Phytoplankton abundances and community structure in the equatorial Pacific

Deep-Sea Research II 49 (2002) 2561–2582

Phytoplankton abundances and community structure in the equatorial Pacific D.J. Mackeya,*, J. Blanchotb, H.W. Higginsa, J. Neveuxc a

CSIRO Marine Research, PO Box 1538, Hobart, TAS 7001, Australia b Station Biologique de Roscoff, BP 74, 29682 Roscoff cedex, France c Observatoire Oc!eanologique de Banyuls UMR 7621, 66651 Banyuls-sur-Mer, France Received 6 June 2000; received in revised form 23 October 2000; accepted 1 November 2000

Abstract In both the warm pool region and the high-nutrient low-chlorophyll (HNLC) region of the equatorial Pacific, the * phytoplankton community is dominated by high concentrations of picophytoplankton regardless of whether El Nino, * or normal conditions prevail. Flow cytometry measurements indicate that the picoplankton community La Nina consists of two species of prokaryotes, Prochlorococcus and Synechococcus, and a diverse mixture of picoeukaryotes that cannot be differentiated. Analyses of chlorophyll and carotenoid pigments, determined by HPLC, confirm the importance of Prochlorococcus and Synechococcus and indicate that the eukaryotes are predominantly haptophytes. There are at least three ecotypes of Prochlorococcus and, possibly, various ecotypes of Synechococcus. These ecotypes have quite different pigment ratios and, possibly, pigment to carbon ratios, which complicates comparisons between cell counts by flow cytometry and the contribution that prokaryotes make to total chlorophyll as calculated from pigment * conditions in the warm pool, analysis. Increased productivity is associated with the transient influx of nutrients, El Nino or surface nutrients in HNLC waters. Under these conditions there seems to be a small increase in the relative abundances of Synechococcus, diatoms and chlorophytes. Grazing of picophytoplankton by macro-grazers may make an important contribution to the ‘biological pump’, which may not be dominated by large phytoplankton such as diatoms. r 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction While new production in the equatorial Pacific is a significant fraction of the global carbon cycle, it has little impact on ameliorating the greenhouse effect unless the carbon fixed by photosynthesis can be moved to deep waters. Subduction of these warm, low-density waters is minimal and so the *Corresponding author. Tel.: +61-3-6232-5280; fax: +61-36232-5000. E-mail address: [email protected] (D.J. Mackey).

major route by which carbon is sequestered over timescales of 102–103 years is via the ‘biological pump’ in which biogenic particles sink as dead organisms, faecal matter or detritus. This, in turn, depends on the nature of the photosynthetic organisms in the surface waters, since it is thought that larger phytoplankton, especially diatoms, tend to contribute more to new production, sink faster when the cells die, and form the basis of higher trophic levels and so get more readily incorporated into larger organic particles. Phytoplankton also contribute indirectly to the ocean’s

0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 0 4 8 - 6

2562

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

capacity to ameliorate the build-up of CO2 in the atmosphere since the formation of calcareous organisms (coccolithophores) lowers the alkalinity of surface waters, which decreases the solubility of CO2 in seawater. Across the entire equatorial Pacific, phytoplankton biomass and chlorophyll a concentrations are higher than the mid-latitude gyres of the major oceans but much less than coastal upwelling regions. Early work identified the three major groups of phytoplankton as diatoms, dinoflagellates and coccolithophorids of relatively large size (Hasle, 1959). However, their cell concentrations did not correlate well with chlorophyll a or with auxiliary pigments (Kaczmarska and Fryxell, 1995). In the last twenty years, it has become clear that most of the chlorophyll is due to nanoand picoplankton (Le Bouteiller and Blanchot, 1991) rather than larger organisms such as diatoms or dinoflagellates (Desrosie" res, 1969). Fortuitously, this awareness of the importance of nano- and picoplankton coincided with the start of the equatorial JGOFS program as well as with the development of techniques such as flow cytometry and HPLC with diode array detection, which greatly improved our ability to investigate phytoplankton community structure. The large data sets collected during the JGOFS oceanographic cruises include equatorial transects and long-duration fixed stations. In this paper, we review the current state of knowledge and compare results on phytoplankton abundances determined by a range of experimental techniques, on samples * and La Nina * collected during normal, El Nino events, and ranging from the warm pool to the colder nutrient-rich waters of the eastern Pacific.

2. Oceanography and productivity The northeast and southeast tradewinds drive a shallow upwelling in the equatorial Pacific. In the eastern Pacific, the thermocline and nutricline are sufficiently shallow (30–50 m) that the upwelled waters are cold and rich in nutrients while, in the west, the thermocline and nutricline are much deeper (70–100 m) and the upwelled waters are nutrient-poor, essentially isothermal, and are

indistinguishable from the waters they displace. West of the Galapagos Islands, the equatorial Pacific can therefore be divided into two regions, the warm pool in the west and the slightly colder upwelled waters, or high-nutrient low-chlorophyll (HNLC) waters, in the east. The warm pool is characterised at the surface by high-temperature (>28.51C), low-salinity (So34:5) and low surfacenutrient concentrations (o0.1 mM NO3). The transition from the warm pool to the HNLC waters is characterised by a marked increase in surface salinity (Picaut et al., 1996), an increase in the concentration of surface nutrients, a decrease in temperature and an increase in the surface partial pressure of CO2 (Inoue et al., 1996; Stoens et al., 1999; Rodier et al., 2000; Le Borgne et al., 2002). In this paper, we will generally use the term ‘upwelling’ to refer to ‘cold’, nutrient-rich waters. The isothermal waters in the warm pool contain a number of haloclines and the mixed-layer depth is generally shallower than the top of the thermocline by up to 40 m. The region between the top of the thermocline, and the bottom of the mixed layer is called the ‘barrier layer’ (Lindstrom et al., 1987; Lukas and Lindstrom, 1991), and it essentially isolates the surface waters from the deep chlorophyll maximum (DCM). In the warm pool, when the mixing layer is deep, the nitracline generally starts at the top of the pycnocline while, when it is shallow, the nitracline is clearly below the top of the pycnocline (Mackey et al., 1995; Radenac and Rodier, 1996). In all cases, the nitracline and the DCM occur at around 70–100 m depth. The warm pool is subject to westerly wind bursts caused by the interaction between warm water (>291C) (McPhaden, 1999) and a periodic (30–60 day) atmospheric wave originating in the Indian Ocean called the Madden–Julian Oscillation (Madden and Julian, 1971, 1972). These westerly wind bursts, which are more common during El * conditions, can cause the surface waters Nino along the equator to reverse and flow to the east causing convergence at the equator. However, this is associated with divergence at 3–4 degrees away from the equator and may be a contributing factor to intermittent increased productivity at these latitudes. Westerly wind bursts have been found to increase mixed-layer phytoplankton pigment

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

concentrations by almost a factor of three (Siegel et al., 1995). Because the upwelled waters are generally rich in CO2 as well as nutrients, the equatorial Pacific is the largest oceanic source of CO2 to the atmosphere (Takahashi et al., 1997). Paradoxically, the equatorial Pacific is also a major sink for CO2, and estimates of gross primary production, new production fuelled by upwelled nitrate (calculated from Redfield C:N ratios) and f ratios are consistent with the equatorial Pacific, from 51N to 51S and 901W to 1801, sequestering 0.65–0.98 Gt C year
1 (Chavez et al., 1996). New production is not limited to the nutrient-rich equatorial waters, and in the warm pool the new production is 0.9 versus 2.1 mmol NO3 m
2 d
1 in the central equatorial waters (Stoens et al., 1999). Production due to upwelling in the equatorial Pacific has been estimated to amount to approximately 34% of the global shallow-water upwelling, with the increased supply of nutrients to the euphotic zone supporting approximately 18% of global new production (Chavez and Toggweiler, 1995). This impressive amount of new production is due largely to the enormous expanse of the Pacific Ocean rather than an intrinsic high productivity because even in nutrient-rich upwelled waters, chlorophyll a concentrations are always relatively low. Most of the equatorial Pacific therefore can be classified as a HNLC system (Minas et al., 1986) and, like other such systems, there is now good evidence that new production is limited by the availability of iron (Coale et al., 1996; Gordon et al.,1997; Landry et al., 1997) and, possibly, other trace elements. Tropical instability waves provoke small iron enrichments, which then promote transient enhancement of primary production (Barber et al., 1996; Foley et al., 1997). The location of the front between the warm pool and HNLC waters varies greatly depending * or La Nina * climatic condion whether El Nino tions prevail (Inoue et al., 1996; Picaut et al., 1996; Delcroix and Picaut, 1998). For example, for two transects between 1651E to 1501W, Le Borgne et al. (1999) found that in October 1994 the * phase: warm pool extended to 1721W (El Nino FLUPAC cruise) whereas it was west of 1651E in

2563

* phase: ZONAL FLUX April–May 1996 (La Nina cruise). Inoue et al. (1996) observed that the western boundary of the equatorial upwelling (as defined by high pCO2 values) was correlated with the southern oscillation index (SOI) and ranged from 1601W to 1501E during 1987–1994. This shift in the boundary between the warm pool and HNLC waters leads to a great deal of variability at 1651E, and Radenac and Rodier (1996) discussed the variability in nitrate and chlorophyll under four different regimes (i) normal (warm pool, non* * El Nino), (ii) warm pool (El Nino), (iii) westerly wind burst (a barrier layer and nitrate depleted surface waters but with meridional advection), and * (HNLC). (iv) La Nina

3. Techniques 3.1. Nomenclature Prochlorococcus contains divinyl-chlorophyll a and, sometimes, divinyl-chlorophyll b rather than monovinyl-chlorophyll a or b. In the equatorial Pacific, Prochlorococcus is abundant, and we will therefore refer to monovinyl-chlorophyll a(b) as Chl a1(b1), divinyl-chlorophyll a(b) as Chl a2(b2) and use the abbreviation Chl a(b) for their sum in those cases where the experimental technique cannot distinguish between them. For comparison, all data are normalised assuming that the specific absorption coefficients of Chl a2 (or b2) and Chl a1 (or b1) are the same at their red peak. For spectrofluorometric measurements, Neveux and Lantoine (1993) used the same specific fluorescence coefficients for Chl a2 (or b2) and Chl a1 (or b1) at their respective excitation and emission fluorescence maxima. Their results can be converted to agree with the normalised absorption values by dividing the spectrofluorometric Chl a2 values by 1.18 and multiplying the spectrofluorometric Chl b2 values by 1.73. 3.2. Comparison of techniques For information on phytoplankton community structure there are now a number of techniques

