MAP Technical Reports Series No. 106 UNEP

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Because of incomplete transfer from one compartment to the next, the total biomass
(standing crop) in each subsequent compartment should be smaller than that of the preceding
one. The superposition of the compartments forms, what is called, a biomass-pyramid. The
biomass-pyramid of marine systems stretches over about 7 logarithmic size classes, i.e., from
micrometers to decameters. This conceptual scheme is often the basis for food-chain
simulation models.
While this conception is correct within very broad terms, it is incorrect in some details.
First, the structural and functional relationships at the lower trophodynamic levels appears to be
more intricate than depicted in the general scheme given above. The primary production level
is surely the most important energy input compartment in both, terrestrial and aquatic selfsufficient
ecosystems. Yet, the environment in which this compartment is embedded in pelagic
systems, appears to be that of a functionally related network of autotrophic and heterotrophic
activities. Excretion, recycling of organic and inorganic substances, resorption and phagotrophy
is particularly intense at this level. Accordingly, the concept of a sequential relationship, as
depicted in the linear model, has to be supplanted by a more diverse one that includes additional
levels arranged as a net of relationships (for more details cf. e.g., Fenchel, 1988).
Second, at higher levels where the situation seems to be more clear, the unequivocal
allocation of species to well defined categories is not straight forward either. Food preference
of species varies with availability, and in some species can be very complex, depending also on
age. Juvenile stages of some species may be herbivorous, while adults may be carnivorous.
Symbiotic relationships between species belonging to different taxonomic groups are numerous
in the marine environment ranging from loose associations to strictly bonded unions; to mention
e.g., symbiosis between Zoantharia and zooxanthellae. Structurally, then, an ecosystem does
not function as a linear sequence, i.e., as a food-chain but rather as a food-web.
Third, the concept of biomass pyramid of decreasing compartment size may apply to
terrestrial ecosystem, but hardly describes aquatic systems correctly. For oligotrophic, more
or less self-contained oceanic steady state systems it has been estimated that the
compartmental biomass amounts of the logarithmically ordered size class spectra, ranging from
bacteria to whales, are in the same order of magnitude, i.e., 0,1 to 0,01 g/m 3 (Sheldon et al.,
1972; Kerr, 1974; Platt and Denman, 1977). Numbers of individuals/volume in each size class
tend to decrease inversely proportional to body weight.
This model does not necessarily apply to more eutrophic systems. The highly dynamic
nature of such systems, particularly those plankton dominated (high rotation at the primary input
level where seasonal inputs of nutrients cause large population oscillations; phase displacement
between compartments because of increasing mean live-span of the component species in
sequential compartments; top-bottom down control, etc.) produces the most varied
combinations of biomass present in the various compartments, both in time and space.
On the other hand, in order to maintain the system, the total energy input at the primary
production level, integrated over time, must exceed the sequential energy transfers to
successive compartments (energy cascade; Odum, 1971), regardless of the variations of the
momentary compartmental biomass spectra. This condition may not hold, however, if the
system receives a substantial fraction of allochthonous organic material (which either may be
imported e.g., by advection from other areas, or by discharge from land) that can directly be
utilized by species of higher trophodynamic levels.