A model for the growth of shelf-sea phytoplankton in summer

The model describes the steady-state vertical distribution of phytoplankton chlorophyll biomass in a shelf sea in summer as a function of vertical turbulent diffusion, surface photosynthetically effective irradiance, the diffuse attenuation of this irradiance, grazing pressure, and deep-water nutrient flux. It includes equations for nutrient uptake and nutrient-controlled growth, net photosynthesis, and the recycling of nutrient by grazers. It differs from related 'vertical-process' models in using cell-quota, threshold limitations, theory for nutrient- and light-limited algal growth. The model was used to predict the distribution of chlorophyll on a section near the Scilly Isles, England, in July 1979, and in the Sound of Jura, Scotland, in July 1983. The former is a mainly stratified region into which is intruded island mixing; the latter is a predominantly mixed strait with a small stratified area. Nitrogen was taken as the limiting nutrient, and the values of most parameters were obtained either from the literature or from independent observations. The model's predictions were particularly sensitive to values chosen for photosynthetic fixation per unit irradiance at low illuminations, nutrient subsistence quota, and the chlorophyll-carbon ratio. Prediction was unsuccessful in the case of the Sound of Jura, probably because of the short time taken by residual flows to cross the stratified region. Results for the Scilly Isles show, however, that despite uncertainties in the parameterization of diffusivity, grazing and benthic nutrient flux, this simple, vertical-diffusion driven, model predicts a substantial part of the distribution of phytoplankton in a typical shelf sea in summer, and can clarify the processes determining the growth and production of planktonic algae. Minimal estimates of primary production near the Scilly Isles in summer range from about 160 mg C m^-2 day^-1 in well-stratified water columns to about 700 mg C m^-2 day^-1 in frontal conditions resulting from island mixing.