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”W, > 1,000 km west of the Costa Rica1 Dome. Although NO, is not depleted in upwelling regions, relationships among nutrient concentrations and temperature in 1986-1988 data from the same area show that NO, is the the first macronutrient to be depleted in adjacent, lessproductive regions. A three-dimensional, two-layer box model of NO, 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 (saturating) NO, 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 NO, utilization and productivity in nutrient-rich oceanic regions of the eastern tropical Pacific. The eastern tropical Pacific contains some of the most productive waters of the world ocean. Concern about atmospheric CO, increase and the role of the deep ocean as a sink in the global C cycle has presented oceanographers with an important question: what are the rates of new production in large, productive areas of the open ocean and what limits those rates? Nutrients are rarely depleted in surface waters of the equatorial Pacific (Thomas 1979) or in other upwelling regions such as the Costa Rica Dome and coastal Peru and Ecuador. The thermocline is shallow in these regions, so that vertical advection and mixing bring cold, nutrient-rich water from below the thermocline (nutricline) into the surface layer. This nutrient input maintains optimal (saturating) concentrations of NO3 at the surface and results in relatively high levels of new production (Chavez and Barber 1987), but not high enough to deplete N03. Persistence of nutrients at the sea surface characterizes the high-nutrient, low-chlorophyll Acknowledgments We thank Stephen Reilly for continued support of this work, and the officers and crew of NOAA Ships McArthur and David Starr Jordan. John Cullen, David Behringer, and two reviewers provided critical comments. (HNLC) condition that is the subject of this symposium (see Cullen 1991). Surface currents and water masses of the eastern tropical Pacific are illustrated schematically in Fig. 1. Both coastal upwelling along eastern boundaries and oceanic upwelling along offshore divergences occur in this region. The effects of equatorial upwelling are evident in the equatorial thermocline ridge (Fig. 2) and in the anomalously cool temperature of equatorial surface water. Sverdrup et al. (1942) presented evidence of surface divergence and upwelling at two sites in the equatorial current system: in tlhe South Equatorial Current along the equator, and between the North Equatorial Countercurrent and North Equatorial Current at lOoN. The countercurrent thermocline ridge along 1O”N is evidence of upwelling in this countercurrent divergence. However, generally warm temperatures and low concentrations of nutrients suggest that the effects of upwelling do not reach the surface except at the eastern end of the ridge in the Costa Rica Dome. Offshore transport of surface waters and coastal upwelling are driven by equatorward, longshore winds along the coasts of Baja California and PeruEcuador and by topographically induced offshore winds at several points along the