Biological and physical aspects of a coastal upwelling event observed during March-April 1974 off northwest Africa

A description of the time scales of the physical and biological fields of a coastal upwelling event observed during March and April 1974 near 21°40′N off the coast of Mauritania is given from concurrent measurements of wind, current, temperature, salinity,chlorophyll-a, primary productivity, and nutrients. At a mid-shelf site the time lag between the onset of strong, equatorward winds and a significant increase in the offshore component of the near-surface currents was about 1 day. Nutrient-rich surface water was observed near the coast about 4 days after the initiation of the wind event, but because of the rates associated with the biological measurements, the time scale for the occurrence of large quantities of nutrient in the surface layer might have been less than 4 days. Despite the high nutrient concentrations in the nearshore region, the primary productivity remained low until the strength of the winds decreased about 10 days after the event began. The strong winds and offshore currents combined to produce a mixed layer considerably deeper than the euphotic zone. When the intensity of the equatoward winds and the offshore component of the near-surface current decreased, the upper ocean became stratified, the nutrients diminished, and the primary productivity increased. The productivity remained high for 2 to 3 days.