Deep Sea Research Part I: Oceanographic Research Papers
Variability of chlorophyll and primary production in the Eastern North Atlantic Subtropical Gyre: potential factors affecting phytoplankton activity
Introduction
Determination of time-varying plankton productivity in the world ocean has been one of the main goals of biological oceanography from its beginning in the mid-19th century (Barber and Hilting, 2002). The oligotrophic gyres of the three major oceans account for about a quarter of global primary production (Longhurst, 1995) and contribute up to 50% of global carbon export (Emerson et al., 1997). However, after more than 50 years of measurements, the magnitude of primary production in the oligotrophic ocean gyres is still a matter of controversy. A major problem lies in the low number of observations and the poor coverage of both temporal and spatial scales. Most investigations of phytoplankton variability in tropical and subtropical seas have covered either the temporal or the spatial scale (e.g. Menzel and Ryther, 1960; Bienfang et al., 1984; Frazel and Berberian, 1990; Malone et al., 1993; Karl et al., 1996; Michaels and Knap, 1996; Goerike and Welschmeyer, 1998; among others). However, both spatial and temporal variability must be addressed concurrently in order to gain an adequate understanding of plankton distribution and activity, and of the functioning of pelagic ecosystems.
Because of the stability of the physical environment, tropical and subtropical regions have been traditionally regarded as the least variable oceanic regions in terms of biological activity (Bienfang et al., 1984) as well as the least productive. Accordingly, phytoplankton biomass and primary production would remain nearly constant over both spatial and temporal scales. By contrast, a more dynamic vision of the phytoplankton physiological state at the Subtropical Gyres has emerged over the past two decades (Platt and Harrison, 1985; Goldman, 1988, Goldman, 1993), and as a consequence the investigations in these oligotrophic regions have received a considerable impulse. Several works have shown a certain degree of spatial or temporal variability in primary productivity (e.g. Bienfang et al., 1984; Marañón and Holligan, 1999; Harrison et al., 2001; Marañón et al., 2000, Marañón et al., 2003) and, to a lesser extent, chlorophyll-a concentration (e.g. Goerike and Welschmeyer, 1998). However, the patterns and mechanisms of variability characteristic of these regions still remain poorly understood. Time-series stations in the Sargasso Sea off Bermuda (BATS) and in the subtropical NE-Pacific Ocean (HOTS) have allowed a good description of the temporal variability over both seasonal and interannual scales (Karl and Lukas, 1996; Michaels and Knap, 1996). By contrast, the characterisation of temporal variability in the subtropical NE-Atlantic Ocean and in southern regions of both the Pacific and Atlantic Oceans has been rather limited. Large-scale surveys, aiming at the adequate description of spatial variability are scarce. Some ocean basin-scale studies have revealed a considerable meridional and zonal variability in the N central Pacific Subtropical Gyre (Hayward, 1987) and in the North and South Atlantic (e.g. Frazel and Berberian, 1990; Strass and Woods, 1991; Buck et al., 1996; Vinogradov et al., 1999; Agustí et al., 2001; Harrison et al., 2001; Marañón et al., 2003). In addition to the rather limited number of observations in subtropical regions, some recent studies confirm that traditional sampling strategies tend to underestimate episodic increments of primary production associated with hydrodynamical singularities occurring at relatively short temporal or spatial scales (McGillicuddy et al., 1998; Oschlies and Garçon, 1998; Garçon et al., 2001) or with the atmospheric deposition of iron and nitrogen (Paerl, 1985; Young et al., 1991; Baker et al., 2003). Karl et al. (2003) suggested that under-sampled episodic events of higher primary production would have a profound effect on the estimation of the metabolic balance of the sea, which demands a very careful interpretation of the available information and also, in the near future, an intensive data collection effort (Lewis, 2002).
During the past 12 years intensive investigations carried out by our group have enabled the collection of a valuable database of phytoplankton chlorophyll-a and production in the Eastern North Atlantic Subtropical Gyral province (NASE) across different seasons of the year. This province represents the poleward part of the North Atlantic anticyclonic gyre, which lies under the influence of the westerly winds which are usually weaker than in the provinces further to the north (Sathyendranath et al., 1995; Longhurst, 1998). Considering the large impact that oligotrophic gyral provinces have on the global cycles, our main objective in this work was to investigate the patterns and causes of both latitudinal and seasonal variability in phytoplankton chlorophyll-a and productivity in the NASE province.
Section snippets
Methods
We sampled 82 oligotrophic sites in the eastern North Atlantic Subtropical Gyral (NASE) biogeochemical province (25–44°N) during 13 cruises (CD66, CD83, AMT-1, 2, 3, 4, 5, 6 and 11, Azores-1, Azores-2, Pos273 and Circana-1) carried out from 1992 to 2001 (see Fig. 1 for station locations). The cruises CD66 and CD83 were conducted on board RRS Charles Darwin during March 1992 and December 1993, respectively. AMT cruises were carried out on board RRS James Clark Ross between September 1995 and
Latitudinal and seasonal variability of thermohaline properties
The vertical distribution of temperature and salinity along a latitudinal section, centred around 20 °W, from ∼20°N to ∼44°N (corresponding to sections carried out during AMT cruises) is shown in Fig. 2. Because of the interannual variability observed in the thermohaline conditions along this section (see, e.g. Marañón et al., 2000), we have chosen two representative sections for the spring period and two for autumn conditions.
Upper layer temperature progressively decreased northwards with
Variability patterns of chlorophyll-a concentration
We have shown that surface chl-a concentrations and the depth of the subsurface chl-a maximum are subjected to high latitudinal and seasonal variability (Fig. 3b). The magnitude of the DCM, which remained rather constant in the province (0.2–0.3 mg chl-a m−3), was in good agreement with previous observations in the Subtropical N Atlantic (e.g Li and Harrison, 2001; Steinberg et al., 2001; Lefèvre et al., 2003). The existence of a DCM constitutes a widespread phenomenon in the open ocean. A DCM
Acknowledgments
This work was supported by EU Contract CANIGO (MAS3CT960060), a MEC Grant (MAR981417E), EU Contract CIRCANA (MAR99-1072-01), Contract CARPOS (REN2003-09532-C03-01), and the PML AMT program. E.T. was funded by a PFPI fellowship from the MEC (Spain) and by an EU Marie Curie Individual Fellowship (HPMF-CT-2002-01738). B. M. was supported by an FPU fellowship from the MEC (Spain) and by a postdoc Fullbright-MECD (Spain). V.P. was supported by an FPI fellowship from the MCYT (Spain). We are indebted
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Now at Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA, USA