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

Continuous measurements of temperature, salinity, nutrients, oxygen, partial pressure of CO2, pH, fluorescence of chlorophyll, and light scattering were undertaken during the Paciprod cruise in August 1986. Underway measurements were performed on two tracks perpendicular and parallel to the Peruvian coast near 15-degrees-S, and continuous vertical profiles were obtained on two transects perpendicular to the coast. The upwelling was intense, as indicated by the thickness of the Eckman layer and the large extent of cold water and nutrients. Offshore upwellings were marked by decreases of temperature and increases in the nutrient content of the mixed layer. The surface water was largely undersaturated with oxygen and supersaturated with respect to CO2. Partial pressures of CO2 as high as 1000 muatm was observed at the source of the upwelling. Oxygen-poor waters were found immediately below the thermocline. 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 westward of upward tilts of the isotherms was linked to the circulation. An increase of fluorescence and light scattering accompanied the decrease in nitrate and nitrite concentrations in oxygen-depleted water. Denitrification was lower in the shelf water and in the less saline subantarctic water. The mixed layer indicated a high nutrient-low chlorophyll situation on the whole area studied. Four main surface zones with different features were distinguished along a transect perpendicular to the coast. The fluorescence is relatively high and increases seaward in the first zone situated between the upwelling center and 22 km offshore. This corresponds to the biological conditioning described by MACISAAC et al. (Deep-Sea Research, 32, 503-529, 1985). In the second and third zones, the biomass is low, silicate or nitrate concentrations are probably limiting, and grazing may control production, especially in the third zone where ammonium appears. In the subtropical water, the fourth zone, the biomass is very low and the N/Si/P ratios in sea water are approximately 4/3/1.