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Silicon limitation of biogenic silica production in the Equatorial Pacific
Deep-Sea Research I 48 (2001) 639}660
Silicon limitation of biogenic silica production in the Equatorial Paci"c A. Leynaert *, P. TreH guer , Christiane Lancelot, Martine Rodier Laboratoire **Flux de matie% re et re& ponses du vivant++, Institut Universitaire Europe& en de la Mer, TechnopoL le Brest-Iroise. Place N. Copernic, 29 280 Plouzane, France Groupe de Microbiologie des Milieux Aquatiques, Campus de la Plaine, CP 221, Boulevard du Triomphe, 1050 Bruxelles, Belgium Institut de Recherche pour le De& veloppement, Station Marine d'Endoume, Chemin de la batterie des lions, 13007 Marseille, France Received 4 August 1999; received in revised form 14 February 2000; accepted 6 March 2000
Abstract During the EBENE cruise (November 1996), distributions of biogenic silica concentration and production rates were investigated in the surface waters of the equatorial Paci"c (1803W, from 83S to 83N), with particular emphasis on the limitation of the biogenic silica production by ambient silicic acid concentrations. Integrated over the depth of the euphotic layer, concentrations of biogenic silica and production rates were maximum at the Equator (8.0 and 2.6 mmol m\ d\) and decreased more or less symmetrically polewards. Contribution of diatoms to the new production was estimated indirectly, comparing biogenic silica production rates and available data of new and export production in the same area. This comparison shows that new production in the equatorial area could mostly be sustained by diatoms, accounting for the major part of the exported #ux of organic carbon. Kinetics experiments of silicic acid enrichment were performed. Half saturation constants were 1.57 lM at 33S and 2.42 lm at the Equator close to the ambient concentrations. The corresponding < values for Si uptake were 0.028 h\ at 33S and 0.052 h\ at the equator.
Experiments also show that in situ rates were restricted to 13}78% of < , depending on ambient silicic acid
concentrations. This work provides the "rst direct evidence that the rate of Si uptake by diatom populations of the equatorial Paci"c is limited by the ambient concentration of silicic acid. However, such Si limitation might not be su$cient in itself to explain the low diatom growth rates observed, and additional limitation is suggested. One hypothesis that is consistent with the results of Fe limitation studies is that Fe and Si limitations may interact, rather than just being a mutually exclusive explanation for the HNLC character of the system. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Silicon cycle; Diatoms; Limiting factor; Equatorial Paci"c
* Corresponding author. Tel.: 33-2-98-49-86-57; fax: 33-98-49-86-95. E-mail address: [email protected] (A. Leynaert). 0967-0637/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 0 0 ) 0 0 0 4 4 - 3
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1. Introduction The carbon biologically "xed in the surface layer and exported to the deep sea (the biological pump of CO ) is one of the major factors controlling CO partial pressure in the atmosphere (Sarmiento and Siegenthaler, 1992). Accurate determination of this #ux and of its controlling factors are therefore critically important for understanding global carbon cycling and its response to climate change. It has been estimated that about half of the export #ux of organic carbon to the deep ocean is synthesized by diatoms (Nelson et al., 1995). Consequently, it is essential to consider the factors in#uencing the relative contribution of diatoms to total primary and export production, to better understand the processes that determine the e$ciency of the biological pump. Most studies of biogenic silica production reported to date have emphasized areas of known or presumed high primary productivity, relatively high diatom abundance, and active accumulation of diatomaceous sediments. The equatorial Paci"c is one of the main areas for opal sediment accumulation from which few measurements of silica production from overlying waters are yet available. The central equatorial Paci"c is known as a `high nutrient low chlorophylla (HNLC) region. If phytoplankton biomass and production rates remain low, diatoms have been implicated to play an important role (Chavez et al., 1990). Among other factors that have been put forth to unravel the processes that maintain the relatively constant HNLC conditions, grazing pressure and iron limitation have been argued (Coale et al., 1996a; Landry et al., 1997), but neither has proven su$cient in itself to explain the characteristics of HNLC regions (Price et al., 1994). Recently, Ku et al. (1995) suggested the availability of `newa silicic acid as a limiting factor controlling production in the upper equatorial Paci"c. This hypothesis, based upon studies of nutrient budgets and Ra distributions, received support from Dugdale et al. (1995) and Dugdale and Wilkerson (1998), who also showed from a simple silicon-cycle model that silicic acid would play a key role in the regulation of new production in the equatorial Paci"c upwelling. However, no direct experimental evidence of silicic acid limitation has yet ever been obtained in this area. As part of a more global investigation of biogenic silica concentration and production rates, this paper reports the "rst experimental evidence of silicic acid limitation of diatom silica production in the equatorial Paci"c. Experiments were conducted during the IRD (ex-ORSTOM) EBENE cruise (1996) in the equatorial Paci"c, as part of the French contribution to the international JGOFS program. It was in continuity with the FLUPAC cruise (1994). Since data on the silica cycle were also collected along the equator (between 1703E and 1553W) during FLUPAC, we will frequently refer to the FLUPAC cruise in this paper.
2. Methods Experiments were conducted during the EBENE cruise, in the equatorial Paci"c (October}November 1996), aboard the French R.V. Atalante. The cruise plan consisted of one transect along the date line, from 83S to 83N, with a station at each degree and two time-series stations, at 33S and at the Equator.
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Chemical measurements (nitrate, nitrite, ammonium, phosphate and silicic acid) were carried out by standard automated colorimetric methods (Strickland and Parsons, 1972). Data sets and analytical details are reported in Le Borgne et al. (1998). 2.1. Biogenic silica concentration Biogenic silica (BSi) was determined on particulate matter collected by "ltration of 2 l of sea water through 0.6 lm polycarbonate membrane "lter. The "lter was folded in quarters, placed in a petri dish, dried at 603C for 12 h and stored at ambient temperature. Later analysis was performed using the alkaline digestion method (Paasche, 1973) modi"ed by Ragueneau and TreH guer (1994). The blank was 6 nmol l\ (SD"3 nmol l\). 2.2. Biogenic silica production rates Biogenic silica production rates (PBSi) were measured at six depths in the euphotic zone. Incubations were generally conducted under simulated in situ conditions, except at the time series stations, where incubations were performed in situ from dawn to dusk (total duration of about 12 h), using a drifting array, and followed by deck incubations for the night, to complete the 24 h. The general procedure was the following: 250 ml samples, in polycarbonate bottles, were spiked with 50 000 dpm (830 Bq) of Si tracer (52 000 Bq/lg Si, Los Alamos National Laboratory). At the end of the incubation period (24 h), each sample was gently vacuum-"ltered through a 0.6 lm polycarbonate membrane "lter (Nuclepore) and rinsed with 10 ml of "ltered seawater. The "lter was then immediately placed in the bottom of a 20 ml plastic vial. 2 ml of 2.9 M HF was added to dissolve biogenic silica. The reaction was complete after 30 minutes, and 10 ml of scintillation cocktail (Ultima Gold XR) was then added to each vial. After shaking, samples were counted on a Tri-carb 1500 TR instrument (Packard) for 60 minutes, or when a counting precision of 0.5% was achieved for cpm in each counting window. An equilibrated Si solution and P standards were used to deconvolve the energy spectra, as described in Leynaert et al. (1996). The biogenic silica production rate (PBSi, in nmol l\ h\) is the fraction between the initially dissolved Si activity added to the sample, and the Si taken up by phytoplankton and counted on the "lter at the end of the incubation, divided by the incubation time. To control for the non-biological uptake of Si, sodium azide and HgCl have been used essentially on seawater samples from turbid environments (North Sea and Black Sea coastal waters). Evidence of adsorption has never been observed, "nal counts being always at the background level. However, because of the extremely high cost of the radionucleide Si, these control experiments are not run each time. The speci"c production rate (<, time\) is the molar ratio of the biogenic silica production rate to the initial biogenic silica standing stock. < represents a minimum estimate of the speci"c production rate by living cells, due to the presence of non-living biogenic silica (empty frustules, shell fragments, etc.). The growth rate (k) is the rate of increase of a cell substance per unit of that cell substance (Eppley, 1972; Schlegel, 1987). In mathematical terms: 1 dN k" ; , N d¹
