ocean
ecosystems cycle energy and material at a faster rate
than most terrestrial ecosystems.
In addition, most of the organic matter that is
synthesized by the autotrophs is removed very rapidly
by grazing and microbial degradation. This not only
keeps the standing stocks of grazers and detritivores
at much lower levels, but it also means that seldom is
there any build-up of large accumulations of organic
matter, as occurs in freshwater wetlands and forests.
Elton (1935), a terrestrial ecologist, argued that
there are global principles governing the functioning of
ecosystems. He claimed that body size is a fundamental
characteristic – “Animals form food chains in which
the species become progressively larger in size or,
in the case of parasites, smaller in size. A little
consideration will show that size is the main reason
underlying the existence of these food chains . . . . We
have very little information as to the exact relative
sizes of enemies and prey, but future work will
no doubt show that the relation is fairly regular
throughout all animal communities”. The size spectra
and spatial distribution of primary producers in openocean ecosystems are strikingly different from those
in terrestrial and even most shallow-water ecosystems.
Standing crops of plants are quite often much smaller
than the annual primary production. Turnover rates are
high and residence times of organic carbon in oceanic
biomass are much shorter, being about 0.08 years
compared with 11.2 years in terrestrial ecosystems
(Harte, 1988) – a 140-fold difference, which is large
enough to be real, even if the data are imprecise.
The small size and unpredictable occurrence of
phytoplankton in the oceans appears to have inhibited
the evolution of the sorts of specific associations
between animal and plant species that are such a
notable feature of terrestrial ecosystems. Away from
shallow coastal waters, plants are seldom large enough
to provide a physical substratum for the herbivores,
Sargassum weed being the obvious exception. In
addition, autotrophs are almost entirely restricted to
the upper sunlit depths, which constitute a very small
fraction (c. 2.5%) of the total living space within
the oceans. This not only limits the distributional
ranges of herbivorous grazers to the upper waters but
also, because their food is so tiny, most grazers are
constrained physiologically to being quite small in size.
Animals of small size have limited ability to regulate
their vertical ranges. Even those which are large enough