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

Abstract

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 μm. Trophic transfers of carbon in the models take place chiefly via one or two steps, after 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 vary 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. © 1992 Taylor & Francis Group, LLC.