The size-based dynamics of plankton food webs. II. Simulations of three contrasting southern Benguela food webs

Abstract

Size-based models are used to simulate planktonic food webs for the euphoric zones of three contrasting southern Benguela ecosystems, coastal stratified waters of the Agulhas Bank, a coastal upwelling area, and an oligotrophic area representative of SE Atlantic oceanic waters. In the models, phytoplankton are divided into four size categories and zooplankton into five All parameters describing flows are derived using body-size relationships Nitrogen is assumed to be limiting in ail three ecosystems, and other physical factors, such as light, are kept constant for simplicity. Initial pulses of new nitrogen in the Agulhas Bank (10 mg-at N m −3 ) and coastal upwelling models (25 mg-at N m −3 ) respectively simulate mixing and upwelling. Continuous inputs of new nitrogen into the Agulhas Bank (0.6 mg-at N m −3 day −1 ) and oceanic models (0 1 mg-at N m −3 day ) simulate turbulent diffusion into the euphotic zone across the nutricline The Agulhas Bank simulations depict a fluctuating standing stock of phytoplankton, dominated by different size classes at different times Large (net-) phytoplankton are important in this model only during periods of enhanced nitrogen supply. The phytoplankton bloom in the upwelling simulation is dominated by nano- and net-phytoplankton. The oceanic simulation depicts a phytoplankton community dominated by small cells (<5 μm), the populations of which remain at a relatively constant biomass of ∼40 mg C m −3 . In all three simulations the smallest autotrophs generally are responsible for the greatest proportion of primary production. The simulated heterotroph communities consist of varying biomasses of different-sized organisms. The Agulhas Bank and coastal upwelling simulations become dominated by micro- and meso-zooplankton, whereas small beterotrophs dominate the oceanic simulation. Different size classes of heterotrophs are important at different times in nitrogen regeneration, although zooflagellates (1–5 μm) were shown to be the most important regenerators on average. In the stratified Agulhas Bank and SE Atlantic oceanic simulations, regenerated production is generally greater than new production. In the Benguela upwelling simulation, new production is replaced by regenerated production as the phytoplankton bloom matures after upwelling and new nitrogen is exhausted. Average f -ratios were greatest (∼0 75) in newly upwelled water, decreasing for coastal stratified (0.25) and oligotrophic oceanic regions (0.14). Simulated model communities compare well with field observations in terms of standing stocks and size composition, suggesting that the model results accurately reflect nature. Up to seven-step ‘food chains’ occur in all three simulated ecosystems, but in practice most of the carbon is transferred via three effective trophic categories Long transfer pathways are important for planktivorous pelagic fish production in the oceanic simulation, short pathways in the coastal upwelling simulation, and intermediate-length pathways in the simulation of stratified coastal waters. Very little (<13%) of total photosynthetically fixed carbon potentially is available to pelagic fish in the simulations, and most carbon is lost through respiration and sinking. Nevertheless, the carbon potentially available to pelagic fish in the upwelling simulation is sufficient to sustain the pelagic fish production measured in the southern Benguela region. A revised food web model of plankton communities depicts the relative importance of different carbon and nitrogen flow pathways in different marine systems.