(1.1)
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which upon integration becomes ln N!ln N k" , t
(1.2)
N could be any cellular constituent (carbon, chlorophyll, biogenic silica, cells number, etc.). In our case, N and N denote the biogenic silica at time t and at time t, respectively. The biogenic silica at time t is predicted from the initial biogenic silica concentration and the uptake rate. However, at steady state, <"k (Eppley, 1972; Schlegel, 1987). The mean doubling time (t ) is de"ned as the time required for the cellular component to increase by a factor 2. The relationship between t and k is derived from Eq. (1.2): ln 2 . t " k Its reciprocal, the number of doublings per unit of time (1/t ), is also commonly used. When comparing the growth rate and the cell division rate, one must bear in mind that mass per cell is not necessarily constant and can change during growth. However, at steady state, a doubling of biomass is accompanied by a doubling of all other measurable constituents of the population. 2.3. Si-limitation experiments The concentration dependence of diatom silicic acid uptake rates was investigated at 33S and at the Equator by conducting Si tracer kinetic experiments at di!erent silicic acid concentrations. For each kinetic experiment, a set of 10 (at 33S) or eight (at the Equator) 250 ml sub-samples were poured into polycarbonate incubation bottles and enriched with Si(OH) , at concentrations ranging from 0 to 10 lM above ambient. A 50 ml subsample was collected for nutrient analysis and immediately processed. Then 200 000 dpm (3300 Bq) of Si tracer (52 000 Bq/lg Si) at 33S, or 125 000 dpm (2100 Bq) at the Equator, was added to each bottle. Incubations were conducted at sunrise for 6 h, at full light and under in situ simulated conditions. After incubation, samples were processed as described above for biogenic silica production experiments. Additional experiments on silicic acid limitation were conducted at two depths (surface and chl a maximum) at each station, according to Glibert and McCarthy (1984) and Brzezinski et al. (1997). Basically, these experiments compare Si incubation performed at ambient concentration with those enriched to 10 lM Si(OH) . Considering the K of 1 lM reported for eastern tropical Paci"c diatoms (Thomas and Dodson, 1975) and assuming that silicic acid uptake obeys the MichaelisMenten equation, more than 90% of < is reached at 10 lM Si(OH) enrichments (10 times the K ). The ratio between in situ and enriched samples gives an indication of the degree to which silicic acid uptake by phytoplankton is limited by ambient silicic acid concentrations. 3. Results 3.1. Climatic regime during EBENE, in October}November 1996 The climate system of the equatorial Paci"c is subject to intermittent variabilities that lead to large sea surface temperature anomalies. Oscillations between unusually warm (El Nin o) and cold
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Fig. 1. TAO (Tropical Atmosphere Ocean) monthly mean sea surface temperature (3C) and winds (m s\), and temperature anomalies of the studied area (printed from http://www.pmel.noaa.gov/tao-bin/tao/cover), in November 1994 (FLUPAC cruise) and in November 1996 (EBENE cruise).
(La Nin a) conditions have now been well documented. Along the Equator, in November 1996, EBENE took place during a `neutral scenarioa between El Nin o and La Nin a, as shown by the plot of sea surface temperature anomalies (Fig. 1). The cruise track crossed the Equator at 1803W. Although fairly to the west, the tongue of cool upwelled water was still perceptible at that longitude (sea surface temperature was close to 28.53C). 3.2. Nutrient distributions Fig. 2 shows section plots of silicic acid and inorganic nitrogen (nitrate#nitrite#ammonium) concentrations along the transect in the upper 400 m, from 83S to 83N. Silicic acid concentrations
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Fig. 2. Vertical nutrient distributions: (a) N in lmol l\, (b) Si(OH) in lmol l\, (c) Si/N molar ratio, from 83S to 83N, at 1803W.
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were relatively constant at the surface, ranging from 1.0 to 2.3 lM. Inorganic nitrogen concentrations were more variable, ranging from complete depletion to 3.5 lM. Contours shoal in response to strong vertical advection in the equatorial upwelling zone. They decrease polewards at the surface. In the water column, silicic acid and nitrate concentrations increased sharply below the 0.1% light depth (150 m) to reach about 25 and 32 lM, respectively, at 400 m. The lack of coupling between nitrate and silicic acid regeneration is evidenced by the vertical distribution of the Si/N molar ratio, considering the value of 1 as typical for diatom growth requirement (Fig. 2c). Values slightly below, but close to 1, are observed in the surface upwelled waters at the equator, whereas the ratio decreases to values lower than 0.5 at the base of the euphotic zone (150 m). At the surface, the ratio increases polewards dramatically to values exceeding 50, resulting from nitrate depletion. 3.3. Biogenic silica distribution across the Equator (83S}83N) along the date line Biogenic silica concentrations were very low (7}80 nmol l\) all along the transect (Fig. 3). The maximum was encountered at the Equator. A two-fold increase of the concentrations was observed in the euphotic zone for stations of the equatorial area (13S}13N), as compared to stations polewards. Vertical distribution of biogenic silica evidenced a maximum at the surface, and often in the subsurface, close to the nitracline, for stations outside the direct in#uence of the upwelling (south of 13S and north of 13N). A slight maximum in biogenic silica concentration was also noticed around 300 m depth (Fig. 3). During the long-term stations at the equator and at 33S, vertical pro"les of BSi were obtained during three successive days. Integrated in the euphotic layer (150 m), BSi concentrations were 6.9, 7.4 and 9.5 mmol m\ at the Equator and 3.4, 4.8, and 3.7 mmol m\, at 33S, giving averages of 8.0 ($1.4) and 4.0 ($0.7) mmol m\. No noticeable variations can be evidenced during the time course of each station, as the maximum standard deviation between the successive experiments (1.4 mmol m\) is close to the detection limit.
Fig. 3. Vertical section of biogenic silica concentrations (nmol-Si l\) along 1803W (negative latitudes are south).
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3.4. Biogenic silica production rates Time course experiments of silicic acid uptake were conducted for 24 h at two stations (33S and the Equator). Sub-samples were taken after 6, 12 and 24 h incubation, and the silicic acid uptake was measured. Results (Fig. 4) clearly show that silicic acid uptake did not proceed at the same rate during day and night. A signi"cant decrease of Si uptake of about 30% was observed during the dark period for both investigated communities. Although we cannot rule out a possible bottle artifact, these results suggest some energy dependence of Si uptake by diatoms. Accordingly, samples collected at 0.1% light depth did not show any Si uptake. Other studies have reported (Brzezinski and Nelson, 1989; Nelson and Brzezinski, 1997) little evidence of systematic day/night di!erences among pro"les of the biogenic silica speci"c production rates for natural phytoplankton population. However diel periodicity in Si uptake was observed by Goering et al. (1973), with
Fig. 4. Time course of biogenic silica production (nmol-Si l\) determined from surface water in situ incubations of 6, 12 and 24 h, in triplicate, (a) at the Equator, and (b) at 33S.
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Fig. 5. Vertical section of biogenic silica production rates (PBSi, in nmol-Si l\ h\), along 1803W (negative latitudes are south).
