The plankton quiz: Parameter studies of a NPZD model in a seasonal coastal upwelling region

This work uses a coupled physical-biological NPZD model in the seasonal upwelling system of the Bonney Coast upwelling region, South Australia, in a range of biological parameter studies, dubbed “plankton quiz”. The model is forced by a constant upwelling favourable wind over 3 months. Discussed are findings of experiments considering halving or doubling of either a) the maximum phytoplankton growth rate μ*, b) the maximum zooplankton grazing rate g*, or c) all rate terms of the biological model. An increase in μ* leads to an increase in phytoplankton and zooplankton biomasses, but at reduced rates for larger values due to nutrient limitation on phytoplankton growth. An increase in g* leads to decrease in phytoplankton biomass, but there is an intermediate value for which zooplankton production is at maximum. Except for low values, variation of the magnitude of all rate terms by the same fraction does not significantly change the final phytoplankton and zooplankton biomasses at the end of the upwelling season. Moreover, the findings show that the settings of the biological model control the characteristic timescale of phytoplankton growth; that is, the timespan over which an equilibrium phytoplankton level establishes. From this, the best model setting can be derived from a comparison of exponential growth rates of observed and predicted chlorophyll-a values for selected upwelling events. For the Bonney Coast upwelling region, this corresponds to minimum chlorophyll-a doubling timescale of 5 days, which is equivalent to a growth rate of ∼0.14 day⁻¹.