2564

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

that are currently being used and which should be seen as providing complementary information. It is therefore worthwhile to summarise the strengths and weaknesses of the various techniques as well as the difficulties involved in comparing results obtained from different techniques. The most widely used technique for estimating phytoplankton biomass is by the measurement of in situ or extracted chlorophyll. Total in situ stimulated Chl a fluorescence (F) represents an energy loss signal issuing from the total phytoplankton community. It is largely used during field studies to assess Chl a concentration (Lorenzen, 1966) and contributes to estimates of growth rate of phytoplankton (Marra, 1997). However, this signal and the F/Chl a ratio are influenced both by optical settings of the measuring instrument and by species composition through their structural (pigment composition and abundance) and physiological properties (photosynthetic electron transfer capability). Moreover, since the discovery that Chl a2 (rather than Chl a1) is produced by Prochlorococcus, Chl a fluorescence has to be considered, in most parts of the equatorial and tropical oceanic areas, as the sum of fluorescence emitted by Chl a1 and by Chl a2. However, the same weight of each Chl a-type pigment does not necessarily correspond to the same signal in terms of fluorescence. The Chl a2 absorption peak is shifted by about 8 nm towards higher wavelengths with respect to that of Chl a1 and the influence of accessory photosynthetic pigments, which photosensitise Chl a2 fluorescence, is different. The estimation of Chl a concentrations from in situ Chl a fluorescence requires frequent calibration by extraction of the Chl a with 90% acetone or methanol and quantitation of the Chl a by absorption or fluorescence measurements. The concentrations of Chl a in the open ocean are low, and fluorometric methods (Holm-Hansen et al., 1965) have generally been preferred to spectrophotometric ones because of their greater sensitivity. However, problems of interference and the need for a rapid assessment of community structure have led to the development of analytical methods such as spectrofluorometry for chlorophylls (including Chl a2 and Chl b2; Neveux and Lantoine, 1993; Neveux et al., 2002), and high

pressure liquid chromatography (HPLC) for determining a large range of chlorophyll and carotenoid pigments (Mantoura and Llewellyn, 1983; Roy, 1987; Wright et al., 1991; Jeffrey et al., 2000). Identification and quantification of phytoplankton often is assisted by analyses of photosynthetic and photoprotective pigments: several pigments (the so-called ‘marker’ pigments) are restricted to one or two taxa and can be used as indicators for those taxa. Analyses of marine ecosystems by the chromatography of pigments was originally rather qualitative and based on thin-layer chromatography (Jeffrey, 1974; Neveux, 1975; Gieskes et al., 1978). The use of pigments in the identification of phytoplankton classes in seawater has increased in the past decade, mainly due to the development of HPLC analytical techniques (Jeffrey and Hallegraeff, 1980, 1987; Hallegraeff and Jeffrey, 1984; Ridout and Morris, 1985; Klein and Sournia, 1987; Wright et al., 1991). Various algorithms have been used to derive quantitative information on the abundances of various phytoplanktonic classes from the concentrations of carotenoid and chlorophyll pigments (Gieskes and Kraay, 1986; Gieskes et al., 1988; Everitt et al., 1990; Letelier et al., 1993). A more robust method, CHEMTAX, has recently been developed based on factor analysis, does not require unique ‘marker pigments’ for class identification (Mackey et al., 1996, 1998; Wright et al., 1996; Pinckney et al., 1998; Higgins and Mackey, 2000), and is able to estimate the contribution of Prochlorococcus to Chl a even when the HPLC technique cannot resolve Chl a1(b1) from Chl a2(b2) (Mackey et al., 1996). Other photosynthetic pigments, the phycobiliproteins (phycoerythrins, phycocyanins, allophycocyanins) are not solubilised in organic solvents but can also provide qualitative and quantitative information on community structure since they are only represented in three taxa (cyanobacteria, cryptophytes and rhodophytes) and each taxon contains species with different types of phycobiliproteins. In the open oceans, the main types of phycobiliproteins are phycoerythrins (Olson et al., 1988; Lantoine and Neveux, 1997). Their estimation requires extraction by a phosphate buffer or

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

the use of an uncoupler like glycerol (Wyman, 1992; Neveux et al., 1999). The direct counting of cells by conventional microscopy provides the most comprehensive information on the taxa in a sample, but this requires extensive time for sample preparation, counting, and determination of size, especially if statistically valid counts of the less abundant plankton classes are required. Smaller phytoplankton, especially the picoplankton, can be difficult to identify since they lack taxonomically useful external morphological features; yet they are now recognised as being significant contributors to the productivity of oceanic waters (Li et al., 1983; Platt et al., 1983; Ituriaga and Mitchell, 1986; Chavez et al., 1990). In addition, many species are very fragile and do not survive sample fixation (Gieskes and Kraay, 1983). The increased resolution of scanning or transmission electron microscopy allows identification of some picoplankton species, but the sample preparation required renders electron microscopy extremely time-consuming for phytoplankton identification and is impractical to do in largescale surveys. Flow cytometry, on the other hand, gives excellent counting statistics even for picophytoplankton that are too small to be counted by conventional microscopy. It is particularly useful for counting the two populations of cyanobacteria (Prochlorococcus and Synechococcus), but it cannot distinguish the various eukaryotes in the sample, which are therefore counted as a composite population of picoeukaryotes. Selecting the correct threshold for detection together with fluorescent amplification and a calibration technique for sample volume are vital for optimising the resolution of populations and determining the concentration of cells. Factors such as coincidence of particles and interference from fluorescent detritus also may result in inaccuracies of the flow cytometric counts, although these effects should be minimal in equatorial waters. Due to the rapidity and the sensitivity of flow cytometry, it has been possible to use the technique to study not only the structure of picophytoplanktonic populations but also their dynamics (growth rate and mortality rates).

2565

4. Community structure 4.1. In situ chlorophyll fluorescence and size structure In the warm pool, oligotrophic conditions prevail at the surface and there is a deep maximum in the in situ fluorescence (DCM) and in the concentration of Chl a. The shape of the fluorescence profiles, the maximum concentrations of Chl a and the depth-integrated concentrations of Chl a were similar along 1551E under normal conditions * (1990) and during the 1992 El Nino (0.4– 0.45 mg l
1 and 20–30 mg m
2), although the DCM was shallower in 1992 (Mackey et al., 1997). In the HNLC region, the Chl a is more uniformly distributed and is shallower although the depth-integrated concentrations (25– 36 mg m
2) are very similar (Murray et al., 1994) to those of the warm pool. In HNLC waters, a maximum of in situ fluorescence and Chl a develops around 50–70 m during daylight hours, but, with sufficiently strong mixing, the vertical profile may become homogeneous. At 1651E, from 201S to 71N, size fractionation of Chl a showed that the o1 mm fraction represented a mean of 60% of total Chl a in the surface oligotrophic waters and 45% in nitrate-replete waters (DCM and HNLC) confirming the importance of picoplankton (picoeukaryotes, Synechococcus, Prochlorococcus) in equatorial waters (Le Bouteiller et al., 1992). 4.2. Flow cytometry Flow cytometry was instrumental in demonstrating that a large part of the equatorial Pacific, which was previously considered to be an ecological desert with very few cells per liter, was in fact populated by picophytoplankton at typical cell densities of 108 cell l
1. The structure of the picoplankton community in the equatorial Pacific is dominated by two populations of cyanobacteria (Prochlorococcus and Synechococcus) and a composite population of picoeukaryotes. Prochlorococcus was only discovered about a decade ago in the Sargasso sea by the use of on-board flow cytometry (Chisholm et al., 1988, 1992). It was

2566

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

reported a little later in the equatorial Pacific at 01, 1401W (Chavez et al., 1991) and is now considered to be of global significance (Partensky et al., 1999b). In the warm pool, flow cytometry has been used to enumerate picophytoplankton on the equator at 1651E and along an equatorial transect from 1671E to 1501W. The integrated cells concentrations were typically 15  1012, 0.13  1012 and 0.15  1012 cell m
2 for Prochlorococcus, Synechococcus and picoeukaryotes, respectively (Partensky et al., 1999a; Blanchot et al., 2001). By using FLUPAC cruise data from September– October 1994, the average carbon content per cell for samples from the warm pool was found to be 40714 fg cell
1 (Prochlorococcus), 183722 fg cell
1 (Synechococcus) and 16267 167 fg cell
1 (picoeukaryotes), while the calculated percentages of biomass of picophytoplankton were 69%, 3%, and 28% for Prochlorococcus, Synechococcus and picoeukaryotes, respectively (Blanchot et al., 2001). In the HNLC region, on the equator at 1501W, the corresponding integrated cells concentrations ranged up to 12  1012, 0.62  1012 and 0.51  1012 cell m
2 for Prochlorococcus, Synechococcus and picoeukaryotes, respectively (Partensky et al., 1999a; Blanchot et al., 2001), while the average carbon content per cell was found to be 9677 fg cell
1 (Prochlorococcus), 225711 fg cell
1 (Synechococcus) and 1361743 fg cell
1 (picoeukaryotes). The calculated percentages of biomass of picophytoplankton were 58%, 7%, and 35% for Prochlorococcus, Synechococcus and picoeukaryotes, respectively. 4.3. Pigment concentrations There have been few papers on estimates of class structure based on pigments, and these have tended to be restricted to the distribution of Chl a1 and Chl a2. The contribution of Prochlorococcus to the phytoplankton community during 1993 and 1994 along 1751E was estimated from the ratio of Chl a2 to total Chl a values obtained by HPLC analysis (Suzuki et al., 1995; Ishizaka et al., 1997). * conditions of 1993, During the warm-pool El Nino 50% of the total integrated Chl a (and carbon) at

the equator was attributed to Prochlorococcus. Spectrofluorometric analyses, capable of distinguishing between Chl a1 and Chl a2, of samples collected along 1651E (FLUPAC cruise October 1994) showed that the percentage of Chl a2 was higher than 50% in the surface 60 m at the equator, with maximum values being near 70% (Fig. 1). In general, the maximum concentration of Chl a2 was slightly shallower than that of Chl a1. Similar abundances of Prochlorococcus have been reported west of 1701W in October 1994 during a transect along the equator from the warm pool to the equatorial upwelling (Dupouy et al., 1997), along 1751E in August–September 1993 (Suzuki et al., 1995), and in the central Pacific (Bidigare and Ondrusek, 1996; Chavez et al., 1991). Chl b1 is a relatively abundant accessory chlorophyll in the equatorial and tropical Pacific, although its origin is not always obvious. The low concentrations of carotenoids related to chlorophytes and prasinophytes led Bidigare and Ondrusek (1996) to conclude that most of Chl b1 at the surface in the enriched equatorial area probably belonged to Prochlorococcus. Some strains indeed are able to synthesise Chl b1 and Chl b2, the proportion of these two pigments depending on the ambient light intensity (Partensky et al., 1993). However, during the EBENE cruise in the upwelled waters of the equatorial Pacific (01, 1801) where specific carotenoid concentrations of Chl b1-containing eukaryotes were low, spectrofluorometric analysis showed a negative correlation between Chl b1 and Chl a2 at the surface and a highly significant positive correlation between Chl b1 and Chl c (Neveux et al., 2002). Some unknown, or poorly known, population of eukaryotes also could be the source of most of the Chl b1 in upwelled waters (prasinophytes, euglenophytes, chlorophytes, etc). However, size-fractionation in stratified oligotrophic tropical areas (161S, 1501W, Neveux, unpublished) showed a clear link between Chl b1 and Prochlorococcus. The depth variation of this Chl b1 and of Chl b2 seems more associated with photoadaptation (and photoacclimation) processes occurring in different Prochlorococcus subpopulations or ecotypes (having different Chl a to Chl b ratios and, possibly, different carbon to Chl a ratios; Moore and

