Phytoplankton and bacterial abundance, size-fractionated phytoplankton chlorophyll-a (Chl-a) and production together with bacterial production, microbial oxygen production and respiration rates were measured along a transect that crossed the Equatorial Atlantic Ocean (10°N-10°S) in September 2000, as part of the Atlantic Meridional Transect 11 (AMT 11) cruise. From 2°N to 5°S, the equatorial divergence resulted in a shallowing of the pycnocline and the presence of relatively high nitrate (>1 μM) concentrations in surface waters. In contrast, a typical tropical structure (TTS) was found near the ends of the transect. Photic zone integrated 14C primary production ranged from ∼200 mg C m-2 d-1 in the TTS region to ∼1300 mg C m-2 d-1 in the equatorial divergence area. In spite of the relatively high primary production rates measured in the equatorial upwelling region, only a moderate rise in phytoplankton biomass was observed as compared to nearby nutrient-depleted areas (22 vs. 18 mg Chl-a m-2, respectively). Picophytoplankton were the main contributors (>60%) to both Chl-a biomass and primary production throughout the region. The equatorial upwelling did not alter the phytoplankton size structure typically found in the tropical open ocean, which suggests a strong top-down control of primary producers by zooplankton. However, the impact of nutrient supply on net microbial community metabolism, integrated over the euphotic layer, was evidenced by an average net microbial community production within the equatorial divergence (1130 mg C m-2 d-1) three-fold larger than net production measured in the TTS region (370 mg C m-2 d-1). The entire region under study showed net autotrophic community metabolism, since respiration accounted on average for 51% of gross primary production integrated over the euphotic layer.
|Original language||English (US)|
|Number of pages||20|
|Journal||Deep-Sea Research Part I: Oceanographic Research Papers|
|State||Published - May 2005|
Bibliographical noteFunding Information:
This study was supported by the UK Natural Environment Research Council through the Atlantic Meridional Transect programme (NER/O/S/2001/00680) and by the Spanish Ministerio de Ciencia y Tecnología (MCyT) through project CIRCANA (MAR1999-1072-C03-01). V.P. was supported by a PFPI fellowship from the MCyT. We are indebted to the captain and crew of the RRS James Clark Ross during the AMT 11 cruise. We acknowledge the collaboration of Begoña Castro during sampling and experimentation with bacteria and the assistance of Jorge Lorenzo in the laboratory work. Comments by three anonymous reviewers helped in improving an earlier version of the manuscript. This is contribution number 86 of the AMT programme.
- Atlantic Meridional Transect
- Equatorial Atlantic
- Primary productivity
ASJC Scopus subject areas
- Aquatic Science