Fig. 6. Latitudinal (from 83S to 73N) pro"le of daily biogenic silica production, integrated in the photic layer (mmol m\ d\), along 1803W.
maximum rates occurring at noon. It implies that diatom speci"c uptake rate derived from short-term incubations might be over-estimated or under-estimated, depending on whether the incubation has been conducted around noon or not. Daily rates of biogenic silica production (Fig. 5) in surface waters were generally very low, ranging from less than 0.2 to about 36 nmol l\ d\. Vertical pro"les of biogenic silica production show maximum rates in surface or subsurface waters, and a general decrease with depth. Integrated in the upper layer, daily rates of biogenic silica production (Fig. 6) were maximal at the Equator (2.58$0.40 mmol m\ d\) and decreased more or less symmetrically polewards, to 0.3 mmol m\ d\ at 73N and less than 0.1 mmol m\ d\ at 83S. During the long-term station at
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Fig. 7. Biogenic silica biomass (lmol l\) and production (nmol l\ h\) pro"les performed during three successive days at the equator (03S, 1803W).
the Equator, biogenic silica production measurements, carried out during three successive days, showed little daily #uctuation (Fig. 7). When integrated in the euphotic layer, they varied from 2.14 to 2.91 and 2.70 mmol m\ d\, suggesting steady state. It is interesting to note that there is one order of magnitude di!erence between the production rate of surface waters at the Equator and the most northern and southern stations, whereas there is only a two-fold increase in biogenic silica concentrations. As a result, diatom communities at the equator are characterized by signi"cantly higher speci"c production rates (<, d\). Indeed, the average daily rates in surface waters are 0.43 ($0.23) d\ for stations in the equatorial band (13N}13S), and 0.21 ($0.21) d\ for stations polewards. In the same way, the mean estimated doubling time of the silica biomass (t ) varied from 2.4 ($1.1) days to 6.2 ($5.6) days in the two areas de"ned above. The vertical pro"les of < (not shown) generally paralleled those of PBSi, displaying a sharp decrease with depth in the upper 100 m and very low values below. The speci"c production rate (<), when averaged over the euphotic layer, varied by one order of magnitude during the whole cruise: from 0.03 to 0.29 d\. In the area between 13N and 13S, < averaged 0.24 d\, consistent with typical values reported for coastal upwelling (0.3 d\ o! Baja California, Nelson and Goering, 1978). The mean for stations situated polewards was 0.13 d\, exceeding mean speci"c production rates reported for HNLC polar waters (generally below 0.1 d\, Nelson and Smith, 1986, 1991; Banahan and Goering, 1986), but in the same range as values reported for oligotrophic mid-ocean gyres (Nelson and Brzezinski, 1997). 3.5. Kinetic studies of silicic acid uptake The silicic acid concentration dependence of Si uptake was investigated at two stations (33S and the Equator). Speci"c uptake rates (<, h\) were plotted as a function of the ambient silicic acid
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Fig. 8. Concentration dependence of silicic uptake rate by natural diatom assemblages (a) at 33S and (b) at the Equator, at 1803W. Data points show the measured values of <; the curve represents the Michaelis}Menten hyperbola "tted to the data by the non-linear regression method of Wilkinson (1961).
concentrations (Fig. 8). The dependence of < upon (Si(OH) ) was determined by "tting the Michaelis}Menten equation to the data < ;[Si(OH) ] , <" K #[Si(OH) ] in which < is the speci"c uptake rate, < the speci"c uptake rate at in"nite [Si(OH) ], and K the half-saturation constant (the silicic acid concentration at which <"< /2). These two constants are determined by "tting the data with the non-linear method of Wilkinson (1961). At 33S, the ambient (Si(OH) ) was 1.45 lM. The Si-enrichment experiment gives some evidence of an hyperbolic response of < to increased silicic acid concentration (Fig. 8a), although the "t to the Michaelis}Menten function is only approximate. K and < values are 1.57 ($1.32) lM and 0.028 ($0.006) h\. As diatoms were taking up silicic acid quite rapidly, as indicated by the relatively high < value, the scatter in the data may be attributed to the very low phytoplankton biomass, and particularly the very low biogenic concentrations (30 nmol l\).
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Fig. 9. Ratio of biogenic silica production rates at ambient silicic acid concentrations to biogenic silica production rates at 10 lM of silicic acid concentration above ambient, versus in situ silicic acid concentrations. Experiments were conducted at each station, at the surface and/or at the chl a maximum.
At the Equator (Fig. 8b), ambient (Si(OH) ) was 2.00 lM, and the biogenic silica standing stock was higher (70 nmol l\). Accordingly, the Si-enrichment experiment was more conclusive, providing a much better description of the dependence of < upon silicic acid concentrations. Calculated K and < values are higher, reaching 2.42 ($0.53) lM and 0.052 ($0.004) h\. Finally, systematic indications of the degree to which Si uptake was limited by ambient silicic acid concentrations were provided by comparing Si uptake rate at ambient concentration and after addition of 10 lM silicic acid, at each station, at the surface or at the depth of the chlorophyll maximum (Fig. 9). Considering that the K determined experimentally at 33S and at the Equator (1.57 and 2.42 lM) are representative for diatom assemblages of the equatorial area, < could theoretically be reached by adding 10 times the K concentration (i.e. 15 to 24 lM of Si(OH) ). The addition of 10 lM of silicic acid, as applied in our experiments, would thus only enhance Si uptake rate up to approximately 85% of < . Therefore, the < /< l+ ratio, experimentally determined at all stations (Fig. 9), probably underestimates the degree to which Si uptake by the diatoms is limited by ambient silicic acid concentration. However, a signi"cant increase of Si uptake was observed at all stations after addition of 10 lM Si(OH) . In situ production rates were restricted to between 13 and 76% of < l+ . A stronger Si limitation was observed for lower ambient concentrations (Fig. 9). 4. Discussion 4.1. Production and export 4.1.1. Biogenic silica stocks and production rates At the Equator (03N, 1803E). The geographical limits of the area in#uenced by the equatorial upwelling vary more or less longitudinally depending upon the oscillation of the system from warm (El Nin o) to cold conditions (La Nin a). Chemical and physical conditions encountered during the EBENE cruise at the Equator (1803W) show a very similar pattern to those recorded during the FLUPAC cruise (Le Borgne et al., 1995; Eldin et al., 1997), at 1633W (station 76). Although our cruise track crossed the Equator more to the west, situations are highly comparable because
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FLUPAC occurred during a warming event (El Nin o), and the warm pool was displaced eastward (Fig. 1). The two sampling areas were situated at the edge of the cool tongue, with a sea surface temperature close to 28.53C. Silicic acid and nitrate concentrations were about 2 lM, thus with a Si/N ratio close to 1. For both cruises, surface biogenic silica concentration and production rates were highly comparable, with values close to 80 nmol l\ and 2 mmol m\ d\ (Blain et al., 1997). Accordingly, similar speci"c rates of biogenic production were 0.5 d\ for surface waters. Although exceptional situations can occur and have been met in frontal structures (Yoder, 1994), our results indicate comparable biogenic silica concentrations and production rates for similar physical and chemical conditions. This low variability reinforces the idea that, because of the quasi stationary equatorial upwelling (driven by trades winds) associated with the surface divergence, the equatorial upwelling area works like a chemostat (Frost and Franzen, 1992). Interesting comparison can also be made with recent estimates calculated indirectly from silicic acid supply rate to the euphotic zone (Dugdale and Wilkerson, 1998). The authors estimated a Si uptake rate of 2.36 mmol m\ d\, i.e. similar to the rates measured directly. However, the reported biogenic silica concentration (43 nmol l\) was substantially lower, which led to higher speci"c uptake rates (0.8 d\) compared to our results. This disagreement could be explained by the uncertainties resulting from their indirect estimate of the silica biomass. Dugdale and Wilkerson (1998) evaluated the biogenic silica biomass from an estimate of diatom chlorophyll contribution to total chl a (12%) by converting the diatom}chlorophyll to biogenic silica, considering that 1 lg l\ Chl-a"1 mmol m\ PON, and then using a 1 : 1 molar ratio for diatom N/Si. This was a stopgap measure in the absence of available data, but it is not surprising to get a two-fold di!erence given the variations that can be observed in Chl-a/PON or N/Si ratios. Hutchins and Bruland (1998), for example, found diatom N/Si ratios two to three times lower in Fe-limited medium than in Fe-enriched incubations. Stations poleward (north of 13N and south of 13S). At stations away from the direct in#uence of the equatorial upwelling, conditions encountered were typical of oligotrophic ecosystems. Integrated production rates were very low (range 0.1}0.7 mmol-Si m\ d\), averaging 0.4 mmol-Si m\ d\. These values are close to the lowest rates reported to date from other low silicic acid environments, like the western Sargasso Sea (range 0.2}1.5 mmol-Si m\ d\, Brzezinski and Nelson, 1996), the BATS site (range 0.1}0.9 mmol-Si m\ d\, Nelson and Brzezinski, 1997) or the western equatorial Paci"c (mean 1.5 mmol-Si m\ d\, Blain et al., 1997). These low silica production rates may partly be the result of the low availability of the major nutrients, as compared to silicic acid, and as shown by the Si/N ratios '1. 4.1.2. Contribution of diatoms to the new production of the equatorial Pacixc The contribution of diatoms to the new production was estimated indirectly from the comparison of biogenic silica production rates to new production measurements reported in the same area (Table 1). In the equatorial area (13S}13N), biogenic silica production rates range between 0.61 and 2.91 mmol-Si m\ d\ (average: 1.78 mmol-Si m\ d\) and can be compared with direct estimation of total new production based upon NO uptake rate measurements. Such a comparison is consistent as, owing to the low ambient ammonium concentration ((0.1 lM), N requirements of diatoms were probably fully met by NO uptake (Dortch, 1990). Furthermore, a strong correlation between NO uptake rates and diatom chlorophyll biomass was found by Landry et al. (1997) on
17 13.5}20 4.8
Ra/nitrate distribution C and upwelled nitrate N uptake, diel periodicity El Nino Post El Nino 2D advective balance between upw and meridional divergence of NO Th/scavenging model Th/Th
02}03/1992 -1992
02}03/1992
16.6 19.1 9}20
C, N, sed. trap Si
10/94 10}11/1996
8.2
N
Th, one-D model
Th/sed. trap
02}03/1992 08}09/1992 02}03/1992 08}09/1992 03}04/1992 10/1992 04/88
18.5 28
15
C;f ratio (0.5)
08}09/1992
New production (mmol-C m\ d\)
Method
Period
12.1 (155 m)/3.7 (320 m)
1.9 (120 m)/0.54 (200 m) 2.4 (120 m)/0.71 (200 m)
4}5 (100 m) 0.6}1.3 (100 m)/0.14}0.5 (850 m) 1.5}5.0 (100 m)/0.5}1.1 (850 m) 4.6}8.3 (150 m)
Export #ux (mmol-C m\ d\)
A constant molar Red"eld ratio of 6.6 was used to convert nitrogen uptake data into carbon uptake.