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

2567

Fig. 1. Concentrations of (a) Chl a2 (mg l
1), (c) Chl a1 (mg l
1) and (e) the ratio of Chl a2 to (Chl a1+Chl a2) found along 1651E in October 1994 and, concentrations of (b) Chl a2 (mg l
1), (d) Chl a1 (mg l
1), and (f) the ratio of Chl a2 to (Chl a1+Chl a2) found along 1501W in November 1994.

Chisholm, 1999; Goericke et al., 2000) than to the biomass of Prochlorococcus. Additional information on the community structure can be obtained by combining pigment analyses with phytoplankton size fractionation on Nuclepore filters. For example, size-fractionation at 161S, 1501W (Neveux, unpublished) showed

that most of the Chl b1 was associated with Chl a2 and was o0.8 mm in size. The Chl a1 concentration was insignificant in this fraction, and so the Chl b1 was due to Prochlorococcus and not to chlorophytes. Mackey et al. (1998) and Higgins and Mackey (2000) measured algal pigments along 1551E

2568

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

* conditions and in the during normal (non-El Nino) * and middle of the prolonged 1991–1993 El Nino found the same suite of pigments. In addition to the chlorophyll pigments, the most abundant carotenoid was zeaxanthin (particularly in the surface waters, but decreasing in concentration just above the DCM), followed by 190 -hexanoyloxyfucoxanthin, 190 -butanoyloxyfucoxanthin and fucoxanthin. Other diagnostic pigments quantified included b,e-carotene, diadinoxanthin, peridinin, violaxanthin, alloxanthin, prasinoxanthin, neoxanthin, and lutein. Low and variable concentrations of antheraxanthin, dinoxanthin, diatoxanthin, and b,b-carotene also were observed. Similar pigments were observed in the warm pool by Everitt et al. (1990) and in HNLC waters by Bidigare and Ondrusek (1996). Higgins and Mackey (2000) found that the dominant warm pool algae on both cruises were haptophytes, Prochlorococcus, and cyanobacteria (Synechococcus) (Fig. 2). Although Prochlorococcus accounted for only about 17% of the total 0–150 m integrated Chl a, the contribution of Prochlorococcus at depth (100–125 m) in the high nitrate waters was 35% in 1990 and 29% in 1992. This is comparable to the contribution (30%) to integrated Chl a (carbon) found by Ishizaka et al. (1997) on the equator at 1751E during the upwelling conditions of 1994 and to the Prochlorococcus carbon contribution of 29% calculated by Blanchot and Rodier (1996) for the transient upwelling at 101S, 1651E. In the surface nutrient depleted waters at 1551E, Prochlorococcus was calculated to contribute only 5–10% of the total Chl a (Figs. 2e and f). In terms of their contribution to total Chl a, CHEMTAX analysis indicated that; (i) cyanobacteria (Synechococcus) were dominant in nutrientdepleted surface waters, (ii) Prochlorococcus were dominant at depths of 100–125 m, and (iii) haptophytes were important throughout the water column. These three algal classes each contributed up to 30–40% of the total Chl a at some depth within the water column and made a significant contribution to the concentration of Chl a at the DCM. The contribution of chrysophytes and dinoflagellates to total Chl a was fairly uniform, particularly at mid-depths, whereas prasinophytes

were localised at the DCM. In 1992 cryptophytes and diatoms were found largely south of the equator, particularly near the DCM between 21S and 51S. In contrast, cryptophytes and diatoms were found largely north of the equator in 1990, particularly near the DCM between 31N and 51N. In both years, these algal classes were found where there was evidence of localised and/or intermittent upwelling (Mackey et al., 1995, 1997). Significant contributions by chlorophytes and chrysophytes were also evident during both cruises. Ishizaka et al. (1997) reported data from a series of cruises along 1751E in September each year from 1990 to 1993 (warm pool) and in April 1994 (HNLC). The upwelling and HNLC status at 1751E was short-lived as it was not observed on the equator at 1751E a few months later during October 1994 (Dupouy et al., 1997; Eldin et al., 1997). During the warm pool conditions of 1993, Prochlorococcus contributed 50% of the total integrated Chl a (and carbon), which dropped to 30% under the equatorial upwelling conditions of April 1994 at 1751E (Ishizaka et al., 1997). From 1993 to April 1994, the integrated carbon biomass (calculations based on microscopy data presented by Ishizaka et al., 1997) of the coccolithophorids also decreased (7–3%), the diatoms did not change (5%), whereas the autotrophic dinoflagellates doubled (8–16%) and the proportion of Synechococcus increased substantially (1–7%). It appears that Synechococcus and the larger dinoflagellates respond fairly rapidly to transient nutrient enrichment. It is surprising, however, that there was no increase in the proportion of diatoms, although this may simply reflect a slower response to nutrient enrichment or, possibly, Si or Fe limitation. During the US JGOFS Equatorial Pacific program (EqPac) at 1401W (from 121S to 121N), Bidigare and Ondrusek (1996) used the pigment algorithms of Letelier et al. (1993) to compare the phytoplanktonic community structure under El * and normal conditions. During the El Nino * Nino phase they found that the contributions of eukaryotic and prokaryotic photoautotrophs were nearly identical all along the transect. It was the same south of 51S and north of 31N during normal conditions. However, around the equator, the

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

2569

Fig. 2. Percentage of Chl a contributed by (a) cyanobacteria (Synechococus), (c) haptophytes, and (e) Prochlorococcus along 1551E in October 1990 and by (b) cyanobacteria (Synechococus), (d) haptophytes, and (f) Prochlorococcus along 1551E in June–July 1992. Percentages deduced from CHEMTAX software and HPLC pigment analysis.

total Chl a biomass was higher during normal * This increase conditions than during the El Nino. was mainly associated with eukaryotic Chl a. In both cases, the main carotenoids associated with the Chl c-containing eukaryotes were 190 hexanoyloxyfucoxanthin (prymnesiophytes) >190 butanoyloxyfucoxanthin (pelagophytes)>fucoxanthin (diatoms). The relative increase in these

pigments during the transition to a normal phase was higher for fucoxanthin than for the two other pigments. This was compatible with the increase of the micro-phytoplankton biomass, which was dominated by the colonial pennate diatom Pseudonitzchia delicatissima, (Iriarte and Fryxell, 1995). A ten-fold increase in diatom concentration also was noted during the equatorial transect of the

2570

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

FLUPAC cruise from the warm pool to the HNLC waters (Blain et al., 1997). Bidigare and Ondrusek (1996) hypothesised that differences in the pigment distributions and phytoplanktonic community structure were related to (i) changes in the depth of the Equatorial Underwater Current core, and (ii) modulation of Tropical Instability Wave activity by the 1991–1992 El Ni*no. During the OLIPAC cruise (November 1994), a transect along 1501W from 151S to 11N showed that the relative contribution of Chl a2 to total Chl a decreased (Fig. 1) from surface nutrient-depleted waters at the south (50–70%) to nutrient-rich equatorial waters (30–40%). Values at the equator were similar to previous observations (Chavez et al., 1991; Bidigare and Ondrusek, 1996). During the same transect, the main carotenoid was 190 hexanoyloxyfucoxanthin, which increased in concentration from 151S to the equator (Cailliau, 1996; Moon-van der Staay et al., 2000).

The ratio of Chl a2 to Chl a1 can be used to estimate the relative importance of Prochlorococcus in the community even if the ratios of Chl a1 and Chl a2 to carbon are not the same. The ratio of Chl b2 to Chl a2 is even more specific since it is clearly related to Prochlorococcus, particularly the low-light adapted strains. In the equatorial Pacific, the maximum of Chl b2 is generally located 10 m deeper than that of Chl a2. However, the maximum value of the ratio of Chl b2 to Chl a2 is deeper still (Figs. 3 and 4). Along 1651E, this maximum reached 3.0–3.3 and was associated with relatively high concentrations of Chl b2 (0.1–0.2 mg l
1). Along 1501W, the ratio was generally lower (o3.0) and associated with relatively low concentrations of Chl b2 (o0.055 mg l
1) except at the equator. The high values at 1651E are probably due to the relative permanence of more stratified waters and of lower dissolved O2 concentrations in the warm pool than in the

Fig. 3. Concentrations of (a) Chl b2 (mg l
1) and (c) the ratio of Chl b2 to Chl a2 found along 1651E in October 1994, and concentrations of (b) Chl b2 (mg l
1), and (d) the ratio of Chl b2 to Chl a2 found along 1501W in November 1994.

