Bottom–up and top–down forcing in a simple size-structured plankton dynamics model

A size-structured plankton dynamics model is developed and used to explore the effects of variations in bottom–up and top–down forcing upon the biomass spectrum, size-structured patterns in primary production, and the flux of energy from primary producers to fish. Parameters and mechanisms controlling the steady-state model response to bottom–up forcing via nutrient enrichment and top–down forcing via fluctuations in planktivorous fish are first diagnosed. Results are then compared with mean observed biomass spectra from three ecosystems spanning a broad range of productivity. Solutions using parameters within empirical ranges can recreate trends in the biomass spectrum across these systems. The zooplankton gross growth efficiency is critical for matching the steady-state slopes of the spectra. Variability in export sources and zooplankton half-saturation constants both provide ways of matching the mean biomass. Results support the model's potential to provide mechanistic insights and testable quantitative hypotheses for the dynamics underlying observed biomass spectra.