1353W 23N-63S 1503W, 03 1803W 03S, 1803W
1403W 93N}123S 1403W 123N-123S 1403W
903W}1803 103N}103S 1403W 903W}1803 53N}53S 1403W, 53N}53S 23N}23S 1503W 13S}13N
Location
Rodier et al. (1997) This study
Pen a et al. (1992)
Bacon et al. (1996)
Murray et al. (1996)
Buesseler et al. (1995) Luo et al. (1995)
Carr et al. (1995)
McCarthy et al. (1996)
Chavez and Barber (1987) Ku et al. (1995) Chavez et al. (1996)
Ref.
Table 1 Summary of reported rates of new production and exported #ux of organic matter from the surface layer of the equatorial Paci"c
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EqPac cruises during El Nin o (January}February 1992), and during normal upwelling conditions (August}September 1992). Our results based upon Si-uptake are consistent with rates of new production reported by McCarthy et al. (1996) and Pen a et al. (1992) from NO uptake (0.72}2.51 mmol-N m\ d\) in the same area. This comparison gives a mean Si/N ratio of 1.1, which is consistent with existing Si/N data for diatoms in culture. These ratios were shown to vary between 0.7 and 2, depending on ambient Fe concentrations (Takeda, 1998). The primary production due to diatoms in the equatorial area (13N}13S) can be estimated from the biogenic silica production and a Si/C diatom stoichiometry ranging from 0.13 (Brzezinski, 1985) to 0.29 (Takeda, 1998). Such a range is used to take into account a possible variation of the degree of silici"cation in response to a potential iron de"ciency (Hutchins and Bruland, 1998). Results of these calculations give a primary production due to diatoms that ranged between 2.10 and 22.38 mmol-C m\ d\ (average: 12.2 mmol-C m\ day\). These values compare well with carbon new production estimates reported from the same area (Table 1), ranging from 4.8 to 28 mmol-C m\ d\. Altogether, these calculations based on direct measurement of Si uptake suggest that new production in the equatorial region (13N}13S) could mostly be sustained by diatoms. 4.1.3. Biogenic silica export yuxes (03S, 1803W) It is generally admitted that the diatom-based production is exported to deeper layers (Dugdale and Goering, 1967; Eppley and Peterson, 1979), either directly (aggregation, sedimentation) or indirectly (after mesozooplankton grazing). An indirect estimate of biogenic silica loss in the surface layer can be derived from biogenic silica production rates measured during three successive days at the equator (Fig. 7), where steady-state conditions were met. Steady state was deduced from the stability of physical (Fig. 10), chemical and biological parameters (Le Borgne et al., 1998). Under such conditions, the biogenic silica produced in the euphotic layer (the gross production, equal to 2.6 mmol m\ d\) is balanced by the biogenic silica loss (i.e. export#recycled production) of the system. An indirect estimate of the contribution of diatom export production to the total carbon export production can be derived from the comparison between the diatom-C loss and carbon export production #uxes reported by others (Table 1) for the same area. The former was estimated from the mean biogenic silica production rate measured at steady state, during the 3-days station at the equator (03S, 1803W), and converted to carbon using Si/C ratios. It ranges between 9 and 20 mmol-C m\ d\. Carbon export production from the euphotic zone was estimated either from
Fig. 10. Evolution of salinity and temperature at 20 m depth, at a station located at 03N, 1803E, during three successive days.
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Th isotopes distribution or from particles interceptor traps (PIT). If we consider C export #uxes from the upper layer (100}150 m), they range altogether between 0.6 and 12.1 mmol-C m\ d\, with most of the data in the lower range (Table 1). This comparison shows that the minimum value calculated for the diatom-C loss is close to or higher than the maximum value of particulate organic carbon exported #ux. This suggests, on the one hand, that diatoms are a major contributor to the export #ux of organic matter from the surface to deeper layers, and, on the other hand, that a signi"cant fraction of the diatom biogenic silica is redissolved in the euphotic layer. 4.1.4. Dissolution of biogenic silica in the surface layer Direct measurements of biogenic silica dissolution rates are seldom made in low chlorophyll environment because of the lack of su$ciently sensitive methods. However, the few existing data (Nelson and Goering, 1977; Nelson et al., 1981; Nelson and Gordon, 1982; Brzezinski and Nelson, 1989) cover a fairly wide range of marine systems. The overall mean resulting from these studies indicates that, in surface waters, dissolution represents 58% of the biogenic silica production (Nelson et al., 1995). Temperature, physiological state of diatom cells (Kamatani, 1982), and bacterial activity (Bidle and Azam, 1999) have been shown to enhance the processes of biogenic silica dissolution. The speci"c value of 0.006 h\ or 0.14 d\ reported by Brzezinski and Nelson (1989) from the relatively warm waters (203C) of a Gulf Stream core ring is the one that could best compare with expected values for the equatorial Paci"c. On this basis, we calculate that biogenic silica dissolution could represent, in the upper 150 m, 33% (0.006 h\/0.018 h\) of the average speci"c production in the 13N}13S area, and 75% for stations polewards (0.006 h\/0.008 h\). In the equatorial Paci"c, biogenic silica dissolution rate can also be estimated from the budget between production rate and downward #ux of biogenic silica measured at 300 m depth with drifting sediment traps (Blain et al., 1997) in October 1994 (FLUPAC cruise). This percentage reaches 84%. It is very high compared to rates calculated above for the upper layer (150 m), but we must note that it is measured at 300 m depth, and dissolution at intervening depths might be high, as evidenced by Bacon et al. (1996). These results suggest strongly that, as in oligotrophic gyres, biogenic silica dissolution is of quantitative signi"cance and must be taken into account in the silicic acid cycle of the equatorial Paci"c. 4.2. Diatom limitation 4.2.1. In situ diatom growth limitation At steady state, the diatom growth rate, calculated from the measurement of Si uptake rate, is equal to the division rate of the diatom assemblage. Steady-state uptake rate is measured when the instantaneous conditions (nutrient concentration, temperature, salinity, light, etc.) are the same as the preconditioning one (Morel, 1987). The equatorial Paci"c has many non-steady-state characteristics. Westward propagating tropical instability waves, and eastward propagating Kelvin waves, are able to induce variability in biogeochemical processes on time scales of days. Their signatures can be seen in modi"cations of sea surface characteristics. However, no such events were observed at the equator over the course of the study (5}7 November 96), as shown by the
