Mass balance models of Ekman transport and nutrient fluxes in coastal upwelling zones

The nutrient cycles of coastal upwelling zones are studied with simple mass balance models of Ekman transport, longshore transport, surface productivity, and dissolved phosphorous. The models are constrained with data from the Peru, northwest Africa, and Oregon upwelling systems. The onshore-offshore mass balance model agrees with published Ekman transport, surface productivity, and nutrient data as well as hypothesized nutrient f-ratios for highly productive coastal settings. The onshore-offshore model suggests that increased primary productivity in glacial-era coastal upwelling zones was not a linear function of Ekman transport, but instead was probably dependent on the physical and chemical dynamics of a specific setting. In the Peru upwelling system, longshore quatorward surface currents and poleward undercurrents produce positive surface nutrient gradients in the equatorward direction and relatively constant gradients in subsurface waters. Longshore nutrient gradients off northwest Africa are positive in the equatorward irection for both surface and subsurface waters. These observations are consistent with the conceptual model of surface and subsurface currents which are moving toward the equator and continually being upgraded by the offshore flux of nutrients. The northwest Africa and Peru data are not consistent with the longshore nutrient model of Redfield et al. (1963).