Latitudinal distribution of microbial plankton abundance, production, and respiration in the Equatorial Atlantic in autumn 2000

Abstract

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.

Introduction

The determination of the role of the tropical open ocean in the global carbon cycle has been the objective of intense research during the past decade (e.g. Longhurst, 1993; Murray et al., 1994; Le Borgne et al., 2002). Much of our understanding of the relevant biological processes in these areas derives from studies conducted in the Pacific Ocean, mainly related to its contribution to ocean–atmosphere CO₂ fluxes or to its response to the interannual variability forced by the El Niño Southern Oscillation (ENSO) (e.g. Murray et al., 1995; Barber et al., 1996; Chavez et al., 1999). Comparatively, much less attention has been paid to the investigation of biologically mediated carbon cycling in the Equatorial Atlantic.

The Equatorial Atlantic, frequently divided into two different biogeographic provinces (Longhurst, 1998), the Eastern Tropical Atlantic (ETRA) and the Western Tropical Atlantic (WTRA), displays a banded current structure resulting from the trade winds regime. The main westward flows at the surface are the North Equatorial Current (NEC; 10°–15°N) and the South Equatorial Current (SEC; 3°N–15°S), balanced by three eastward currents: the highly seasonal North Equatorial Countercurrent (NECC), located at the surface between 3° and 10°N, the South Equatorial Countercurrent (SECC), also superficial, weak and variable, and the Equatorial Undercurrent (EU), centred at the Equator at ca. 100 m depth (Tomczak and Godfrey, 1994). The different direction of the Ekman transport on both sides of the Equator causes a divergence southwards of the Equator bringing into the surface layer relatively nutrient-rich water from the EU. This results in an enhancement of phytoplankton biomass, which is permanently visible, albeit with seasonally changing intensity, in global maps of surface ocean chlorophyll (e.g. Signorini et al., 1999).

A characteristic feature of the tropical and subtropical Atlantic is the presence of a quasi-permanent “Typical Tropical Structure” (TTS; Herbland and Voituriez, 1979), whereby a nutrient-depleted upper mixed layer is separated from a light-limited lower layer by a strong density gradient. A deep chlorophyll maximum (DCM) typically develops at or near the pycnocline depth (e.g. Agustí and Duarte, 1999). The equatorial upwelling can modify this two-layer structure by introducing nutrients into the euphotic layer and causing a shallowing of the DCM (Herbland et al., 1987; Longhurst, 1993; Monger et al., 1997). Phytoplankton biomass and primary production in the equatorial region show a large degree of variability, with averaged C fixation rates ranging from ∼120 to ∼1000 mg C m⁻² d⁻¹ (Minas et al., 1983; Marañón et al., 2001, Marañón et al., 2000; Serret et al., 2001). This variability could derive from short-term changes in the current system (Herbland and Le Bouteiller, 1982; Herbland et al., 1983) and also from interannual differences in atmospheric forcing. The relationship between the equatorial upwelling and enhanced rates of primary production seems to be far from simple. Thus, some studies have reported high levels of primary production and phytoplankton biomass associated with the seasonal advection of subsurface nutrient rich waters (e.g. Bauerfeind, 1987). Other studies, however, found no differences in primary production between the strong upwelling and weak upwelling periods (Oudot and Morin, 1987), and Herbland et al. (1987) suggested that the upwelling only produces an upward displacement of the chlorophyll maximum without increasing the production or biomass of phytoplankton. It is also well known that phytoplankton size structure remains rather constant, dominated by small phytoplankton cells, in spite of the wide range of primary production rates measured in the region (Herbland et al., 1985; Bauerfeind, 1987).

Little is known about other components of the planktonic community in the Equatorial Atlantic. Zubkov et al., 2000, Zubkov et al., 2000a measured heterotrophic bacterial abundances and production during two meridional transects in the Atlantic Ocean. Their results in the equatorial region (10°N–10°S) showed up to two-fold variations in bacterial abundances and production rates, probably related to seasonal variability (Zubkov et al., 2000). Measurements of net community production in this region are also scarce and derive from large-spatial-scale studies as well (González et al., 2002; Serret et al., 2001). González et al. (2002) visited the 10°N–10°S region in Spring and Autumn 1997 and found a balanced or net heterotrophic community metabolism. Serret et al. (2001) found a negative production/respiration balance in June 1998 in the same region, but this study was located closer to the African coast and did not include the equatorial divergence. The reduced number of measurements together with their large variability both in time and space are likely to be responsible for the observed differences.

Studies focused on the quantification of the biomass and distribution patterns of specific microbial compartments and/or fluxes between compartments in the Equatorial Atlantic have been conducted over the past decades (see references above). However, to the best of our knowledge, a comprehensive study based on the concurrent measurement of the biomass of, and fluxes between, the main components of the microbial community has not been attempted previously in this oceanic region. The objective of this investigation was to construct a carbon budget for the microbial plankton of the Equatorial Atlantic. With this aim, we concurrently measured size-fractionated phytoplankton Chl-a and production together with bacterial biomass and production, and microbial oxygen production and respiration along a latitudinal section crossing the eastern Equatorial Atlantic. This data set, collected in contrasting thermohaline environments, allowed further study of the relationships between nutrient supply, phytoplankton community structure and microbial community metabolism in the open ocean.