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temperature and salinity evolution (Fig. 10) or as described in more detail in the cruise report (Le Borgne et al., 1998). In this context, the Si uptake rate was measured at steady state, and the deduced diatom growth rate (k ) should be equal to the division rate of the diatom assemblage. In the equatorial zone (13S}13N), mean uptake rates measured under simulated in situ conditions at the surface imply a mean growth rate (k ) of 0.35 d\, yielding 0.5 doubling of biogenic silica per day. These growth rates are substantially lower than the 2.5 doublings of biomass per day predicted by the temperature-dependent equation of Eppley (1972), i.e., in non-limiting light and nutrient conditions, and suggest that diatom growth is limited. 4.2.2. Direct evidence of Si limitation The positive response of Si uptake rates by natural diatom communities of the central equatorial Paci"c after silicic acid enrichments, provides the "rst direct evidence of a widespread silicic acid limitation of biogenic silica production rates in this area. In both areas, half saturation constants of the diatom population are close to the ambient silicic acid concentration. At 33S, the measured K is 1.57 ($1.32) lM when ambient concentration is 1.45 lM, whereas at the Equator, the K is 2.42 ($0.53) lM in ambient concentration of 2 lM. These half saturation constants of Si uptake are in the range of values reported for other natural diatoms assemblages. Previously measured K values for silicic acid uptake vary widely, at least by one order of magnitude, from 0.53 lM (in a Gulf Stream warm-core ring, where ambient silicic acid concentration was below the detection limit, Nelson and Brzezinski, 1990) to 5.3 lM (in the Mississippi river plume, Nelson and Dortch, 1996). Higher K values are generally characteristic of silicic-acid-enriched environments. One example is the Ross Sea, where a K of 4.6 lM has been measured by Nelson and TreH guer (1992), in waters where silicic acid concentration seldom decrease below 5 lM. In situ measured K do not refer to a speci"c species, but rather to an assemblage of di!erent diatoms and could re#ect some adaptation of the diatom assemblage to in situ conditions. These results are in good agreement and support the Dugdale and Wilkerson (1998) modi"ed Si(OH) pump model, which states that the diatom population is regulating at about half the maximal uptake, with a substrate concentration near the half saturation constant, K . The < values reported from the two Si uptake kinetic studies (0.028 h\ at 33S and 0.052 h\ at the Equator) are comparable to those previously reported for natural population of the oligotrophic waters of the Sargasso Sea and the Gulf Stream warm-core ring (Brzezinski and Nelson, 1996; Nelson and Brzezinski, 1990), but lower than in the Peru upwelling region, where Goering et al. (1973) reported a < as high as 0.075 h\. Furthermore, the ratio < /< provides an indication of the degree to which the diatom Si uptake is limited by ambient silicic acid concentrations. That limitation is evaluated at 54% of the maximal speci"c uptake rate at 33S and 42% at the Equator. This direct evidence of Si uptake rate limitation does not necessarily imply that diatom growth rate is limited by Si. It is known that diatoms can maintain division rates close to k , even when the rate of Si uptake is signi"cantly less than < , by decreasing the Si content of the frustule. Conversely, physiological adaptation can take place in response to nutrient abundance (increase in cellular quota), in the course of the experiment, while division rate remains approximately constant. In other words, diatoms are able to maintain a growth rate signi"cantly lower than k , even when the rate of Si uptake is close to < , by increasing the Si content of their frustule. Then the diatom division rate (0.8 d\, or 1.15 doubling of the biomass per day) that we can predict from < is certainly an over-estimation of
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the maximal growth rate that diatoms could reach in their environment if ambient silicic acid was not limiting. However, this predicted and over-estimated daily growth is still signi"cantly lower than the 2.5 doublings of biomass per day predicted by the temperature-dependent equation of Eppley (1972), i.e., under non-limiting light and nutrient conditions. Comparing doubling times reported for equatorial Paci"c diatoms and those expected from temperature (Eppley, 1972) could appear bold. Eppley's curve is the upper limit of a scatter plot that has tremendous variations, and it does not exclude the possibility that many phytoplankton assemblages may not be physiologically capable of reaching this maximum. Nevertheless, Latasa et al. (1997) have measured, from dilution experiments in the central equatorial Paci"c, diatom growth rates of 1.7 d\. This result implies 2.4 doublings per day of the diatom population. Even if Latasa et al. (1997) did not rule out a possible containment or contamination artifact in their experiments to explain such high growth rates, this result shows that some diatom assemblages of the central equatorial Paci"c are physiologically capable of reaching the maximum predicted by Eppley's equation. As far as silicic acid limitation in the central equatorial Paci"c is concerned, we can conclude that ambient silicic acid is limiting biogenic silica production, but the silicic acid limitation of diatom growth is an hypothesis that remains to be tested. However, even if diatom growth was limited by ambient silicic acid, such Si limitation would certainly not be su$cient in itself to explain the low diatom growth rates observed in situ. 4.2.3. Co-limitation These results suggest that diatoms in the central equatorial Paci"c might be co-limited simultaneously by some other resource than silicic acid. In this area of the Paci"c, when looking for additional potentially co-limiting factor, we evidently think about iron, because iron limitation has been extensively reported as a controlling factor of phytoplankton growth. For instance, the Ironex II experiment (Coale et al., 1996b) resulted in a 85-fold increase in diatom biomass within a 10 km patch of surface water over 10 days, corresponding to a growth rate of about 1 db d\ (Coale et al., 1996a), which is also signi"cantly lower than the maximum expected from Eppley's formula (i.e. 2.5 db d\). If, with Fe addition alone, or Si addition alone, diatom growth does not even approach the upper limit imposed by the temperature dependence relation, then one explanation could be that Fe and Si limitations interact, rather than just being opposing explanations for the HNLC character of the system. Although little is known on the synergy between iron and silicic acid limitation, processlevel experiments performed at non-limiting silicic acid concentrations (Hutchins and Bruland, 1998; Takeda, 1998) have shown that Fe de"ciency could induce increases in cellular silica or a decrease in the cellular N content of diatoms. It is not clear, however, what the relationship between Fe limitation and cellular Si content would be once silicic acid is depleted to levels that limit the rate of Si uptake. All experiments to date that show increases in cellular Si under Fe limitation have been preformed at non-limiting silicic acid concentrations, and once Si uptake is limited by silicic acid, it would probably be more di$cult for diatom cells to produce frustules with elevated Si content. We could envision, as suggested in Ragueneau et al. (in press), that when both Si and Fe are depleted, diatom Si uptake rate is limited by silicic acid concentrations, but NO uptake rate or photosynthesis are limited by Fe.
