THE PERUVIAN UPWELLING NEAR 15-DEGREES-S IN AUGUST 1986 - RESULTS OF CONTINUOUS MEASUREMENTS OF PHYSICAL AND CHEMICAL-PROPERTIES BETWEEN 0 AND 200 M DEPTH
C. Copinmontegut et P. Raimbault, THE PERUVIAN UPWELLING NEAR 15-DEGREES-S IN AUGUST 1986 - RESULTS OF CONTINUOUS MEASUREMENTS OF PHYSICAL AND CHEMICAL-PROPERTIES BETWEEN 0 AND 200 M DEPTH, Deep-sea research. Part 1. Oceanographic research papers, 41(3), 1994, pp. 439-467
Continuous measurements of temperature, salinity, nutrients, oxygen, p
artial pressure of CO2, pH, fluorescence of chlorophyll, and light sca
ttering were undertaken during the Paciprod cruise in August 1986. Und
erway measurements were performed on two tracks perpendicular and para
llel to the Peruvian coast near 15-degrees-S, and continuous vertical
profiles were obtained on two transects perpendicular to the coast. Th
e upwelling was intense, as indicated by the thickness of the Eckman l
ayer and the large extent of cold water and nutrients. Offshore upwell
ings were marked by decreases of temperature and increases in the nutr
ient content of the mixed layer. The surface water was largely undersa
turated with oxygen and supersaturated with respect to CO2. Partial pr
essures of CO2 as high as 1000 muatm was observed at the source of the
upwelling. Oxygen-poor waters were found immediately below the thermo
cline. Water layers with near zero nitrate concentration show that the
denitrification was intense in the equatorial subsurface water beyond
the shelf. The occurrence of these low nitrate cores situated westwar
d of upward tilts of the isotherms was linked to the circulation. An i
ncrease of fluorescence and light scattering accompanied the decrease
in nitrate and nitrite concentrations in oxygen-depleted water. Denitr
ification was lower in the shelf water and in the less saline subantar
ctic water. The mixed layer indicated a high nutrient-low chlorophyll
situation on the whole area studied. Four main surface zones with diff
erent features were distinguished along a transect perpendicular to th
e coast. The fluorescence is relatively high and increases seaward in
the first zone situated between the upwelling center and 22 km offshor
e. This corresponds to the biological conditioning described by MACISA
AC et al. (Deep-Sea Research, 32, 503-529, 1985). In the second and th
ird zones, the biomass is low, silicate or nitrate concentrations are
probably limiting, and grazing may control production, especially in t
he third zone where ammonium appears. In the subtropical water, the fo
urth zone, the biomass is very low and the N/Si/P ratios in sea water
are approximately 4/3/1.