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

2571

Fig. 4. Concentrations of (a) Chl b2 (mg l
1) and (b) the ratio of Chl b2 to Chl a2 found along the equator from 1651E to 1701W October–November 1994.

eastern Pacific. However, they were clearly lower than those observed in low-light suboxic environments (0.8–1 mg l
1 for Chl b2 concentration and 3.6–4.6 for the Chl b2/Chl a2 ratio, Goericke et al., 2000). The variations of this ratio essentially reflect photoacclimation processes and are in agreement with calculations showing that, at 1551E as the depth increased from 0–25 m to 125–150 m, the calculated ratio of Chl b to Chl a increased by a factor of 2.1 in 1990 (Mackey et al., 1998) and 2.8 in 1992 (Higgins and Mackey, 2000). 4.4. Growth and mortality The growth of picophytoplankton has been well studied in the equatorial Pacific, and numerous papers allow us to draw some general conclusions. Vaulot et al. (1995) reported the synchronous cell cycling of Prochlorococcus in the field. DNA replication occurs during the afternoon and the division during the first half of the night. The maximum growth rate measured was one doubling per day (Vaulot et al., 1995; Binder et al., 1996). The Prochlorococcus growth rate is very similar in the warm pool and in HNLC waters (Liu et al., 1997) and the mortality rates are balanced by the growth rate, so that cell abundances are rather constant from day to day (Landry et al., 1995, 1997; Latasa et al., 1997). At 51S, 1501W and a depth of 40 m, Vaulot and Marie (1999) reported

that the growth rate of Prochlorococcus was slightly higher than one doubling per day. At 01, 1501W, Andre! et al. (1999) calculated that the integrated values (0–40 m) of the growth rates for Prochlorococcus, Synechococcus and picoeukaryotes were 0.4–0.7, 0.4–0.8 and 0.3–0.5 d
1, respectively, and that the growth rate was balanced by mortality. The diel variability of cell abundances of the three populations was tightly synchronised. Nevertheless some variations of the timing of cells division have been reported. For instance, Vaulot and Marie (1999) reported that Synechococcus divided first at around dusk, Prochlorococcus divided 2 h later, and picoeukaryotes 7 h later. Blanchot et al. (1997) reported that, at the equator, Prochlorococcus and the picoeukaryotes divided at the same time. Whatever the time of division, each population divides synchronously and, in the equatorial Pacific Ocean, at a rate of 0.5–1.0 doublings per day. However, growth rates higher than one doubling per day have been reported in the Indian Ocean (Shalapyonok et al., 1998). 4.5. Grazers and export Little is known about the grazers but they can be divided into two groups, the micro- and the macro-grazers. Micro-grazers have a size smaller than 5 mm and their growth rate is very high

2572

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

(Caron et al., 1999). Picophytoplankton seem to be grazed by flagellates and ciliates, and Christaki et al. (1999) found that a species of ciliate, Strombidium sulcatum, preferentially grazes on Synechococcus. Recently a new micro-grazer, Picophagus flagellatus (E2–3 mm in size) has been isolated from a depth of 15 m at 121S, 1501W, in the transition zone between the equatorial upwelling and the oligotrophic gyre (Guillou et al., 1999b). This micro-predator grazes on cyanobacteria (Prochlorococcus and Synechococcus), has a higher growth rate than Prochlorococcus, and so has the capacity to regulate efficiently the abundance of picoplankton. Macro-grazers belong to the mesozooplankton and have a huge size by comparison to flagellates and ciliates. For instance, the appendicularian Megalocercus huxleyi, which is known to ingest pico- and nanoplankton, has a trunk length of 1–2 mm (Gorsky et al., 1999). At 21N, 1651E, Blanchot and Gorsky (2000) showed that Synechococcus were actively ingested, not digested and compacted into faecal pellets, while other species of similar size were not present in faecal pellets. In laboratory experiments with the species O.ıkopleura dioica, they demonstrated that Prochlorococcus and the nanoeukaryote Isochrysis galbana were efficiently digested whereas Synechococcus transited through the gut without any apparent degradation and aggregated in the faecal pellets (Blanchot and Gorsky, 2000). The general overall relative constancy of phytoplankton abundances, particularly of the smaller size categories, implies that phytoplankton growth and loss processes (grazing and loss through sinking or viral lysis) must be in some sort of a quasi-steady-state relationship. Indeed, Landry et al. (1997), suggested that grazer regulation of small phytoplankton is a general feature of openocean oligotrophic and HNLC waters. However, under certain circumstances, the quasi-steady-state conditions can be disturbed when growth and loss processes get out of phase. In the Iron EX II study in the eastern equatorial Pacific, Landry et al. (2000) observed that iron fertilisation led to both an increase in phytoplankton biomass and an increase in the pigment content per cell (about equal effects). The larger pennate diatoms increased by about 70-fold

over a 6-day period whereas other algal groups only increased about 2-fold. There was little effect on the composition and size structure of the o10 mm autotrophic and heterotrophic protist cells, although there was a slow increase in the heterotrophic bacteria and heterotrophic flagellates as well as an increase in the abundance and biomass of the large protistan consumers (heterotrophic dinoflagellates and cilliates). They suggested that there was a close balance of growth and loss processes for the smaller phytoplankton, whereas the larger pennate diatoms temporarily escaped grazer control, leading to a dramatic increase in biomass during the first 6 days of the experiment before grazer control was re-established. 4.6. Species diversity The diversity of picoplankton is very different for prokaryotes and eukaryotes. Picoplanktonic prokaryotes in open ocean are essentially limited to two genera (Prochlorococcus and Synechococcus) and, although other cyanobacteria have occasionally been observed (Ishizaka et al., 1994; Neveux et al., 1999), their occurrence is unusual and their contribution to biomass is insignificant in the equatorial Pacific. Nevertheless, it has been demonstrated that the genus Prochlorococcus is a mixture of genetically different populations adapted for growth at different light intensities (Moore et al., 1998; Moore and Chisholm, 1999; Goericke et al., 2000; Garczarek et al., 2000). Different lineages of marine Synechococcus also have been reported (Urbach et al., 1998), but the offshore Synechococcus belong mainly to the highphycourobilin fluorescence group (Neveux et al., 1999). The relative contribution of Synechococcus to the phytoplankton biomass is calculated to be small in offshore equatorial and western Pacific waters from the warm pool to 1501W (Blanchot et al; 2001). However, such results rely on the carbon cell content being determined for each hydrographic situation. The integrated cell concentrations of Prochlorococcus are rather constant whatever the nutrient availability, in contrast to the integrated abundances of Synechococcus and

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

picoeukaryotes, which increase markedly in the presence of nutrients at the surface. The diversity of picoplanktonic eukaryotes in the equatorial Pacific (and in the open ocean) is much more diverse and still largely unknown. It appears that numerous algal phyla (heterokonts, chlorophytes, prasinophytes, and prymnesiophytes) have picoplanktonic species. In recent years, several new taxa of picoeukaryotes have been described. For instance, a new algal class recently has been described, the Bolidophyceae, with a new species from the intertropical Pacific Bolidomonas pacifica (Guillou et al., 1999a). In the equatorial Pacific, Bolidophyceae contribute o4% to the total concentration of Chl a in the picoplanktonic fraction (Guillou et al., 1999c). While flow cytometry cannot distinguish the classes of picoeukaryotes, HPLC analysis of equatorial samples attributes most of the Chl a from eukaryotes to haptophytes (prymnesiophytes), with smaller contributions from chrysophytes and chlorophytes (Mackey et al., 1998, Higgins and Mackey, 2000). New techniques, based on the analysis 18S rDNA, can be used not only to discover new species, but also to assess the abundance of a group (Moon-van der Staay et al., 2000). This technique suggests that prymnesiophytes are less abundant in equatorial waters than indicated from pigment analyses. However, this may be due to the fact that, in the equatorial Pacific, several prymnesiophyte lineages have no equivalent among cultivated species used for phylogenic analysis based on 18S rDNA (Moonvan der Staay et al., 2000). For the larger phytoplankton, the biodiversity is easier to assess since species can be seen using an optical microscope and can be identified by their morphological characteristics. The biodiversity is relatively high: 224 (Kaczmarska and Fryxell, 1995) and 169 taxa (Iriarte and Fryxell, 1995) were observed at different periods of 1992 at 1401W during cruises related to the EqPac program (essentially Chl c containing eukaryotes: diatoms, dinoflagellates and coccolithophorids). The species are not specific to the equatorial Pacific and have been encountered in other part of the Pacific as well as in other warm waters (Kaczmarska and Fryxell, 1995). The variability

2573

in micro-phytoplankton was related to variations in environmental conditions such as the influence * and Tropical Instability waves (Iriarte of El Nino and Fryxell, 1995). However, as previously noted, microscopic examination is time-consuming and is not adapted to high-frequency sampling.

5. Discussion Apart from the inherent variability of the distribution of phytoplankton (Figs. 1–5), there are a number of other factors that can bias comparisons between individual data sets, including diel variations, cell division, and the possible presence of different ecotypes. Diel variations of Chl a fluorescence are particularly well marked in HNLC waters. At the surface, a 4–5-fold variation in fluorescence occurs, which is much larger than the variations in Chl a concentration. The diel curve is asymmetric with regard to the minimum fluorescence that is observed around noon. This minimum seems to be largely related to nonphotochemical quenching at high irradiance (Kiefer, 1973). At greater depths, a diel variation in intensity also is observed but with a lower amplitude, while below 30 m the time at which the minimum value of fluorescence occurs shifts from noon to dawn (Dandonneau and Neveux, 1997). At these depths, the effect of non-photochemical quenching is more limited and the variations seem to be essentially dependent on cell growth (chlorophyll accumulation in cells) and mortality (grazing). However, recent measurements at hourly intervals (EBENE cruise, October–November 1996) showed that the diel maximum of extracted Chl a generally preceded the fluorescence maximum (Neveux et al., 2002). In the warm pool, diel variations are generally weaker than in the upwelled waters. They are larger at the depth of the maximum in in situ fluorescence than in shallower or deeper waters (Dandonneau and Neveux, 1997) and seem to be largely related to the effects of growth and grazing. The pigment concentration per cell also can be strongly affected by the ambient light intensity (depth), physiological state of the organism, and nutrient status of the waters. In stratified water

2574

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

Fig. 5. Cell densities (106 cells l
1) for (a) Synechococcus, (b) picoeukaryotes, and (c) Prochlorococcus found along 1651E during October 1994.

