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

Abstract

New Production (NP) is limited by NO3−, NH4+, and light (hv). Here we use a model derived from Michaelis-Menten kinetics to calculate NP from euphotic-zone phytoplankton NR activity and hv. NP = − ∂[NO3−]/ ∂t = [NR] ∗ [hv]/(Klt + [hv]) We calculated Peruvian upwelling NP at 15° S (C-Line) during austral fall, March-April-May 1977 from R/V WECOMA data of the Coastal Upwelling Ecosystem Analysis program. NP, at 50% hv, ranged from 1.37 μM C h−1 at the upwelling centre, to 0.15 μMC h−1 9 km downstream, to 0.37 μM C h−1 57 km further downstream over the Peru Trench. It compared well with 14C carbon productivity measurements ranging from 0.29 - 2.65 μMC h−1 and 0.04 - 1.37 μM C h−1 for the 6 h (gross) and 24 h (net) productivity. Oceanographic conditions during April 1977 made the C-Line an ideal site to compare spring 1977 NP with fall 1976 NP data collected by the R/V Eastward. Those 1976 NP values ranged slightly higher (0.15 to 3.49 μM C h−1) than those we found. The surface temperature background at the upwelling-centre in April 1977 reached 16.41 °C whereas in September 1976 it was 14.07 °C. For the C-Line, NO3− stayed above 10 μM, and NH4 + stayed below 0.1 μM. C-Line Chlorophyll, averaging 1.39 μg L−1 in April 1977, was lower than what it was for the same stations 6 months earlier (3.85 μg L−1). NR, averaging 0.045 μM h−1 for C-Line stations in April 1977, was a fourth of what it was 6 months earlier in September 1976 (0.20 μM h−1). In conclusion, overall NP for austral fall 1977, in nitrogen units, ranged from 22.6 to 206.8 nM N h−1. In carbon units, median NP in austral fall was only 42% (0.76 μM C h−1) of NP in austral spring (1.82 μM C h−1).