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At one equatorial fixed position occupied 4 times in 1 yr, 31 in situ uptake experiments were performed with nitrate or ammonium in parallel with carbon fixation incubations. When NO; concentration exceeds 0.1 ymole in seawater, nitrate uptake is strongly related to the chlorophyll a abun4ance. Carbon fixation is also related to chlorophyll a. Hence, chlorophyll a Would be the best index to predict both new and total production. From 5 to 25 % of the phytoplankton nitrogen needs would be satisfied by nitrate uptake according to the chlorophyll abundance. Specific nitrate uptake rate is correlated to both light and temperature, but there is no significant relation between nitrate uptake and NO; concentration, which suggests no nitrate limitation when ambient NO; exceeds about 0.2 ymole. In the mixed layer without detectable nitrate, results suggest only low nitrate assimilation. For high 15N enrichment, the maximum nitrate uptake rate was 0.0016 mmole m-3 h-' on average in such waters. Total nitrate uptake rate in the photis zone ranges from 1 to 5 mmole m-2 d-l. Specific ammonium uptake rate is closely coupled to tracer addition. However, it is generally maximum in subsurface and decreases downwards for a fixed 15N enrichment. Size fractionation experiments suggest that the < 3 pm phytoplankton have a preference for ammonium, and the larger ones a preference for nitrate. Previous studies have shown that the > 3 pm organisms are only relatively numerous in the chlorophyll maximum whereas picoplankton predominates in the mixed layer but also in nitrate rich waters. Consequently, in this region ammonium would be the principal source of nitrogen for natural assemblages of phytoplankton within the whole euphotic layer.

Environmental control of nitrate and ammonium uptake by phytoplankton in the Equatorial Atlantic Ocean