columns, photoacclimation processes (an increase in cellular light-harvesting pigment concentration with decreasing light) will cause the maximum in a pigment biomass distribution to be deeper than that of the corresponding distribution of algal abundance or carbon biomass. As discussed earlier, there is also a well-established diel variability in all three types of picophytoplankton due to synchronous cell division, and hence the pigment concentration per cell can vary by up to a factor of two in the absence of any other type of forcing. The results in the warm pool reported here suggest that flow cytometry attributes less importance to Synechococcus and more importance to Prochlorococcus, particularly in surface waters, compared to the results from CHEMTAX calculations. Such comparisons have assumed that there

is only one ecotype of Prochlorococcus and Synechococcus, and it is now clear that this is not true. Some of the apparent discrepancy may be due to the presence of different ecotypes of each prokaryote, particularly if the ecotypes have very different pigment ratios or pigment concentrations per cell. In the case of Prochlorococcus, there is a low pigment content isolate that is found in high light environments, and thus the proportion of carbon accounted for by Prochlorococcus in surface waters may be much higher than indicated by the percentage of Chl a. Comparison between estimates of algal class abundances from pigment analyses, and estimates of cell numbers from flow cytometry, therefore can be problematic. As an example, in the western equatorial Pacific, Blanchot and Rodier (1996)

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

reported that Prochlorococcus represented 54% of the biomass of phytoplankton and there was good agreement with spectrofluorometric data (see Fig. 1), while in the same area but during other cruises, Mackey et al. (1998) and Higgins and Mackey (2000) estimated that Prochlorococcus contributed 17% of the total concentration of Chl a. However, the two sets of results actually report totally different things. Blanchot and Rodier measured Prochlorococcus cell numbers by flow cytometry and converted this to a mean Chl a concentration by using a factor of 1.1 fg Chl a cell
1 (range 0.2–6.4 fg Chl a cell
1; note that the range is a factor of 32). They then assumed a factor of 55 g C/g Chl a (Chavez et al., 1996, report a range from o40 to >250) to estimate the contribution of Prochlorococcus to the total particulate carbon integrated over the top 160 m of the water column to arrive at their estimate that Prochlorococcus contributes 54% of the picophytoplankton biomass at the equator. On the other hand, Mackey et al. (1998) estimated the amount of Chl a contributed by Prochlorococcus, i.e. only the first step in the calculations done by Blanchot and Rodier. Comparison between pigment analyses and cell counts by flow cytometry is particularly difficult for Prochlorococcus in surface waters since the fluorescence per cell depends on the ambient light intensity and may be so low that direct counting is not possible, while the low absolute cell numbers and pigment concentrations introduce significant errors into the techniques of flow cytometry and HPLC using spectrophotometric detection. However, the error in Chl a1 and a2 concentrations can be reduced by using spectrofluorometry or HPLC with fluorometric detection, which is capable of discriminating Chl a1 and a2 (Goericke and Repeta 1993). Blanchot and Rodier (1996) reported that their abundances of Prochlorococcus ranged from 29% at 101S to 78% at 141S, and this difference is certainly more significant than the difference between the two methods. In general, temporal or spatial differences reported for algal class abundances or cell counts will be more significant when the same technique is used for the measurements rather than using some conversion factor to compare results measured using different techni-

2575

ques. While there is a clear discrepancy between CHEMTAX and spectrofluorometric analysis in the surface waters of the warm pool, it is not clear whether this is due to inherent differences in the samples or to methodological problems in the determination of the biomass of Prochlorococcus using CHEMTAX in the absence of direct measurements of Chl a2 concentrations. The estimated contribution to Chl a made by Prochlorococcus at 1551E of 5–10% (Fig. 2) is comparable to values found (Neveux, unpublished) from spectrofluorometric measurements of Chl a1 and Chl a2 in the oligotrophic waters of the eastern Mediterranean Sea (Group for Aquatic Productivity cruise, September 1989). At the edge of a semi-permanent warm-core eddy, the ratio of Chl a2 to Chl a ranged from 5–10% in the surface waters (Chl a was 0.025–0.035 mg l
1) to 50–65% at the DCM (Chl a was 0.22–0.25 mg l
1), which was located at 100–110 m. It was even lower at the surface in the centre of the eddy (o5%). The concentration of Chl a2 and the concentration of Chl a2 per cell appeared higher in surface HNLC waters than in the warm pool. If the proportion of the low (Chl a2) pigment content Prochlorococcus ecotype (Moore et al., 1998; Moore and Chisholm, 1999) was higher in the warm pool than in the HNLC waters, particularly at the surface, then this would account for the apparent discrepancy between the low importance attributed to Chl a2 from Prochlorococcus in the surface waters of the warm pool (Fig. 2) and the large number of cells found by flow cytometry (Fig. 5). Nevertheless, the Chl a concentration and the percentage of Chl a2 (50–70%) measured in surface waters by spectrofluorometry along 1651E during the FLUPAC cruise (Fig. 1) were associated with cell numbers (Fig. 5) that were not strongly different from those reported in Blanchot and Rodier (1996) at the same longitude during the SURTROPAC 17 cruise (August–September 1992). 5.1. Oceanographic controls on phytoplankton The vertical profiles of picophytoplankton are directly linked to hydrological conditions and have been discussed in various papers. In both the warm

2576

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

* conditions pool and HNLC waters, La Nina promote an increase in the abundance of Synechococcus and picoeukaryotes (Blanchot et al., 1992; Landry et al., 1996). Prochlorococcus is more abundant when oligotrophic conditions prevail (Blanchot and Rodier, 1996; Landry et al., 1996; Partensky et al., 1999a). In the warm pool, the vertical distributions of the cell abundances of the three picoplanktonic populations appear different. The maximum of Prochlorococcus cells (1–2  108 cell l
1) occurs in the nitrate-depleted layer, whereas the picoeukaryotes maximum (2  106 cell l
1) is deeper and more related to the DCM (Blanchot and Rodier, 1996; Navarette, 1998). Synechococcus has a more regular profile and disappears around 100 m. Its abundance in surface waters seems related to the 0.1 mM nutrient isopleths, the minimum at the equator (o2.5  106 cell l
1) being observed when the 0.1 mM nitrate isopleth was at a depth of 100–110 m. Measurements of phycoerythrins confirmed the presence of Synechococcus (Neveux et al., 1999). In HNLC waters, there is a well-mixed upper layer and all the prokaryotes and picoeukaryotes have similar profiles with homogeneous distributions from the surface to the base of the mixed layer followed by a rapid decrease in cell numbers from the bottom of the mixed layer to 100 m. The transition from the oligotrophic warm pool to the mesotrophic upwelled waters leads to modifications in structure and optical properties of the phytoplankton community (Dupouy et al., 1997). The percentage of Chl a due to Chl a2 decreased due to an increase in the number of Synechococcus (from 8 to 20  106 cell l
1) and eukaryotes (from 3 to 6  106 cell l
1) relative to the abundance of Prochlorococcus, which was approximately the same in the two types of waters. The Chl b2 concentration was very low or absent in surface waters and reached a maximum around 100–120 m depth whatever the region. The intensity of this maximum was higher in the warm pool than in the upwelling region as logically suggested by the relative abundance of Prochlorococcus cells. Both flow cytometry and HPLC analyses suggested that the relative importance of Synechococcus increased under conditions of nutrient

enrichment due to: HNLC waters compared to the warm pool (Partensky et al., 1999a; Blanchot * conditions in the western and et al., 2001); La Nina central equatorial Pacific (Blanchot et al., 1992; Landry et al., 1996); or transient nutrient enrichment due to westerly wind bursts in the warm pool (Mackey et al., 1997, 1998; Higgins and Mackey, 2000). Synechococcus tends to be more abundant in coastal waters than in the open ocean, and it is possible that the high abundance of Synechococcus along 1551E could be due to the influence of Papua New Guinea and the Solomon Islands. Although the larger land masses are about 500 km to the south, waters flowing along the north coast of New Ireland in the New Ireland Coastal Undercurrent (NICU) contribute to about 20% of the flow in the EUC (Butt and Lindstrom, 1994) and may influence the nutrient status of the equatorial waters. Higgins and Mackey (2000) calculated that larger phytoplankton, typical of nutrient-enriched waters, diatoms and chlorophytes, were more abundant in 1992 than in 1990, which is consistent with an increased nutrient supply in the warm pool * conditions. We have assumed that under El Nino fucoxanthin is an indicator of diatoms, but the discovery of fucoxanthin in Bolidophyceae may cast doubt on the relative importance of diatoms. Guillou et al. (1999a, b) showed that Bolidophyceae contributed o4% of the total Chl a in the picoplanktonic fraction in the equatorial Pacific, a concentration that is comparable to the abundance of diatoms (2–3%) estimated by Higgins and Mackey (2000). While the enhanced productivity at 1551E observed in 1992, compared to that in 1990, was attributed to the phytoplankton residing at shallower depths within the water column (Mackey et al., 1998), there also may be a contribution from the intermittent injection of nutrients into the euphotic zone. These effects are opposite to those that occur in the central and * condieastern equatorial Pacific under El Nino tions. However, as prolonged surface equatorial upwelling does not usually extend as far west as * conditions, it is not 1551E, even under La Nina surprising that there was not a large increase in diatoms as is typical of the strong upwelling in the eastern Pacific.

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

Higgins and Mackey (2000) found that the Chl a attributed to chlorophytes almost doubled from 7.8% during 1990 to 14.8% in 1992. The greater proportion of chlorophytes in 1992 was particularly noticeable in the surface waters south of the equator where there was evidence of upwelling. Chlorophytes were also more abundant around 41N during 1990 where there was evidence for transient nutrient enrichment (Mackey et al., 1997). While chlorophytes are fairly ubiquitous, they are often found in great abundance in inshore and coastal waters where inorganic and organic enrichment is high. Chlorophytes may therefore be indicative of increased nutrient status in the euphotic zone of the western equatorial Pacific * conditions. under El Nino It is generally assumed that the export of new production is dominated by the grazing and sinking of large diatoms and dinoflagellates. However, large macro-grazers could play a key role in the equatorial Pacific by efficiently regulating picophytoplankton cell abundances and by removing a sizeable part of the primary production from the microbial food web. Thus, the picophytoplankton could be removed from the euphotic zone and contribute to export to the deep ocean via aggregation in rapidly sinking faecal pellets. In the presence of abundant gelatinous plankton, the ‘biological pump’ in the equatorial Pacific may not be dominated by large phytoplankton.

