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

Abstract

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−3, 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−3) and in August (0.24 mg m−3), 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 depth-integrated 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−1, which represents 16% of the open ocean primary production estimated for the whole Atlantic Ocean.