Chlorophyll- a distribution and associated oceanographic conditions in the upwelling region off northern Chile during the winter and spring 1993

Abstract

The mesoscale distributions of chlorophyll- a (Chl- a) and associated physical and chemical variables off the northern coast of Chile are described using quasi-synoptic data obtained during the austral winter (August–September) and spring (November–December) of 1993. Surface temperature, salinity and chlorophyll- a data were collected in the region bounded by 18–24°S and 70–72°W. An area of persistent coastal upwelling (19–22°S, out to 74 km from the coast) was selected to examine the vertical distribution (0–100 m) of these variables together with dissolved oxygen (O 2) and nutrient concentrations (phosphate, nitrate and nitrite). Surface temperature indicated that the occurrence of coastal upwelling was not well defined during the winter compared to the spring, but the vertical distribution of temperature, density, dissolved Oz and nutrients within the selected area confirmed that there was activity during both seasons. Low dissolved O 2 ( < 0.5 ml 1- −1) concentrations and high nitrite levels ( > 2 μM within the upper 100 m, together with the vertical salinity distribution, suggested that the main subsurface component of the upwelling water within the selected area was water of equatorial subsurface origin. Surface Chl- a distribution in the region was mostly heterogeneous during the winter, with higher concentrations ( > 1 mg m −3) both inshore and offshore, while during the spring these values were confined to within 37 km of the coast, in association with the upwelling band. Surface concentrations, however, were not significantly different between the two seasons over the whole of the region, and maximum values (> 10 mg m −3) were found in the same zone, inshore off Iquique (20°S), on both occasions. Within the selected upwelling area, most of the Chl- a was present in the upper 25 m layer during both seasons, and surface values were correlated with Chl- a integrated over this layer. Water column Chl- a concentrations in this area were significantly higher during the spring, and, during both periods, the highest Chl- a concentrations ( > 2 mg m −3) were located inshore (<37 km from the coast), with maximum values off Iquique. The predominance of net-phytoplankton was mostly associated with the highest total Chl- a concentration, and it increased towards the coast except inshore off Iquique and offshore off Arica-Iquique. These results are discussed in terms of their implications for the overall production of the pelagic system and the important fisheries that it sustains.