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Additional data on those interactions would be of great value. But already existing models and studies of nutrient limitation should be reconsidered. They generally assume a single-controlling nutrient (Hecky and Kilham, 1988; Lancelot et al., 1997; Dugdale and Wilkerson, 1998), which is determined as the most limiting nutrient. The latter is "xed by the comparison between the ambient concentrations and the Michaelis}Menten kinetic parameters, characterizing the uptake of each nutrient. All nutrient uptake rates are then set proportional to that nutrient (Liebig model). O'Neill et al. (1989) show that although the Liebig model remains useful when one nutrient limitation predominates, it fails to converge to reasonable numbers when several nutrients are reaching sub-optimal concentrations. He studied and compared the behaviour of di!erent models: additive, multiplicative, and others. Among them, the additive model appears to have the best properties to serve as a general description for multiple nutrient limitation. In the equatorial Paci"c, conditions might be those reported by O'Neill et al. (1989) as ambient silicic acid and iron concentrations have both been reported as being at sub-optimal levels. Future studies should take into account simultaneous limitation by di!erent nutrients reaching sub-optimal concentrations, and new models (such the one proposed by O'Neill et al., 1989) should be tested. The development of a high sensitivity method to measure in situ silica dissolution rates and process-level studies to better understand the links between the silica and carbon cycles are necessary. Acknowledgements We especially thank Robert Le Borgne for his leadership as chief scientist, the captain and crew of the R.V. Atalante, and Annick Masson for technical assistance in sample analysis. This research was supported by CNRS-INSU and ORSTOM. References Bacon, M.P., Cochran, J.K., Hirschberg, D., Hammar, T.R., Fleer, A.P., 1996. Export #ux of carbon at the Equator during the Eqpac time-series cruises estimated from Th measurments. Deep-Sea Research II 43 (4}6), 1133}1154. Banahan, S., Goering, J.J., 1986. The production of biogenic silica and its accumulation on the southeastern Bering Sea shelf. Continental Shelf Research 5, 199}213. Bidle, K.D., Azam, F., 1999. Accelerated dissolution of diatom silica by marine bacterial assemblages. Nature 397, 508}512. Blain, S., Leynaert, A., TreH guer, P., ChreH tiennot-Dinet, M.J., Rodier, M., 1997. Biomass, growth rates and limitation of equatorial Paci"c diatoms. Deep-Sea Research I 44, 1255}1275. Brzezinski, A.M., 1985. The Si : C : N ratio of marine diatoms: interspeci"c variability and the e!ect of some environmental variables. Journal of Phycology 21, 347}357. Brzezinski, M.A., Nelson, D.M., 1989. Seasonal changes in the silicon cycle within a Gulf Strean warm-core ring. Deep-Sea Research I 36, 1009}1030. Brzezinski, M.A., Nelson, D.M., 1996. Chronic substrate limitation of silicic acid uptake rates in the western Sargasso Sea. Deep-Sea Research II 43, 437}453. Brzezinski, M.A., Phillips, D.R., Chavez, F.P., Friederich, G.E., Dugdale, R.C., 1997. Silica production in the Monterey, California, upwelling system. Limnology and Oceanography 42 (8), 1694}1705. Buesseler, K.O., Andrews, J.A., Hartman, M.C., Belastock, R., Chai, F., 1995. Regional estimates of the export #ux of particulate organic carbon derived from thorium-234 during the JGOFS Eqpac program. Deep-Sea Research II 42 (2}3), 777}804.
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Carr, M., Lewis, M.R., Kelley, D., Jones, B., 1995. A physical estimate of new production in the equatorial Paci"c along 1503W. Limnology and Oceanography 40 (1), 138}147. Chavez, F.P., Buck, K.R., Service, S.K., Newton, J., Barber, R.T., 1996. Phytoplankton variability in the central and eastern tropical Paci"c. Deep-Sea Research II 43 (4}6), 835}870. Chavez, F.P., Barber, R.T., 1987. An estimate of new production in the equatorial Paci"c. Deep Sea Research 34, 1229}1243. Chavez, F.P., Buck, K.R., Barber, R.T., 1990. Phytoplankton taxa in relation to primary production in the equatiorial Paci"c. Deep-Sea Research 37, 1733}1752. Coale, K.H., Fitzwater, S.E., Gordon, R.M., Johnson, K.S., Barber, R.T., 1996a. Control of community growth and export production by upwelled iron in the equatorial Paci"c Ocean. Nature 379, 621}624. 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., 1996b. A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Paci"c Ocean. Nature 383, 495}501. Dortch, Q., 1990. The interaction between ammonium and nitrate uptake in phytoplankton. Marine Ecology Progress Series 61, 183}201. Dugdale, R.C., Goering, J.J., 1967. Uptake of new and regenerated form of nitrogen in primary productivity. Limnology and Oceanography 12, 196}206. Dugdale, R.C., Wilkerson, F.P., Minas, H.J., 1995. The role of silicate pump in driving new production. Deep-Sea Research I 42, 697}719. Dugdale, R.C., Wilkerson, F.P., 1998. Silicate regulation of new production in the equatorial Paci"c upwelling. Nature 391, 270}273. Eldin, G., Rodier, M., Radenac, M.-H., 1997. Physical and nutrient variability in the upper equatorial Paci"c associated with westerly wind forcing and wave activity in October 1994. Deep-Sea Research II 44, 1783}1800. Eppley, R.W., 1972. Temperature and phytoplankton growth in the sea. Fisheries Bulletin 70, 1063}1085. Eppley, R.W., Peterson, B.J., 1979. Particulate organic matter #ux and planktonic new production in the deep ocean. Nature 282, 677}680. Frost, B.W., Franzen, N.C., 1992. Grazing and iron limitation in the control of phytoplankton stock and nutrient concentration: a chemostat analogue of the Paci"c equatorial upwelling zone. Marine Ecology Progress Series 83, 291}303. Glibert, P.M., McCarthy, J.J., 1984. Uptake and assimilation of ammonium and nitrate by phytoplankton: indices of nutritional status for natural assemblages. Journal of Plankton Research 6, 677}697. Goering, J.J., Nelson, D.M., Carter, J.A., 1973. Silicic acid uptake by natural population of marine phytoplankton. Deep-Sea Research I 20, 777}789. Hecky, R.E., Kilham, P., 1988. Nutrient limitations of phytoplankton in freshwater and marine environments; a review of recent evidence on the e!ects of enrichment. Limnology and Oceanography 33, 796}822. Hutchins, D.A., Bruland, K.W., 1998. Iron-limited diatom growth and Si : N uptake ratios in a coastal upwelling regime. Nature 393, 561}564. Kamatani, A., 1982. Dissolution rates of silica from diatoms decomposing at various temperatures. Marine Biology 68, 91}96. Ku, T.L., Luo, S., Kusakabe, M., Bishop, J.K.B., 1995. Ra-derived nutrient budgets in the upper equatorial Paci"c and the role of `newa silicate in limiting productivity. Deep-Sea Research II 42 (2}3), 479}498. Lancelot, C., Rousseau, V., Billen, G., Van Eeckhout, D., 1997. Coastal eutrophication of the Southern Bight of North Sea: assessment and modeling. In: Ozsoy, E., Mikaelyan, A. (Eds.), Sensitivity to Change: Black Sea, Baltic Sea and North Sea. NATO-ASI Series, Vol. 2, pp. 439}454. 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 Paci"c: an Eqpac synthesis. Limnology and Oceanography 42, 405}418. Latasa, M., Landry, M.R., SchluK ter, L., Bidigare, R.R., 1997. Pigment-speci"c growth and grazing rates of phytoplankton in the central equatorial Paci"c. Limnology and Oceanography 42 (2), 289}298.
