A simple numerical model for predicting vertical distribution of phytoplankton on the continental shelf

A simple one-dimensional ecosystem model of phytoplankton production and nitrogen recycling is used to predict the steady-state vertical distribution of phytoplankton on the continental shelf (i.e. water depths <100 m). For an homogenously mixed water-column, close-to-uniform profiles of phytoplankton biomass are simulated in the model at mixing rates greater than 3 × 10−3 m−2 s−1. When mixing is reduced, sub-surface maxima develop either in mid-water or at the seabed depending on the rate of light attenuation. Nitrogen exchange between the water-column and seabed drives a positive feedback in the model between settling velocity and surface phytoplankton productivity. The model is tested against observations made along the Western Australian shelf. Despite the model's simplicity, from 42 profiles in depths between 20 and 100 m, we are able to predict vertical distributions of phytoplankton and nitrate to within 1 standard deviation. The model is shown to reproduce observed cross-shelf gradients in depth integrated quantities.