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correspondence with reality in that the size categories were composed of
very different groups. The picoplankton, measuring just a micron or a few
microns, were very small photosynthetic cells. The nanoplankton, up to
about 20 µm, were fairly small phytoplankton cells, some of which were
heterotrophic, feeding on each other. The 20-200-µm range was usually
considered the realm of the microplankton, including both larger
phytoplankton cells and small zooplankton, mostly protistan groups.
Between about 200 µm and 2 millimetres were macroplankton or
mesoplankton, of which copepods were the dominant group among a
number of other things. Above 2 mm were a host of larger plankton such as
krill, euphausids, gelatinous plankton and so on.
Epipelagic ecosystems
He turned next the trophodynamics of epipelagic ecosystems, or
how these systems functioned. Well into the 1960s and 1970s, people
thought that what had been termed the classic ecosystem structure was
fairly universal. However, it turned out to apply primarily to coastal areas,
upwelling zones and regions with high nutrient input. In such places, a
substantial supply of nutrients led to the production of relatively large
phytoplankton that were grazed predominantly by macrozooplankton such
as copepods, and these were then grazed directly by fish such as clupeids,
herrings and anchovies. This was a fairly efficient and short food chain,
dominated by what was called new production. New production was,
basically, production with new nutrients that had entered the system.
Increasingly, however, it had come to be realised that, particularly in areas
such as the open ocean with low nutrient input, this was not the real
structure of the production system. Those areas tended to incorporate what
was known as a microbial loop, in which bacteria regenerated nutrients,
small phytoplankton cells in the pico- to nanoplankton range were the
predominant primary producers, microzooplankton grazed on both the small
phytoplankton and the bacteria, and they in turn were grazed by
macrozooplankton and fish.
If production at the primary level amounted to 100 grams of carbon,
for example, typically about 10-15 g would be produced at the secondary
level, with a loss of about 85-90 percent in going from one step to the next.
Adding a further step below the fish level meant that overall productivity at
that higher level was down by about 90%. This was the system that existed
in the open ocean, which depended predominantly on regenerated
nutrients.
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