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Le Borgne, R., Brunet, C., Eldin, G., Radenac, M.H., Rodier, M., 1995. Campagne oceH anographique FLUPAC a` bord du N.O. l'Atalante. Du 23 septembre au 29 octobre 1994. Recueil des donneH es, Tome 1. ORSTOM, NoumeH a, Archives Sciences de la Mer, oceH anographie, No. 1. Le Borgne, R., Langlade, M.J., Polidori, P., Rodier, M., 1998. Campagne oceH anographique EBENE a` bord du N.O. l'Atalante. Du 21 octobre au 20 novembre 1996. Recueil des donneH es, Tome 1. ORSTOM, NoumeH a, Archives Sciences de la Mer, oceH anographie, No. 3. Leynaert, A., TreH guer, P., Nelson, D.M., Del Amo, Y., 1996. Si as a tracer of biogenic silica production: methodological improvements. In: Baeyens, J., Dehairs, F., Goeyens, L. (Eds), Integrated Marine System Analysis, Minute of the First Workshop Meeting. VUB, Brussels, Belgium, pp. 29}35. Luo, S., Ku, T.L., Kusakabe, M., Bishop, J.K.B., Yang, Y.L., 1995. Tracing particle cycling in the upper ocean with Th and Th: An investigation in the equatorial Paci"c along 1403W. Deep-Sea Research II 42 (2}3), 805}830. McCarthy, J.J., Garside, C., Nevins, J.L., Barber, R.T., 1996. New production along 1403W in th equatorial Paci"c during and following the 1992 El Nino event. Deep-Sea Research II 43 (4}6), 1065}1094. Morel, F.M.M., 1987. Kinetics of nutrient uptake and growth in phytoplankton. Journal of Phycology 23, 137}150. Murray, J.W., Young, J., Newton, J., Dunne, J., Chapin, T., Paul, B., McCarthy, J.J., 1996. Export #ux of particulate organic carbon from the central equatorial Paci"c determined using a combined drifting trap - Th approach. Deep-Sea Research II 43 (4-6), 1095}1132. Nelson, D.M., Brzezinski, M.A., 1990. Kinetics of silicic acid uptake by natural diatom assemblages in two Gulf Stream warm-core rings. Marine Ecology Progress Series 62, 283}292. Nelson, D.M., Brzezinski, M.A., 1997. Diatom growth and productivity in an oligotrophic midocean gyre: a 3-yr record from the Sargasso Sea near Bermuda. Limnology and Oceanography 42, 473}486. Nelson, D.M., Dortch, Q., 1996. Silicic acid depletion and silicon limitation in the plume of the Mississippi river: evidence from kinetics studies in spring and summer. Marine Ecology Progress Series 136, 163}178. Nelson, D.M., Goering, J.J., 1977. Near-surface silica dissolution in the upwelling region o! northwest Africa. Deep-Sea Research I 24, 65}73. Nelson, D.M., Goering, J.J., 1978. Assimilation of silicic acid by phytoplankton in the Baja California and northwest Africa. Limnology and Oceanography 23 (3), 508}517. Nelson, D.M., Goering, J.J., Boisseau, D.W., 1981. Consumption and regeneration of silicic acid in three coastal upwelling systems. In: Richards, F.A. (Ed.), Coastal Upwelling, Vol. 1. AGU, Washington DC, pp. 242}256. Nelson, D.M., Gordon, L.I., 1982. Production and pelagic dissolution of biogenic silica in the southern Ocean. Geochimica et Cosmochimica Acta 46, 491}501. Nelson, D.M., Smith, W.O., 1986. Phytoplankton bloom dynamics of the western Ross Sea ice-edge. II. Mesoscale cycling of nitrogen and silicon. Deep-Sea Research I 33, 1389}1412. Nelson, D.M., Smith, W.O., 1991. Sverdrup revisited: critical depths, maximum chlorophyll levels, and control of Southern Ocean productivity by the irradiance-mixing regime. Limnology and Oceanography 36 (8), 1650}1661. Nelson, D.M., TreH guer, P., 1992. Role of silicon as a limiting nutrient to Antarctic diatoms: evidence from kinetic studies in the Ross Sea ice-edge zone. Marine Ecology Progress Series 80, 255}264. Nelson, D.M., TreH guer, P., Brzezinski, M.A., Leynaert, A., QueH guiner, B., 1995. Production and dissolution of biogenic silica in the ocean: revised estimates, comparison with regional data and relationship to biogenic silica sedimentation. Global Biogeochemical Cycle 9, 359}372. O'Neill, R.V., DeAngelis, D.L., Pastor, J.J., Post, W.M., 1989. Multiple nutrient limitation in ecological models. Ecological Modelling 46, 147}163. Paasche, E., 1973. Silicon and the ecology of marine plankton diatoms. 1. Thalassiosira pseudonana (Cyclotella nana) grown in a chemostat with silicate as a limiting nutrient. Marine Biology 19, 117}126. Pen a, M.A., Harrison, W.G., Lewis, M.R., 1992. New production in the central equatorial Paci"c. Marine Ecology Progress Series 80, 265}274. Price, N.M., Ahner, B.A., Morel, F.M.M., 1994. The equatorial Paci"c ocean: grazer-controlled phytoplankton population in an iron-limited ecosystem. Limnology and Oceanography 39, 520}534. Ragueneau, O., TreH guer, P., 1994. Determination of biogenic silica in coastal waters: applicability and limits of the alkaline digestion method. Marine Chemistry 45, 43}51.
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Silicon limitation of biogenic silica production in the Equatorial Paci"c A. Leynaert *, P. TreH guer , Christiane Lancelot, Martine Rodier Laboratoire **Flux de matie% re et re& ponses du vivant++, Institut Universitaire Europe& en de la Mer, TechnopoL le Brest-Iroise. Place N. Copernic, 29 280 Plouzane, France Groupe de Microbiologie des Milieux Aquatiques, Campus de la Plaine, CP 221, Boulevard du Triomphe, 1050 Bruxelles, Belgium Institut de Recherche pour le De& veloppement, Station Marine d'Endoume, Chemin de la batterie des lions, 13007 Marseille, France Received 4 August 1999; received in revised form 14 February 2000; accepted 6 March 2000
Abstract During the EBENE cruise (November 1996), distributions of biogenic silica concentration and production rates were investigated in the surface waters of the equatorial Paci"c (1803W, from 83S to 83N), with particular emphasis on the limitation of the biogenic silica production by ambient silicic acid concentrations. Integrated over the depth of the euphotic layer, concentrations of biogenic silica and production rates were maximum at the Equator (8.0 and 2.6 mmol m\ d\) and decreased more or less symmetrically polewards. Contribution of diatoms to the new production was estimated indirectly, comparing biogenic silica production rates and available data of new and export production in the same area. This comparison shows that new production in the equatorial area could mostly be sustained by diatoms, accounting for the major part of the exported #ux of organic carbon. Kinetics experiments of silicic acid enrichment were performed. Half saturation constants were 1.57 lM at 33S and 2.42 lm at the Equator close to the ambient concentrations. The corresponding < values for Si uptake were 0.028 h\ at 33S and 0.052 h\ at the equator.
Experiments also show that in situ rates were restricted to 13}78% of < , depending on ambient silicic acid
concentrations. This work provides the "rst direct evidence that the rate of Si uptake by diatom populations of the equatorial Paci"c is limited by the ambient concentration of silicic acid. However, such Si limitation might not be su$cient in itself to explain the low diatom growth rates observed, and additional limitation is suggested. One hypothesis that is consistent with the results of Fe limitation studies is that Fe and Si limitations may interact, rather than just being a mutually exclusive explanation for the HNLC character of the system. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Silicon cycle; Diatoms; Limiting factor; Equatorial Paci"c
* Corresponding author. Tel.: 33-2-98-49-86-57; fax: 33-98-49-86-95. E-mail address: [email protected] (A. Leynaert). 0967-0637/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 0 0 ) 0 0 0 4 4 - 3
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1. Introduction The carbon biologically "xed in the surface layer and exported to the deep sea (the biological pump of CO ) is one of the major factors controlling CO partial pressure in the atmosphere (Sarmiento and Siegenthaler, 1992). Accurate determination of this #ux and of its controlling factors are therefore critically important for understanding global carbon cycling and its response to climate change. It has been estimated that about half of the export #ux of organic carbon to the deep ocean is synthesized by diatoms (Nelson et al., 1995). Consequently, it is essential to consider the factors in#uencing the relative contribution of diatoms to total primary and export production, to better understand the processes that determine the e$ciency of the biological pump. Most studies of biogenic silica production reported to date have emphasized areas of known or presumed high primary productivity, relatively high diatom abundance, and active accumulation of diatomaceous sediments. The equatorial Paci"c is one of the main areas for opal sediment accumulation from which few measurements of silica production from overlying waters are yet available. The central equatorial Paci"c is known as a `high nutrient low chlorophylla (HNLC) region. If phytoplankton biomass and production rates remain low, diatoms have been implicated to play an important role (Chavez et al., 1990). Among other factors that have been put forth to unravel the processes that maintain the relatively constant HNLC conditions, grazing pressure and iron limitation have been argued (Coale et al., 1996a; Landry et al., 1997), but neither has proven su$cient in itself to explain the characteristics of HNLC regions (Price et al., 1994). Recently, Ku et al. (1995) suggested the availability of `newa silicic acid as a limiting factor controlling production in the upper equatorial Paci"c. This hypothesis, based upon studies of nutrient budgets and Ra distributions, received support from Dugdale et al. (1995) and Dugdale and Wilkerson (1998), who also showed from a simple silicon-cycle model that silicic acid would play a key role in the regulation of new production in the equatorial Paci"c upwelling. However, no direct experimental evidence of silicic acid limitation has yet ever been obtained in this area. As part of a more global investigation of biogenic silica concentration and production rates, this paper reports the "rst experimental evidence of silicic acid limitation of diatom silica production in the equatorial Paci"c. Experiments were conducted during the IRD (ex-ORSTOM) EBENE cruise (1996) in the equatorial Paci"c, as part of the French contribution to the international JGOFS program. It was in continuity with the FLUPAC cruise (1994). Since data on the silica cycle were also collected along the equator (between 1703E and 1553W) during FLUPAC, we will frequently refer to the FLUPAC cruise in this paper.