6. Conclusions While we have learned much about the phytoplankton population of the equatorial Pacific, it is also clear that the finer details of the observed variability in phytoplankton abundances and class composition will only be resolved when we have more data on samples where we have simultaneous measurements by a range of techniques including flow cytometry, HPLC pigment analyses, and chlorophyll-specific spectrofluorometry. Ideally, these measurements would cover high- and lownutrient conditions. New techniques, and the refinement of existing ones, will be needed to identify and characterise the new species or

2577

ecotypes that are currently thought to be present. Improved methods for the HPLC analyses of pigments are required so that we can determine all types of Chl a, Chl b and Chl c while still being able to determine the more important carotenoids. More information is also required on the pigment ratios of relevant algal classes and ecotypes. Flow cytometry measurements would be of more value if methods could be found for identifying picoeukaryotes. One possibility is a new technique that uses whole-cell hybridization with taxonspecific oligonucleotide fluorescent probes (Simon et al., 1995, 1997; Marie et al., 2000). However, it is a difficult technique and, for natural samples, it has been limited to hybridization on microscope slides. The isolation of new strains and their analyses by molecular biology, particularly DNA directly extracted from the field, holds great potential for the characterisation of new strains and phylogenetic lineages and will allow us to investigate their contributions to the natural environment. On a broader scale, the picoplanktonic distribution of the equatorial Pacific is reasonably well established, and very high abundances of cells have been reported whatever the nutrient concentration. The regular offshore community structure is mainly composed of three groups of picophytoplankton. Cell division in each group is tightly synchronised, and growth generally occurs at a maximum rate of one doubling per day. Cyanobacteria, with only two taxa but with different and highly variable ecotypes, occupy all ecological niches from oligotrophic to mesotrophic waters and from the well-lit near-surface layers down to the depth of 0.1% light penetration. Picoeukaryotes are preferentially found in mesotrophic situations under all light intensities. However, in contrast to the cyanobacteria, many taxa with numerous lineages seem to occur and, although only a few species are known, it is likely that there are many, as yet unidentified, species present. Techniques based on pigment analyses indicate that haptophytes are the major class of eukaryotes. Since picoplankton contribute most of the Chl a found in these waters, it is probable that the picoeukaryotes enumerated by flow cytometry are predominantly haptophytes. Pigment data on

2578

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

size-fractionated samples are needed to determine whether this is indeed the case. Apart from light, the major forcing factor in determining community structure is nutrient availability. Major nutrients are always available in the HNLC region but become less available under El * conditions and more available under La Nino * conditions or as a result of tropical Nina instability waves. In the warm pool, increased availability is associated with a shallowing of the * and thermocline, as occurs during an El Nino, * or with upwelling at low possibly during La Nina, latitudes due to westerly wind bursts and current shear. The effects of increased nutrient concentration or availability, regardless of the cause or the duration of the increase, are remarkably consistent across the entire equatorial Pacific. There is an increase in the cell concentrations of Synechococcus and picoeukaryotes and an increase in the contribution of Synechococcus, diatoms, chlorophytes and picoeukaryotes to the total concentration of Chl a.

Acknowledgements This work was supported by JGOFS-France, IRD, INSU-CNRS and UMR 7621.

References Andr!e, J.M., Navarette, C., Blanchot, J., Radenac, M.H., 1999. Picophytoplankton dynamics in the equatorial Pacific: growth and grazing rates from cytometric counts. Journal of Geophysical Research 104, 3369–3380. Barber, R.T., Sanderson, M.P., Lindley, S.T., Chai, F., Newton, J., Trees, C.C., Foley, D.G., Chavez, F.P., 1996. Primrary productivity and its regulation in the equatorial * DeepPacific during and following the 1991–1992 El Nino. Sea Research II 43, 933–969. Bidigare, R.R., Ondrusek, M.E., 1996. Spatial and temporal variability of phytoplankton pigment distributions in the Central Equatorial Pacific Ocean. Deep-Sea-Research II 43, 809–833. Binder, B.J., Chisholm, S.W., Olson, R.J., Frankel, S.L., Worden, A.Z., 1996. Dynamics of picophytoplankton, ultraphytoplankton and bacteria in the central equatorial Pacific. Deep-Sea Research II 43, 907–931. Blain, S., Leynaert, A., Treguer, P., Chr!etiennot-Dinet, M.-J., Rodier, M., 1997. Biomass, growth rates and limitation of

equatorial Pacific diatoms. Deep-Sea Research I 44, 1255–1275. Blanchot, J., Gorsky, G., 2000. Picophytoplankton ingestion by the appendicularia Oikopleura dioica in natural and artificial environment. ISAC2000 Congress. Blanchot, J., Rodier, M., 1996. Picophytoplankton abundance and biomass in the western tropical Pacific Ocean during the * year: results from flow cytometry. Deep-Sea 1992 El Nino Research I 43, 877–895. Blanchot, J.B., Rodier, M., Le Bouteiller, A., 1992. Effect of El * southern oscillation events on the distribution and Nino abundance of phytoplankton in the western Pacific tropical ocean along 165 E. Journal of Plankton Research 14, 137–156. Blanchot, J., Andr!e, J.-M., Navarette, C., Neveux, J., 1997. Picophytoplankton dynamics in the equatorial Pacific: diel cycling from flow-cytometer observations. Comptes Rendus De L’Acad!emie Des Sciences Paris, Sciences De La Vie 320, 925–931. Blanchot, J., Andr!e, J.-M., Navarette, C., Neveux, J., Radenac, M.-H., 2001. Picophytoplankton in the equatorial Pacific: vertical distributions in the warm pool and in the high nutrient low chlorophyll conditions. Deep-Sea Research I 48, 297–314. Butt, J., Lindstrom, E., 1994. Currents off the east coast of New Ireland, Papua New Guinea, and their relevance to regional undercurrents in the Western Equatorial Pacific Ocean. Journal of Geophysical Research 99, 12503–12514. Cailliau, C., 1996. Les pigments comme biomarquers spe! cifiques des stocks et flux de mati"eres autotrophes en milieu marin: application a" l’oc!ean austral et l’oc!ean Pacifique e! quatorial. Th"ese de Doctorat de L’Universit!e Pierre et Marie Curie, Paris, 209pp. Caron, A.C., Peele, E.R., Lin Lim, E., Dennett, M.R., 1999. Picoplankton and nanoplankton and their trophic coupling in surface waters of the Sargasso Sea south of Bermuda. Limnology and Oceanography 44, 259–272. Chavez, F.P., Toggweiler, J.R., 1995. Upwelling in the ocean: modern processes and ancient records. In: Summerhayes, C.P., Emeis, K.C., Angel, M.V., Smith, R.L., Zeitschel, B. (Eds.), Physical Estimates of Global New Production: the Upwelling Contribution. Wiley, Chichester, pp. 313–320. Chavez, F.P., Buck, K.R., Barber, R.T., 1990. Phytoplankton taxa in relation to primary production in the Equatorial Pacific. Deep-Sea Research 37A, 1733–1752. Chavez, F.P., Buck, K.R., Coale, K.H., Martin, J.H., DiTullio, G.R., Welschmeyer, N.A., Jacobson, A.C., Barber, R.T., 1991. Growth rates, grazing, sinking, and iron limitation of equatorial Pacific phytoplankton. Limnology and Oceanography 36, 1818–1833. Chavez, F.P., Buck, K.R., Service, S.R., Newton, J., Barber, R.T., 1996. Phytoplankton variability in the central and eastern tropical Pacific. Deep-Sea Research II 43, 835–870. Chisholm, S.W., Olson, R.J., Zettler, E.R., Goericke, R., Waterbury, J.B., Welschmeyer, N.A., 1988. A novel freeliving prochlorophyte abundant in the oceanic euphotic zone. Nature 334, 340–343.

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582 Chisholm, S.W., Frankel, S.F., Goericke, R., Olson, R.J., Palenik, B., Waterbury, J.B., West-Johnsrud, L., Zettler, E.R., 1992. Prochlorococcus marinus nov. gen. nov. sp: an oxyphototrophic marine prokaryote containing divinyl chlorophyll a and b. Archives of Microbiology 157, 297–300. Christaki, U., Jacquet, J.R., Dolan, D., Vaulot, D., Rassoulzadegan, F., 1999. Growth and grazing on Prochlorococcus and Synechococcus by two marine ciliates. Limnology and Oceanography 44, 52–61. Coale, K.H., Johnson, K.S., Fitzwater, S.E., Gordon, R.M., Tanner, S., Chavez, F.P., Ferioli, L., Sakamoto, C., Rogers, P., Millero, F., Steinberg, P., Nightingale, P., Cooper, D., Cochlan, W.P., Landry, M.R., Constantinou, J., Rollwagen, G., Trasvina, A., Kudela, R., 1996. A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean. Nature (London) 383, 495–501. Dandonneau, Y., Neveux, J., 1997. Diel variations of in vivo fluorescence in the eastern equatorial Pacific: an unvarying pattern. Deep-Sea Research I 44, 1869–1880. Delcroix, T., Picaut, J., 1998. Zonal displacement of the western equatorial Pacific ‘‘fresh pool’’. Journal of Geophysical Research C103, 1087–1098. Desrosi"eres, R., 1969. Surface macrophytoplankton of the Pacific Ocean along the Equator. Limnology and Oceanography 14, 626–632. Dupouy, C., Neveux, J., Andr!e, J.M., 1997. Spectral absorption coefficient of photosynthetically active pigments in the equatorial Pacific Ocean (1651E–1501W). Deep-Sea Research II 44, 1881–1906. Eldin, G., Rodier, M., Radenac, M.-H., 1997. Physical and nutrient variability in the upper equatorial Pacific associated with westerly wind forcing and wave activity in October 1994. Deep-Sea Research II 44, 1783–1800. Everitt, D.A., Wright, S.W., Volkman, J.K., Thomas, D.P., Lindstrom, E.J., 1990. Phytoplankton community compositions in the western equatorial Pacific determined from chlorophyll and carotenoid pigment distribution. Deep-Sea Research 37A, 975–997. Foley, D.G., Dickey, T.D., McPhaden, M.J., Bidigare, R.R., Lewis, M.R., Barber, R.T., Lindley, S.T., Garside, C., Manov, D.V., McNeil, J.D., 1997. Longwaves and primary productivity variations in the equatorial Pacific at 01, 1401W. Deep-Sea Research II 44, 1801–1826. Garczarek, L., Hess, W.R., Holtzendorff, J., Van der Staay, G., Partensky, F., 2000. Multiplication of antenna genes as a major adaptation to low light in marine prokaryote. Proceedings National Academy of Sciences USA 97, 4098–4101. Gieskes, W.W.C., Kraay, G.W., 1983. Dominance of Cryptophyceae during the phytoplankton spring bloom in the central North Sea by HPLC analysis of pigments. Marine Biology 75, 179–185. Gieskes, W.W.C., Kraay, G.W., 1986. Floristic and physiological differences between the shallow and deep nanoplankton community in the euphotic zone of the open tropical

