A model of phytoplankton plume formation during variable Oregon upwelling

A time dependent, two-dimensional, marine ecosystem model relates wind events, upwelling, and primary production off the Oregon coast. Model predictions of daily primary production (mg N m__,, day-') increase soon after an intensification of the northerly component of the wind stress. Paradoxically the highest phytoplankton concentration occurs upon relaxation of winds after a major upwelling event. When northerly winds are strong, phytoplankton are supplied with limiting nutrient but the cells experience a short euphotic zone residence time. The phytoplankton are advected offshore and down to light limiting depths by the lower, cyclonically rotating gyre of a two-cell, zonal circulation. After relaxation of the wind, downwelling is not as prevalent and the plants remain longer in the nutrient-rich, lighted zone. The ecosystem dependent variables (phytoplankton nitrogen, zooplankton nitrogen, nitrate, ammonia and detrital nitrogen) are advected by an explicitly modeled flow which is influenced by bottom topography and a variable wind stress. Simulations predict a phytoplankton and detritus plume for which considerable observational evidence exists. It is concluded that during summer, advection by a two-cell, upwelling circulation is the major physical mechanism leading to mesoscale patchiness in the plankton and nutrient fields.

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