A lagrangian model of phytoplankton dynamics in the Northwest African coastal upwelling zone

The growth and cross-shelf advection of phytoplankton in a coastal upwelling zone has been modelled using a time-dependent, two-dimensional Lagrangian-particle approach where the velocity and temperature fields were predicted by a three-dimensional, baroclinic hydrodynamic model. Individual phytoplankton cell parcels are tracked as they move offshore during the development of an upwelling episode. To allow for possible sinking of phytoplankton cells, neutrally buoyant nutrient particles are treated separately in the model. Phytoplankton specific growth is assumed to be limited by a combination of available nutrients, light and temperature. The Lagrangian method allows the inclusion of the results of recent drogue experiments on the variation of nutrient uptake rate as phytoplankton cells reach the euphotic zone and adjust their physiological status to the high light regime. The model sensitivity to the strength of wind mixing, characteristics of the upwelled source water and initial nutrient and phytoplankton distributions have been investigated. Model predictions suggest nitrate will be depleted seaward from the shelf break and that nutrients will limit phytoplankton growth in offshore waters. Under favourable wind conditions phytoplankton filaments up to 100 km offshore can be formed.