FEATURE EQPAC: A PROCESS STUDY IN THE CENTRAL EQUATORIAL PACIFIC By James W. Murray, Margaret W. Leinen, Richard A. Feely, J.R. Toggweiler and Rik Wanninkhof •.. why the equatorial region is not "greener" and more productive. EQPAC IS the United States-Joint Global Ocean Flux Study (US-JGOFS) process study in the cen- tral equatorial Pacific• The first EqPac cruises sailed in January 1992 during a moderately strong E1 Nino. This was fortuitous for our studies of chemical and biological distributions because E1 Nino events are difficult to predict, and the lead time for a project of this size is long. There was virtually no previous upper-water-column chem- ical or biological data for E1 Nino conditions in the central equatorial Pacific. Now an E1 Nino has been studied in considerable detail, and it will be easy to sample the extremes in environmental conditions by sampling non-E1 Nino conditions (including La Nina) in 1993 and the years there- after. The implementation of EqPac illustrates how difficult it is to mount a large-scale interdis- ciplinary study of the ocean when the interannual variability is large. The equatorial Pacific is one of the more-or- less independent ocean "ecosystems" that have 1) characteristic trophic structure, 2) characteristic material cycles, and 3) recognizable boundaries (Barber, 1988). The region has distinct chemical and biological character compared with the trop- ical waters of the relatively barren, oligotrophic central gyres of the north and south Pacific. Up- welling of nutrient-rich water at the equatorial di- vergence in the Pacific supports a highly produc- tive phytoplankton community that contributes significantly to global new production (Chavez and Barber, 1987). Vertical motion is constrained to within a degree of the equator (Halpern and Freitag, 1987), and surface divergence of the up- welled water results in a broader band of nutrient- J.W. Murray, School of Oceanography, University of Washington, Seattle, WA 98195, USA; M.W. Leinen, Graduate School of Oceanography, University of Rhode Island, Narra- gansett, RI 02882-i 197, USA; R.A. Feely, NOAA Pacific Ma- rine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115, USA; J.R. Toggweiler, NOAA Geophysical Fluid Dynamics Laboratory, Princeton University, Princeton, NJ 08542, USA: and R. Wanninkhof, NOAA Atlantic Ocean- ographic and Meterological Laboratory, 4301 Rickenbacker Causeway, Miami, FL 33149, USA. rich water. The resulting nutrient-rich swath straddles the equator and extends across the Pa- cific basin to at least the dateline. Nutrients are the prime factor governing the large-scale pattern of spatial heterogeneity in productivity of the contemporary ocean and thus also the long-term temporal variations observed in the sedimentary record. Until recently primary production in the equa- torial Pacific was thought to be supported chiefly by the upwelling of"new nutrients" (Chavez and Barber, 1987), such as nitrate (NO3) as defined by Dugdale and Goering (1967). The new pro- duction, driven by these new nutrients was thought to be a large proportion of the total pri- mary production. In oligotrophic regions of the ocean, "regenerated production" driven by nu- trients such as ammonia (NH +) and urea is most important. However, the phytoplankton biomass and primary productivity in the equatorial Pacific are not as high as the flux of nutrients could po- tentially support. Primary productivity controls many oceano- graphic biogeochemical processes. Thus, a key to understanding the present and past ocean carbon cycles is to learn the factors that regulate primary and new productivity in this environment and to learn why the equatorial region is not "greener" and more productive (Barber, 1992a). Previous Studies The central equatorial Pacific was chosen for EqPac for two reasons: 1) the processes that con- trol primary and new productivity and 2) scale and global mass balances. Understanding the basic controls or master variables that determine the spatial heterogeneity and magnitude of primary and new productivity is one of the main goals of US-JGOFS (US- JGOFS, 1990). Macronutrients (e.g., NO3) and grazing have long been thought to determine ex- isting large-scale patterns of productivity. Some recent work has shown that the problem is not so simple. For example, Murray et al. (1989) showed that surface NO5 was a poor predictor of the re- 134 OCEANOGRAPHY-Vol. 5, No. 3.1992