The Interaction of Physical and Biological Processes in a Model of the Vertical Distribution of Phytoplankton Under Stratification

Abstract A time-dependent adveetion-diffusion model of the vertical distribution of phytoplankton and a single nutrient in a stratified hydrodynamic regime is described. Its properties under summer conditions typical of a temperate shelf sea or a deep ocean situation are analysed. In the shelf case the system tends rapidly to a steady state with a thermocline peak of phytoplankton. The model allows for partial nutrient recycling; the effect of this is found to have a generally antisymmetric relation to that of nutrient uptake by phytoplankton. Increasing recycling increases steady state phytoplankton concentrations but has negligible effect on nutrient, increasing uptake reduces steady-state nutrient but leaves phytoplankton unaltered. As either nutrient uptake or recycling is reduced a tendency to oscillate appears. Explicit relations for steady state concentrations and conditions for oscillations are derived for a simplified model of phytoplankton over an infinite nutrient pool. Increased incident light is found to reduce surface phytoplankton, whose nutrient supply is pre-empted by increased growth in the thermocline. This effect is demonstrated in a simple two-layer model, and the determinants of vertical distribution discussed. The diurnal light cycle is shown to be equivalent to a reduction in mean light intensity. Day-night light variation is only weakly reflected in the phytoplankton but produces around 25% nutrient variation in the mixed layer. Diurnal variation in turbulence due to reduced day time convection has negligible effect on steady state mean levels, but somewhat enhances this day-night nutrient variation.

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