2579

Atlantic revealed by HPLC analysis of pigments. Marine Biology 91, 567–576. Gieskes, W.W.C., Kraay, G.Y., Tijssen, S.B., 1978. Chlorophylls and their degradation products in the deep pigment maximum layer of the tropical North Atlantic. Netherlands Journal of Sea Research 12, 195–204. Gieskes, W.W.C., Kraay, G.W., Nontji, A., Setiapermana, D., Sutmono, 1988. Monsoonal alteration of a mixed and a layered structure in the phytoplankton of the euphotic zone of the Banda Sea (Indonesia): a mathematical analysis of algal fingerprints. Netherlands Journal of Sea Research 22, 123–137. Goericke, R., Repeta, D.J., 1993. Chlorophylls a and b and divinyl chlorophylls a and b in the open sub-tropical North Atlantic Ocean. Marine Ecology Progress Series 101, 307–313. Goericke, R., Olson, R.J., Shalapyonok, A., 2000. A novel niche for Prochlorococcus sp. in low-light suboxic environments in the Arabian Sea and the Eastern Tropical North Pacific. Deep-Sea Research I 47, 1183–1205. Gordon, R.M., Coale, K.H., Johnson, K.S., 1997. Iron distributions in the equatorial Pacific: implications for new production. Limnology and Oceanography 141, 419–431. Gorsky, G., Chr!etiennot-Dinet, M.-J., Blanchot, J., Palazzoli, I., 1999. Picoplankton and nanoplankton aggregation by appendicularians: fecal pellet contents of Megalocercus huxleyi in the equatorial Pacific. Journal of Geophysical Research 104, 3381–3390. Guillou, L., Chr!etiennot-Dinet, M.-J., Medlin, L.K., Claustre, H., Loiseaux de Goer, S., Vaulot, D., 1999a. Bolidomonas: a new genus with two species belonging to a new algal class, the Bolidophyceae (Heterokonta). Journal of Phycology 35, 368–381. Guillou, L., Chr!etiennot-Dinet, M.-J., Moon-van der Staay, S.Y., Boulben, S., Vaulot, D., 1999b. Symbiomonas scintilans gen. and sp. nov. and Picophagus flagellatus gen. et sp. nov. (Heterokonta): two new heterotrophic flagellates with picoplanktonic size. Protist 150, 383–398. Guillou, L., Moon-van der Staay, S.Y., Claustre, H., Partensky, F., Vaulot, D., 1999c. Diversity and abundance of Bolidophyceae (Heterokonta) in two oceanic regions. Applied Environmental Microbiology 65, 4528–4536. Hallegraeff, G.M., Jeffrey, S.W., 1984. Tropical phytoplankton species and pigments of continental shelf waters of North and North-West Australia. Marine Ecology Progress Series 20, 59–74. Hasle, G.R., 1959. A quantitative study of phytoplankton from the equatorial Pacific. Deep-Sea Research 6, 38–59. Higgins, H.W., Mackey, D.J., 2000. Algal class abundances, estimated from chlorophyll and carotenoid pigments, in the * and non-El Nino * western Equatorial Pacific under El Nino conditions. Deep-Sea Research I 47, 1461–1483. Holm-Hansen, O., Lorenzen, C.J., Holmes, R.W., Strickland, J.D.H., 1965. Fluorometric determination of chlorophyll. Journal du Conseil, Conseil Permanent International pour Explorer de la Mer 30, 3–15.

2580

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

Inoue, H.Y., Ishii, M., Matsueda, H., Ahoyama, M., 1996. Changes in longitudinal distribution of the partial pressure of CO2 (pCO2) in the central and western equatorial Pacific, west of 1601W. Geophysical Research Letters 23, 1781–1784. Iriarte, J.L., Fryxell, G.A., 1995. Microphytoplankton at the equatorial Pacific (1401W) during the JGOFS EqPac time series: March to April and October 1992. Deep-Sea Research II 42, 559–583. Ishizaka, J., Kiyosawa, H., Ishida, K., Ishikawa, K., Takahashi, M., 1994. Meridional distribution and carbon biomass of autotrophic picoplankton in the Central North Pacific Ocean during the late Northern summer 1990. Deep-Sea Research II 41, 1745–1766. Ishizaka, J., Harada, K., Ishikawa, K., Kiyosawa, H., Furusawa, H., Watanabe, Y., Ishida, H., Suzuki, K., Handa, N., Takahashi, M., 1997. Size and taxonomic plankton community structure and carbon flow at the equator, 1751E during 1990–1994. Deep-Sea Research II 44, 1927–1949. Ituriaga, R., Mitchell, B.G., 1986. Chroococcoid cyanobacteria: a significant component of the food web dynamics. Marine Ecology Progress Series 28, 291–297. Jeffrey, S.W., 1974. Profiles of photosynthetic pigments in the ocean using thin-layer chromatography. Marine Biology 26, 101–110. Jeffrey, S.W., Hallegraeff, G.M., 1980. Studies of phytoplankton species and photosynthetic pigments in a warm core eddy of the east australian current I. Summer populations. Marine Ecology Progress Series 3, 285–294. Jeffrey, S.W., Hallegraeff, G.M., 1987. Phytoplankton pigments, species and light climate in a complex warm-core eddy of the East Australian Current. Deep-Sea Research 34, 649–673. Jeffrey, S.W., Wright, S.W., Zapata, M., 2000. Recent advances in HPLC pigment analysis of phytoplankton. Marine and Freshwater Research 50, 879–896. Kaczmarska, I., Fryxell, G.A., 1995. Micro-phytoplankton of the equatorial Pacific: 1401W meridional transect during the * Deep-Sea Research 42, 535–538. 1992 El Nino. Kiefer, D.A., 1973. Chlorophyll a fluorescence in marine centric diatoms: responses of chloroplasts to light and nutrient stress. Marine Biology 23, 39–46. Klein, B., Sournia, A., 1987. A daily study of the diatom spring bloom at Roscoff (France) in 1985. II. Phytoplankton pigment composition studied by HPLC analysis. Marine Ecology Progress Series 37, 265–275. Landry, M.R., Constantinou, J., Kirshtein, J., 1995. Microzooplankton grazing in the equatorial Pacific during February and August. Deep-Sea Research II 42, 657–671. Landry, M.R., Kirsten, J., Constantinou, J., 1996. Abundances and distributions of picoplankton populations in the central equatorial Pacific from 121N to 121S, 1401W. Deep-Sea Research II 2–3, 657–671. Landry, M.R., Barber, R.T., Bidigare, R.R., Chai, F., Coale, K.H., Dam, H.G., Lewis, M.R., Lindley, S.T., McCarthy,

J.J., Roman, M.R., Stoecker, D.K., Verity, P.G., White, J.R., 1997. Iron and grazing constraints on primary production in the central equatorial Pacific: an EqPac synthesis. Limnology and Oceanography 42, 405–418. Landry, M.R., Ondrusek, S.J., Tanner, S.J., Brown, S.L., Constantinou, J., Bidigare, R.R., Coale, K.H., Fitzwater, S., 2000. The biological response to iron fertilization in the eastern equatorial Pacific (Iron Ex II). 1. Microplankton community abundances and biomass. Marine Ecology Progress Series 201, 27–42. Lantoine, F., Neveux, J., 1997. Spatial and seasonal variations in abundance and spectral characteristics of phycoerythrins in the tropical Northeastern Atlantic Ocean. Deep-Sea Research 44, 223–246. Latasa, M., Landry, M.R., Schluter, L., Bidigare, R.R., 1997. Pigment-specific growth and grazing rates of phytoplankton in the central equatorial Pacific. Limnology and Oceanography 42, 289–298. Le Borgne, R., Rodier, M., Le Bouteiller, A., Murray, J.W., 1999. Zonal variability of plankton and particle export flux in the equatorial Pacific upwelling between 1651E and 1501W. Oceanologica Acta 22, 57–66. Le Borgne, R., Barber, R.T., Delcroix, T., Inoue, H.Y., Mackey, D.J., Rodier, M., 2002. Pacific warm pool and divergence: temporal and zonal variations on the equator and their effects on the biological pump. Deep-Sea Research II 49, 2471–2512. Le Bouteiller, A., Blanchot, J., 1991. Size distribution and abundance of phytoplankton in the Pacific equatorial upwelling. La Mer 29, 175–179. Le Bouteiller, A., Blanchot, J., Rodier, M., 1992. Size distribution patterns of phytoplankton in the western Pacific: towards a generalization for the tropical open ocean. Deep-Sea Research 39A, 805–823. Letelier, R.M., Bidigare, R.R., Hebel, D.V., Ondrusek, M., Winn, C.D., Karl, D.M., 1993. Temporal variability of phytoplankton community structure based on pigment analysis. Limnology and Oceanography 38, 1420–1437. Li, K.W.H., Subba-Rao, D.V., Harrison, W.G., Smith, J.C., Cullen, J.J., Irwin, B., Platt, T., 1983. Autotrophic picoplankton in the tropical ocean. Science 219, 292–295. Lindstrom, E., Lukas, R., Fine, R., Firing, E., Godfrey, S., Meyers, G., Tsuchiya, M., 1987. The western equatorial Pacific Ocean circulation study. Nature (London) 330, 533–537. Liu, H., Nolla, H.A., Campbell, L., 1997. Prochlorococcus growth rate and contribution to primary production in the equatorial and subtropical North-Pacific Ocean. Aquatic Microbial Ecology 12, 39–47. Lorenzen, C.J., 1966. A method for the continuous measurement of in vivo chlorophyll concentration. Deep-Sea Research 13, 223–227. Lukas, R., Lindstrom, E., 1991. The mixed layer of the western equatorial Pacific Ocean. Journal of Geophysical Research 96, 3343–3357. Mackey, D.J., Parslow, J., Higgins, H.W., Griffiths, F.B., O’Sullivan, J.E., 1995. Plankton productivity and biomass

