Wind strength and biological productivity in upwelling systems: an idealized study

Biological production in wind-driven coastal upwelling systems is of global importance. To understand better how wind speed and shelf width affect productivity, we formulated a simple model of cross-shelf transport that included the dynamics of one potential mechanism: increasing nutrients with increasing winds, with possible loss of productivity due to off-shelf Ekman transport. We used this model to determine how shelf productivity would depend on wind speed and shelf width using several ecosystem ( Nutrient - Phytoplankton - Zooplankton, NPZ) models that reflect different trophic pathways and differences in model stability. For constant wind speeds, shelf productivity for all of the NPZ models declined beyond a certain wind speed. The shelf transit time beyond which productivity declines can be determined graphically from the time course of cumulative production in the NPZ model. Shelf width and latitude then determine the corresponding wind speed. The predicted points of decline in productivity with windspeed were consistent with data from local observations of higher trophic levels on the central California coast. Only shelf productivity of the NPZ model reflecting new production was dome-shaped. Shelf productivity of models of regenerated production remained constant with wind speed at low winds. Artificial instability in NPZ models can lead to an inaccurate dependence of productivity on wind. Our model of coastal upwelling, which represents production as cross-shelf transport of parcels containing developing NPZ models, exhibits a different dependence on wind forcing than previous models that represented upwelling production as a fully mixed shelf. The relative degree of mixing versus transport in upwelling systems is a critical factor underlying the dependence of productivity on wind.