Chlorophyll-a distribution and mesoscale physical processes in upwelling and adjacent oceanic zones off northern Chile (summer–autumn 1994)

Abstract

Chlorophyll-a (chl-a) distribution and associated physical (temperature, salinity) and chemical (dissolved oxygen) conditions off northern Chile (Humboldt Current System), during the austral summer (February–March) and autumn (May) of 1994, were studied in the region bounded by ∼18–24°S and 70–72°W (out to ∼200 km from the coast; 0–100 m depth); within this region, nutrients were measured in an area of persistent coastal upwelling (∼19–22°S, out to 80 km from the coast). Temperature and salinity distributions, as well as nutrient concentrations, indicated the occurrence of active upwelling during both cruises. Also, and together with maps of geopotential anomaly (0/200 dbar) and depth of the thermocline (15°C isotherm), their distribution suggested the presence of a mainly equatorward flow, anticyclonic eddy-like structures, and intrusions of warm (>19°C), high salinity (>35·0 psu), subtropical water towards the coast. A tongue of cooler and lower salinity water, and of lower flow fields, extended from the coast towards the offshore zone during both sampling periods, in association with higher chl-a concentrations (>1 mg m⁻³, >20 mg m⁻² between 0 and 25 m depth) and predominance of net-phytoplankton (>20 μm). The comparison of these results with those for the winter and spring of 1993 in the same area suggest a relatively weak seasonal signal in chl-a concentration during the 1993–1994 period, with higher water column concentrations during the summer and spring periods in the selected upwelling area, though surface chl-a concentrations for the whole of the area did not vary significantly.