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582 in the western equatorial Pacific: biological and physical controls. Deep-Sea Research II 42, 499–533. Mackey, M.D., Mackey, D.J., Higgins, H.W., Wright, S.W., 1996. CHEMTAX—a program for estimating class abundances from chemical markers: application to HPLC measurements of phytoplankton. Marine Ecology Progress Series 144, 265–283. Mackey, D.J., Parslow, J.S., Griffiths, F.B., Higgins, H.W., Tilbrook, B., 1997. Phytoplankton productivity and the carbon cycle in the western equatorial Pacific under ENSO and non-ENSO conditions. Deep-Sea Research II 44, 1951–1978. Mackey, D.J., Higgins, H.W., Mackey, M.D., Holdsworth, D., 1998. Algal class abundances in the western equatorial Pacific: estimation from HPLC measurements of chloroplast pigments using CHEMTAX. Deep-Sea Research I 45, 1441–1468. Madden, R.A., Julian, P.R., 1971. Detection of a 40–50 day oscillation in the zonal wind in the tropical Pacific. Journal of Atmospheric Science 28, 702–708. Madden, R.A., Julian, P.R., 1972. Description of global-scale circulation cells in the tropics with a 40–50 day period. Journal of Atmospheric Research 29, 1109–1123. Mantoura, R.F.C., Llewellyn, C.A., 1983. The rapid determination of algal chlorophyll and carotenoid pigments and their breakdown products in natural waters by reversephase high-performance liquid chromatography. Analytica Chimica Acta 151, 297–314. Marie, D., Simon, N., Guillou, L., Partensky, F., Vaulot, D., 2000. DNA, RNA analysis of phytoplankton by flow cytometry. In: Robinson, J.P., Darzynkiewicz, Z., Dean, P.N., Orfao, A., Rabinovitch, P.S., Stewart, C.C., Tanke, H.J., Wheeless, L. L. (Eds.), Current Protocols in Cytometry. Wiley, New York, pp. 11.12.1–11.12.18. Marra, J., 1997. Analysis of diel variability in chlorophyll fluorescence. Journal of Marine Research 55, 767–784. McPhaden, M., 1999. Genesis and evolution of the 1997–98 El Nino. Science 283, 950–954. Minas, H.J., Minas, M., Packard, T.T., 1986. Productivity in upwelling areas deduced from hydrographic and chemical fields. Limnology and Oceanography 31, 1182–1206. Moon-van der Staay, S.Y., van der Staay, G.W.M., Guillou, L., Claustre, H., Vaulot, D., 2000. Abundance and diversity of prymnesiophytes in the picoplankton community from the equatorial Pacific Ocean inferred from 18S rDNA sequences. Limnology and Oceanography 45, 98–109. Moore, L.R., Chisholm, S.W., 1999. Phytophysiology of the marine cyanobacterium Prochlorococcus: ecotypic differences among cultured isolates. Limnology and Oceanography 44, 628–638. Moore, L.R., Rocap, G., Chisholm, S.W., 1998. Physiology and molecular phylogeny of coexisting Prochlorococcus ecotypes. Nature 393, 464–467. Murray, J.M., Barber, R.T., Roman, M.R., Bacon, M.P., Feely, R.A., 1994. Physical and biological controls on carbon cycling in the Equatorial Pacific. Science 266, 58–65.

2581

Navarette, C., 1998. Dynamique du Phytoplancton en ocean equatorial: mesures cytom!etriques et mesure durant la campagne FLUPAC, en octobre 1994 dans la partie ouest du Pacifique. Th"ese de Doctorat de L’Universit!e Pierre et Marie Curie, Paris, 313pp. Neveux, J., 1975. Analyse chromatographique des pigments du phytoplancton en M!editerran!ee nord-occidentale et pr"es des # cotes atlantiques marocaines. International Revue Der Gesamten Hydrobiologie 60, 675–690. Neveux, J., Lantoine, F., 1993. Spectrofluometric assay of chlorophylls and pheophytins using the least squares approximation technique. Deep-Sea Research I 40, 1747–1765. Neveux, L., Lantoine, F., Vaulot, D., Marie, D., Blanchot, J., 1999. Phycoerythrins in the southern tropical and equatorial Pacific: evidence for new cyanobacterial types. Journal of Geophysical Research 104, 3311–3321. Neveux, J., Dupouy-Douchement, C., Blanchot, J., Le Bouteiller, A., Landry, M.R., Brown, S.L., 2002. Diel dynamics of chlorophylls in HNLC waters of the equatorial Pacific (1801): interactions of growth, grazing, physiological responses and mixing. Journal of Geophysical Research, submitted for publication. Olson, R.J., Chisholm, S.W., Zettler, E.R., Armbrust, E.V., 1988. Analysis of Synechococcus pigment types in the sea using single and dual beam flow cytometry. Deep-Sea Research 35A, 425–440. Partensky, F., Hoepffner, N., Li, W.K.W., Ulloa, O., Vaulot, D., 1993. Photoacclimation of Prochlorococcus sp. (Prochlorophyta) strains isolated from the North Atlantic and the Mediterranean Sea. Plant Physiology 101, 295–296. Partensky, F., Blanchot, J., Vaulot, D., 1999a. Differential distribution and ecology of Prochlorococcus and Synechococcus in oceanic waters: a review. In: Charpy, L., Larkum, A.W.D. (Eds.), Marine Cyanobacteria. Mus!ee Oce! anographique, Monaco, pp. 457–475. Partensky, F., Hess, W.R., Vaulot, D., 1999b. Prochlorococcus, a marine photosynthetic prokaryote of global significance. Microbiological Molecular Biology Review 63, 106–127. Picaut, J., Ioualalen, M., Menkes, C., Delcroix, T., McPhaden, M.J., 1996. Mechanism of the zonal displacements of the Pacific warm pool: implications for ENSO. Science 274, 1486–1489. Pinckney, J.L., Paerl, H.W., Harrington, M.B., Howe, K.E., 1998. Annual cycles of phytoplankton community-structure and bloom dynamics in the Neuse River Estuary, North Carolina. Marine Biology 131, 371–381. Platt, T., Rao, D.V.S, Irwin, B., 1983. Photosynthesis of picoplankton in the oligotrophic ocean. Marine Ecology Progress Series 10, 105–110. Radenac, M.-H., Rodier, M., 1996. Nitrate and chlorophyll distributions in relation to thermohaline and current structures in the western tropical Pacific during 1985–1989. Deep-Sea Research II 43, 725–752. Ridout, P.S., Morris, R.J., 1985. Short-term variations in the pigment composition of a spring phytoplankton bloom

2582

D.J. Mackey et al. / Deep-Sea Research II 49 (2002) 2561–2582

from an enclosed experimental ecosystem. Marine Biology 87, 7–11. Rodier, M., Eldin, G., Le Borgne, R., 2000. The western boundary of the equatorial Pacific upwelling: some consequences of climatic variability on hydrological and planktonic properties. Journal of Oceanography 56, 463–471. Roy, S., 1987. High performance liquid chromatographic analysis of chloropigments. Journal of Chromatography 391, 19–34. Shalapyonok, A., Olson, R.J., Shalapyonok, L.S., 1998. Ultradian growth in Prochlorococcus spp. Environmental Microbiology 64, 1066–1069. Siegel, D.A., Ohlman, J.C., Washburn, L., Bidigare, R.R., Nosse, C.T., Fields, E., Zhou, Y., 1995. Solar radiation, phytoplankton pigments and the radiant heating of the equatorial Pacific warm pool. Journal of Geophysical Research C100, 4885–4891. Simon, N., Le Bot, N., Marie, D., Partensky, F., Vaulot, D., 1995. Fluorescent in situ hybridization with rRNA-targeted probes for identifying small phytoplankton by flow cytometry. Applied Environmental Microbiology 61, 2506–2513. Simon, N., Brenner, J., Edwarsen, B., Medlin, L.K., 1997. The identification of Chrysochromulina and Prymnesium species (Haptophyta, Prymnesiophyceae) using fluorescent or chemiluminescent oligonucleotide probes: a means for improving studies on toxic algae. European Journal of Phycology 32, 393–401. Stoens, A., Menk"es, C., Radenac, M.-H., Dandonneau, Y., Grima, N., Eldin, G., M!emery, L., Navarette, C., Andr!e, J.M., Moutin, T., Raimbault, P., 1999. The coupled physicalnew production system in the equatorial Pacific during the * Journal of Geophysical Research 104, 1992–1995 El Nino. 3323–3339. Suzuki, K., Handa, N., Kiyosawa, H., Ishizaka, J., 1995. Distribution of Prochlorophyte Prochlorococcus in the

central Pacific Ocean as measured by HPLC. Limnology and Oceanography 40, 983–989. Takahashi, T., Feely, R.A., Weiss, R.F., Wanninkhof, R., Chipman, D.W., Sutherland, S.C., Takahashi, T.T., 1997. Global air–sea-flux of CO2: an estimate based on measurements of sea–air pCO2 difference. Proceedings of the National Academy of Science USA 94, 8292–8299. Urbach, E., Scanlan, D.J., Distel, D.L., Waterbury, J.B., Chisholm, S.W., 1998. Rapid diversification of marine picophytoplankton with dissimilar light harvesting structures inferred from sequences of Prochlorococcus and Synechococcus (Cyanobacteria). Journal of Molecular Evolution 46, 188–201. Vaulot, D., Marie, D., 1999. Diel variability of photosynthetic picoplankton in the equatorial Pacific. Journal of Geophysical Research 104, 3297–3310. Vaulot, D., Marie, D., Olson, R.J., Chisholm, S.W., 1995. Growth of Prochlorococcus, a photosynthetic prokaryote, in the equatorial Pacific. Oceanographic Science 268, 1480–1482. Wright, S.W., Jeffrey, S.W., Mantoura, R.F.C., Llewellyn, C.A., Bjornland, T., Repeta, D., Welschmeyer, N., 1991. Improved HPLC method for the analysis of chlorophylls and carotenoids from marine phytoplankton. Marine Ecology Progress Series 77, 183–196. Wright, S.W., Thomas, D.P., Marchant, H.J., Higgins, H.W., Mackey, M.D., Mackey, D.J., 1996. Analysis of phytoplankton in the Australian sector of the Southern Ocean: comparisons of microscopy and size frequency data with interpretations of pigment HPLC data using the ‘CHEMTAX’ matrix factorisation program. Marine Ecology Progress Series 144, 285–298. Wyman, M., 1992. An in vivo method for the estimation of phycoerythrin concentrations in marine cyanobacteria (Synechococcus spp.). Limnology and Oceanography 37, 1300–1306.