New production modeled from nitrate reductase activity in the Peru current upwelling

Abstract

We have developed alight-dependent, nitrate and ammonium independent model of New Production based on phytoplankton nitrate reductase (NR) activity that predicts strong New Production off Peru. The model is based on measurements from the Coastal Upwelling Ecosystem Analysis(CUEA) JASON expedition from September 1976. The samples were taken along a transect line (C-Line) across the Peru current at 15°S, which extended from the coast, at position C-1, across the Peru trench to position C-14, 200 km offshore. Sampling depths were established according tolightpenetration; euphotic zone samples were taken at depths where the light was 100, 50, 30, 15, 5, 1 and 0.1% of the surface incident radiation. The model responded to the equation:

=·[ℎ]/67+[ℎ]

which depends on the NR activity [NR] expressed in μM h-1, the amount of light [hv] expressed in % of incident radiation (I0) and the Michaelis-Menten constant(KLT),the amount of light [ℎ] at which half of themaximum NR activity is reached, which took a value of2.4% of I0. The developed model showed that 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 14C carbon productivity measurements whose range was 0-4.2 μM C h−1and 0-1.5 μM C h−1for 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 site in the Peruvian upwelling an ideal one to model new production. Temperature in the center of the upwelling in Septemberof 1976 reached 14.07°C, while NO3−and NO2−ranged from 6.65 to 7.5 and 0.51 to 1.6 μM respectively. Chlorophyll, averaging 3.85 μg L−1for 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−1for 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)