A plankton population model with biomechanical descriptions of biological processes in an idealised 2D ocean basin

A five component pelagic ecosystem model is coupled to a two dimensional configuration of the Princeton Ocean Model (POM), representing an idealised ocean basin with upwelling and downwelling regions. The formulation of the biological equations is based on biomechanical descriptions of the processes of nutrient uptake, light capture, sinking and predator–prey encounter rates. The biological equations have mathematical similarities to existing process-based models which use empirical descriptions of biological processes. These similarities are exploited to determine the planktonic sizes which best correspond to the microzooplankton parameter set in the Edwards et al. (J. Plankton Res. 22 (2000) 1619) modelling study that uses the Franks et al. NPZ model (Mar. Biol. 91 (1986) 121). Simulations show the biomechanical model produces a deep chlorophyll maximum (DCM) when a stable surface mixed layer is present, and a surface bloom during wind-driven coastal upwelling. The Franks biological model is coupled to the physical configuration used for the biomechanical model, and the output from the two models compared in the coastal upwelling region. The behaviour of the biomechanical model is further investigated by undertaking supplementary simulations with the biological parameter values determined (1) using size-based relationships only, (2) using size-based relationships without sinking of phytoplankton and zooplankton, (3) by doubling the cell radii. These simulations provide a preliminary assessment of the biomechanical, size-based approach, and shed light on physical processes at the scale of individual planktonic cells that are determining the rates of biological processes.