Calculating new production from nitrate reductase activity and light in the Peru current upwelling

Abstract We have calculated new production from phytoplankton nitrate reductase (NR) activity and light in the euphotic zone of the Peruvian upwelling system at 15° S. The calculation is based on unique measurements from the Coastal Upwelling Ecosystem Analysis (CUEA) JASON expedition from September 1976. The new production at the 50% light level in the euphotic zone ranged from 3.49 µM C h −1 , 12 km downstream from the upwelling center to 0.15 µM C h −1 , 46 km further downstream over the 4000 m deep Peru Trench where the upwelling was relatively weak. It compared well with 14 C carbon productivity measurements whose range was 0–4.2 µM C h −1 and 0–1.5 µM C h −1 for the 6 h (gross) and 24 h (net) productivity, respectively. In nitrogen units, the overall new production ranged from 4 to 510 nM of N h −1 . The oceanographic conditions found during September 1976 made this upwelling site an ideal one to calculate new production. Temperature in the center of the upwelling in September of 1976 reached 14.07 °C, while NO 3 − ranged from 6.65 to 7.5 µM, and NH 4 + stayed below 0.1 µM. Chlorophyll, averaging 3.85 µg L −1 for the section stations in September 1976, was similar to what it was for all the stations 6 months later in March 1977 (3.23 µg L −1 ). NR, averaging 0.20 µM N h −1 for the section stations in September 1976, was twice what it was for all stations, 6 month later in March 1977 (0.09 µM N h −1 ).