Primary production studies were conducted during September-October 1990 at Mejillones del Sur Bay. The main objective of this study was to analyze the influence of the upwelling events on the spring primary productivity of the Bay, and the potential rol of atmospheric nitrate. The concentration of nitrate was measure in aerosols at Mejillones; oceanographic conditions were studied in two stations, one in the Bay and the other at Punta Angamos. Coastal upwelling was also studied using hourly wind data from Aeropuerto Cerro Moreno and wind records from the Bay. Results show that the spring dynamic of the primary production of the bay is controlled by the upwelling events occurring at Punta Angamos, the availability of nitrogen is the most important factor. The highest production rate (2342 mg C m-3 hr-1) was measured during the development of a red tide of the autotrophic ciliate Mesodinium rubrum. The nitrate contained in aerosols plays a minor rol in the primary productivity during spring.