SHORT COMMUNICATION Seasonal and interannual variability of chlorophyll a and primary production in the Equatorial Atlantic: in situ and remote sensing observations

The seasonal variability of phytoplankton in the Equatorial Atlantic was analysed using Sea-viewing Wide Field-of-view Sensor (SeaWiFS)-derived chlorophyll a (Chl a) concentration data from 1998 to 2001, together with in situ Chl a and primary production data obtained during seven cruises carried out between 1995 and 2000. Monthly averaged SeaWiFS Chl a distributions were in agreement with previous observations in the Equatorial Atlantic, showing marked differences between 10 W in the Eastern Tropical Atlantic (ETRA) and 25 W in the Western Tropical Atlantic (WTRA) provinces (Longhurst et al. 1995. J. Plankton Res., 17, 1245–1271). The seasonal cycle of SeaWiFS-derived Chl a concentration calculated for 0–10 S, 0–20 W (ETRA) is consistent with in situ Chl a measurements, with values ranging from 0.16 mg m , from February to April, to 0.52 mg m 3 in August. Lower variability was observed in 10 N 10 S, 20–30 W (WTRA) where minimum and maximum concentrations occurred in April (0.15 mg m ) and in August (0.24 mg m ), respectively. A significant empirical relationship between depth-integrated primary production and in situ measured sea surface Chl a was found for ETRA, allowing us to estimate the seasonal cycle of depthintegrated primary production from SeaWiFS-derived Chl a. As for Chl a, this model was verified in a small area of the Eastern Equatorial Atlantic (0–10 S, 0–20 W), although in this instance it was not completely able to describe the magnitude and temporal variability of in situ primary production measurements. The annual euphotic depth-integrated primary production rate estimated for ETRA by our empirical model was 1.4 Gt C year , which represents 16% of the open ocean primary production estimated for the whole Atlantic Ocean.