Variability of chlorophyll and primary production in the Eastern North Atlantic Subtropical Gyre: potential factors affecting phytoplankton activity

https://doi.org/10.1016/j.dsr.2004.11.007Get rights and content

Abstract

Size-fractionated chlorophyll-a and carbon incorporation rates were determined on a series of 13 cruises carried out from 1992 to 2001with the aim of investigating the patterns and causes of variability in phytoplankton chlorophyll and production in the Eastern North Atlantic Subtropical Gyral Province (NASE). Averaged (±SE) integrated chlorophyll-a concentration and primary production rate were 17±1 mg m−2 and 253±22 mg C m−2 d−1. Small-sized cells (<2 μm) formed the bulk of phytoplankton biomass (71%) and accounted for 54% of total primary production. A clear latitudinal gradient in these variables was not detected. By contrast, large seasonal variability was detected in terms of primary production, although integrated phytoplankton biomass, as estimated from chlorophyll-a concentration, remained rather constant and did not display significant changes with time. Variability in primary production (PP) was related mainly to variability in surface temperature and surface chlorophyll-a concentration. The control exerted by surface temperature was related to nutrient availability. By contrary, euphotic-zone depth, depth of maximum concentration of chlorophyll-a and integrated chlorophyll-a did not contribute significantly to the high variability in primary production observed in this oligotrophic region.

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

References (78)

  • D.M. Karl et al.

    Seasonal and interannual variability in primary production and particle flux at Station ALOHA

    Deep-Sea Research II

    (1996)
  • D.M. Karl et al.

    Long-term changes in plankton community structure and productivity in the North Pacific Subtropical Gyrethe domain shift hypothesis

    Deep-Sea Research II

    (2001)
  • T. Kiørboe

    Turbulence, phytoplankton cell size and the structure of pelagic food webs

    Advances in Marine Biology

    (1993)
  • W.K.W. Li et al.

    Chlorophyll, bacteria and picophytoplankton in ecological provinces of the North Atlantic

    Deep-Sea Research II

    (2001)
  • A.R. Longhurst

    Seasonal cycles of pelagic production and comsumption

    Progress in Oceanography

    (1995)
  • E. Marañón et al.

    Basin-scale variability of phytoplankton from 50° N to 50° S in the Atlantic Ocean

    Deep-Sea Research Part I

    (2000)
  • D.W. Menzel et al.

    The annual cycle of primary production in the Sargasso Sea off Bermuda

    Deep-Sea Research

    (1960)
  • A.F. Michaels et al.

    Overview of the US JGOFS Bermuda Atlantic Time-series study and the Hydrostation S program

    Deep-Sea Research II

    (1996)
  • J.A. Raven

    Responses of aquatic photosynthesis organisms to increased solar UVB

    Journal of Photochemical Photobiology B

    (1991)
  • C. Robinson et al.

    Plankton Respiration in the North and South Eastern Atlantic

    Deep-Sea Research I

    (2002)
  • S. Sathyendranath et al.

    Regionally and seasonally differenciated primary production in the North Atlantic

    Deep-Sea Research I

    (1995)
  • D.K. Steinberg et al.

    Overview of the US JGOFS Bermuda Atlantic Time-series Study (BATS)a decade-scale look at ocean biology and biogeochemistry

    Deep-Sea Research II

    (2001)
  • V.H. Strass et al.

    New production in the summer revealed by the meridional slope of the deep chlorophyll maximum

    Deep-Sea Research

    (1991)
  • N.A.D. Waser et al.

    Geographic variations in 15N natural abundance of surface particulate nitrogen and new production across the North Atlantic Ocean

    Deep-Sea Research I

    (2000)
  • M.V. Zubkov et al.

    Picoplanktonic community structure on an Atlantic transect from 50°N to 50°S

    Deep-Sea Research I

    (1998)
  • N.S.R. Agawin et al.

    Nutrient and temperature control of the contribution of picoplankton biomass and production

    Limnology and Oceanography

    (2000)
  • N.S.R. Agawin et al.

    Nutrient and temperature control of the contribution of picoplankton biomass and production (Errata)

    Limnology and Oceanography

    (2000)
  • S. Agustí et al.

    Food-web structure and elemental (C, N and P) fluxes in the eastern tropical North Atlantic

    Deep-Sea Research II

    (2001)
  • L. Arin et al.

    Phytoplankton size distribution and growth rates in the Alboran Sea (SW Mediterranean)short term variability related to mesoscale hydrodynamics

    Journal of Plankton Research

    (2002)
  • A.R. Baker et al.

    Atmospheric deposition of nutrients to the Atlantic Ocean

    Geophysical Research Letters

    (2003)
  • K. Banse

    Zooplanktonpivotal role in the control of ocean production

    ICES Journal of Marine Science

    (1995)
  • R.T. Barber et al.

    History of the study of plankton productivity

  • E.S. Barbieri et al.

    Experimental assessment of UV effects on temperate marine phytoplankton when exposed to variable radiation regimes

    Limnology and Oceanography

    (2002)
  • M.J. Behrenfeld et al.

    A consumer's guide to phytoplankton primary productivity models

    Limnology and Oceanography

    (1997)
  • P.K. Bienfang et al.

    Temporal and spatial variability of phytoplankton in a subtropical ecosystem

    Limnology and Oceanography

    (1984)
  • K.R. Buck et al.

    Basin-wide distributions of living carbon components and the inverted trophic pyramid of the central gyre of the North Atlantic, summer 1993

    Aquatic Microbial Ecology

    (1996)
  • C.M. Duarte et al.

    Evidence for a heterotrophic subtropical northeast Atlantic

    Limnology and Oceanography

    (2001)
  • S. Emerson et al.

    Experimental determination of the organic carbon flux from open-ocean surface waters

    Nature

    (1997)
  • E. Fernández et al.

    Potential causes for the unequal contribution of picophytoplankton to total biomass and productivity in oligotrophic waters

    Marine Ecology Progress Series

    (2003)
  • Cited by (0)

    1

    Now at Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA, USA

    View full text