Oceanic upwelling and productivity in the eastern tropical Pacific

An oceanographic survey of the eastern tropical Pacific Ocean in August-November 1990 found a productive, nutrient-rich, moderately high-chlorophyll surface layer in two oceanic upwelling regions: the equatorial divergence, especially east of the Galapagos, and the countercurrent divergence out to 105 degree W, > 1,000 km west of the Costa Rica Dome. Although NO3 is not depleted in upwelling regions, relationships among nutrient concentrations and temperature in 1986-1988 data from the same area show that NO3 is the first macronutrient to be depleted in adjacent, less-productive regions. A three-dimensional, two-layer box model of NO3 flux within and into the euphotic zone gives estimated rates of new production that are ∼29% of measured rates of 14C phytoplankton production. Persistence of excess NO3 in the euphotic zone exceeds 1 yr under high-nutrient, low-chlorophyll conditions off the equator where weak upwelling, or downwelling, occurs. These results indicate substantial control or limitation of NO3 utilization and productivity in nutrient-rich oceanic regions of the eastern tropical Pacific