Simulation studies of trophic flows and nutrient cycles in Benguela upwelling foodwebs

  • Moloney C
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The traditional model of short, efficient food chains in upwelling
regions is revised to incorporate microplanktonic processes and near-exclusive
carnivory by anchovy. The new model is based on analyses of outputs
of simulation models. These include a size-based micro/mesoplankton
model of a Benguela upwelling community and a model of euphausiid
grazing. On average, large-celled phytoplankton are responsible for
only one-quarter to one-third of total primary production, the remainder
occurring by cells < 25 mu m. Trophic transfers of carbon in the
models take place chiefly via one or two steps, after which which
carbon is lost through respiration and the sinking of faecal material.
Trophic efficiencies can be high, generally 40-50 per cent for the
first transfer, 15-20 per cent for the second, and 5-10 per cent
for remaining transfers. A first attempt is made at linking two models
of processes occurring on different scales of time and space. Of
the problems encountered, potentially the most difficult to overcome
appears to be the manner in which feedbacks can be incorporated from
the large to the small scale. C:N ratios of predator and prey are
important in determining C:N ratios of faecal material. In a model
of a euphausiid population, these ratios very from 6 to 17 depending
on whether animal or plant material respectively is being consumed.
This is important in determining rates of carbon export from surface
waters through sinking of faecal pellets. The effect of the mesoscale
physical environment on plankton distributions and transport needs
to be addressed through modelling studies, in order to link planktonic
processes to macrozooplankton- and fish-feeding

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  • C. L. Moloney

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