Nitrate use by plankton in the eastern subtropical North Atlantic, March‐April 1989

Nitrate concentration was measured in seawater samples from the euphotic zone at the beginning and end of 12‐h, daytime, in situ incubations. The changes in concentration are considered to be measurements of new production. During periods of 2–3 weeks in March–April 1989, important time scales for NO 3 − input to the euphotic zone (i.e. residence times) and new production were ~26 d at 18°N, 31°W and ~10 d near 33°N, 21°W. The average rate of NO 3 − use in the two areas was 2.63 and 0.62 mmol N m −2 (12 h) −1 , or, in carbon equivalents 209 and 49 mg C m −2 d −1 , respectively. These values bracket the large‐scale estimate by Jenkins of new production in the nearby beta triangle of 150 mg C m −2 d −1 .