The effect of packaging of chlorophyll within phytoplankton and light scattering in a coupled physical–biological ocean model

A coupled physical–biological model forced with spectrally resolved solar radiation is used to investigate the effect of packaging of pigment and light scattering on physical and biological properties in the open ocean. Simulations are undertaken with three alternate formulations of vertical attenuation, which consider: (1) chlorophyll as dissolved in the water column; (2) chlorophyll packaged into phytoplankton cells with no scattering; and (3) packaged chlorophyll with scattering. In the coupled model, depth-resolved solar heating depends on the vertical profile of phytoplankton concentration, creating a feedback mechanism between the physical and biological states. The particular scenario investigated is a northerly wind off the coast of south-east Australia. The packaging of chlorophyll approximately halves the attenuation rate of 340–500 nm light and a phytoplankton maximum forms ~10 m deeper than in the dissolved chlorophyll case, with a corresponding adjustment of the dissolved inorganic nitrogen and zooplankton fields. Scattering approximately doubles the vertical attenuation of 340–600 nm light, lifting the phytoplankton maximum by ~10 m when compared with the packaged chlorophyll case. Additionally, strong horizontal gradients in chlorophyll distribution associated with filaments of upwelled water inshore of the East Australian Current, when modelled with alternate formulations of vertical light attenuation, result in circulation changes. The explicit representation of the packaging of pigment and light scattering is worth considering in coupled physical–biological modelling studies.

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