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

Chlorophyll-a (chi-a) distribution and associated physical (temperature, sa linity) and chemical (dissolved oxygen) conditions off northern Chile (Humb oldt Current System), during the austral summer (February-March) and autumn (May) of 1994, were studied in the region bounded by similar to 18-24 degr eesS and 70-72 degreesW (out to similar to 200 km from the coast; 0-100 m d epth); within this region, nutrients were measured in an area of persistent coastal upwelling (similar to 19-22 degreesS, out to 80 km from the coast) . Temperature and salinity distributions, as well as nutrient concentration s, indicated the occurrence of active upwelling during both cruises. Also, and together with maps of geopotential anomaly (0/200 dbar) and depth of th e thermocline (15 degreesC isotherm), their distribution suggested the pres ence of a mainly equatorward flow, anticyclonic eddy-like structures, and i ntrusions of warm (> 19 degreesC), high salinity (> 35.0 psu), subtropical water towards the coast. A tongue of cooler and lower salinity water, and o f lower flow fields, extended from the coast towards the offshore zone duri ng both sampling periods, in association with higher chl-a concentrations ( >1 mg m(-3), > 20 mg m(-2) between 0 and 25 in depth) and predominance of n etphytoplankton (> 20 mum). The comparison of these results with those for the winter and spring of 1993 in the same area suggest a relatively weak se asonal signal in chl-a concentration during the 1993-1994 period, with high er 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.