LAUSR

The 2013 U.S. GEOTRACES Eastern Pacific Zonal Transect (EPZT) traversed the highly productive Peruvian coastal upwelling (PCU) region. In this work, the flux of nitrate into the euphotic zone is derived for stations within the PCU using a previously developed method whereby dilution of the water column 7Be inventory by upwelled 7Be-free water provides a means to infer upwelling rates. Furthermore, with knowledge of upwelling rates, 7Be profiles are used to constrain vertical diffusivity within the upper thermocline. These transport terms are applied to nitrate profiles to estimate net community production between 79°W and 104°W along the EPZT, which includes the zone of active upwelling to the edge of the oligotrophic gyre. With a simple, one-dimensional model, the calculated upwelling rates were inversely related to mixed layer temperature and ranged from 0 to 3.0 m/d. Results using a depth-dependent upwelling rate with a component of horizontal advection are also described. Vertical diffusivities near the base of the euphotic zone were in the range 1.7–4.5 × 10−4 m2/s. These values are compared to those generated by analysis of temperature profiles. Net community production averaged 15 mmol C/m2/d for stations between 84°W and 104°W and was 134 mmol C/m2/d for the furthest inshore station at 79°W which displayed the lowest SST and greatest rate of upwelling.