2. Methods Experiments were conducted during the EBENE cruise, in the equatorial Paci"c (October}November 1996), aboard the French R.V. Atalante. The cruise plan consisted of one transect along the date line, from 83S to 83N, with a station at each degree and two time-series stations, at 33S and at the Equator.
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Chemical measurements (nitrate, nitrite, ammonium, phosphate and silicic acid) were carried out by standard automated colorimetric methods (Strickland and Parsons, 1972). Data sets and analytical details are reported in Le Borgne et al. (1998). 2.1. Biogenic silica concentration Biogenic silica (BSi) was determined on particulate matter collected by "ltration of 2 l of sea water through 0.6 lm polycarbonate membrane "lter. The "lter was folded in quarters, placed in a petri dish, dried at 603C for 12 h and stored at ambient temperature. Later analysis was performed using the alkaline digestion method (Paasche, 1973) modi"ed by Ragueneau and TreH guer (1994). The blank was 6 nmol l\ (SD"3 nmol l\). 2.2. Biogenic silica production rates Biogenic silica production rates (PBSi) were measured at six depths in the euphotic zone. Incubations were generally conducted under simulated in situ conditions, except at the time series stations, where incubations were performed in situ from dawn to dusk (total duration of about 12 h), using a drifting array, and followed by deck incubations for the night, to complete the 24 h. The general procedure was the following: 250 ml samples, in polycarbonate bottles, were spiked with 50 000 dpm (830 Bq) of Si tracer (52 000 Bq/lg Si, Los Alamos National Laboratory). At the end of the incubation period (24 h), each sample was gently vacuum-"ltered through a 0.6 lm polycarbonate membrane "lter (Nuclepore) and rinsed with 10 ml of "ltered seawater. The "lter was then immediately placed in the bottom of a 20 ml plastic vial. 2 ml of 2.9 M HF was added to dissolve biogenic silica. The reaction was complete after 30 minutes, and 10 ml of scintillation cocktail (Ultima Gold XR) was then added to each vial. After shaking, samples were counted on a Tri-carb 1500 TR instrument (Packard) for 60 minutes, or when a counting precision of 0.5% was achieved for cpm in each counting window. An equilibrated Si solution and P standards were used to deconvolve the energy spectra, as described in Leynaert et al. (1996). The biogenic silica production rate (PBSi, in nmol l\ h\) is the fraction between the initially dissolved Si activity added to the sample, and the Si taken up by phytoplankton and counted on the "lter at the end of the incubation, divided by the incubation time. To control for the non-biological uptake of Si, sodium azide and HgCl have been used essentially on seawater samples from turbid environments (North Sea and Black Sea coastal waters). Evidence of adsorption has never been observed, "nal counts being always at the background level. However, because of the extremely high cost of the radionucleide Si, these control experiments are not run each time. The speci"c production rate (<, time\) is the molar ratio of the biogenic silica production rate to the initial biogenic silica standing stock. < represents a minimum estimate of the speci"c production rate by living cells, due to the presence of non-living biogenic silica (empty frustules, shell fragments, etc.). The growth rate (k) is the rate of increase of a cell substance per unit of that cell substance (Eppley, 1972; Schlegel, 1987). In mathematical terms: 1 dN k" ; , N d¹
(1.1)
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which upon integration becomes ln N!ln N k" , t
(1.2)
N could be any cellular constituent (carbon, chlorophyll, biogenic silica, cells number, etc.). In our case, N and N denote the biogenic silica at time t and at time t, respectively. The biogenic silica at time t is predicted from the initial biogenic silica concentration and the uptake rate. However, at steady state, <"k (Eppley, 1972; Schlegel, 1987). The mean doubling time (t ) is de"ned as the time required for the cellular component to increase by a factor 2. The relationship between t and k is derived from Eq. (1.2): ln 2 . t " k Its reciprocal, the number of doublings per unit of time (1/t ), is also commonly used. When comparing the growth rate and the cell division rate, one must bear in mind that mass per cell is not necessarily constant and can change during growth. However, at steady state, a doubling of biomass is accompanied by a doubling of all other measurable constituents of the population. 2.3. Si-limitation experiments The concentration dependence of diatom silicic acid uptake rates was investigated at 33S and at the Equator by conducting Si tracer kinetic experiments at di!erent silicic acid concentrations. For each kinetic experiment, a set of 10 (at 33S) or eight (at the Equator) 250 ml sub-samples were poured into polycarbonate incubation bottles and enriched with Si(OH) , at concentrations ranging from 0 to 10 lM above ambient. A 50 ml subsample was collected for nutrient analysis and immediately processed. Then 200 000 dpm (3300 Bq) of Si tracer (52 000 Bq/lg Si) at 33S, or 125 000 dpm (2100 Bq) at the Equator, was added to each bottle. Incubations were conducted at sunrise for 6 h, at full light and under in situ simulated conditions. After incubation, samples were processed as described above for biogenic silica production experiments. Additional experiments on silicic acid limitation were conducted at two depths (surface and chl a maximum) at each station, according to Glibert and McCarthy (1984) and Brzezinski et al. (1997). Basically, these experiments compare Si incubation performed at ambient concentration with those enriched to 10 lM Si(OH) . Considering the K of 1 lM reported for eastern tropical Paci"c diatoms (Thomas and Dodson, 1975) and assuming that silicic acid uptake obeys the MichaelisMenten equation, more than 90% of < is reached at 10 lM Si(OH) enrichments (10 times the K ). The ratio between in situ and enriched samples gives an indication of the degree to which silicic acid uptake by phytoplankton is limited by ambient silicic acid concentrations. 3. Results 3.1. Climatic regime during EBENE, in October}November 1996 The climate system of the equatorial Paci"c is subject to intermittent variabilities that lead to large sea surface temperature anomalies. Oscillations between unusually warm (El Nin o) and cold
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Fig. 1. TAO (Tropical Atmosphere Ocean) monthly mean sea surface temperature (3C) and winds (m s\), and temperature anomalies of the studied area (printed from http://www.pmel.noaa.gov/tao-bin/tao/cover), in November 1994 (FLUPAC cruise) and in November 1996 (EBENE cruise).
(La Nin a) conditions have now been well documented. Along the Equator, in November 1996, EBENE took place during a `neutral scenarioa between El Nin o and La Nin a, as shown by the plot of sea surface temperature anomalies (Fig. 1). The cruise track crossed the Equator at 1803W. Although fairly to the west, the tongue of cool upwelled water was still perceptible at that longitude (sea surface temperature was close to 28.53C). 3.2. Nutrient distributions Fig. 2 shows section plots of silicic acid and inorganic nitrogen (nitrate#nitrite#ammonium) concentrations along the transect in the upper 400 m, from 83S to 83N. Silicic acid concentrations
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Fig. 2. Vertical nutrient distributions: (a) N in lmol l\, (b) Si(OH) in lmol l\, (c) Si/N molar ratio, from 83S to 83N, at 1803W.
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were relatively constant at the surface, ranging from 1.0 to 2.3 lM. Inorganic nitrogen concentrations were more variable, ranging from complete depletion to 3.5 lM. Contours shoal in response to strong vertical advection in the equatorial upwelling zone. They decrease polewards at the surface. In the water column, silicic acid and nitrate concentrations increased sharply below the 0.1% light depth (150 m) to reach about 25 and 32 lM, respectively, at 400 m. The lack of coupling between nitrate and silicic acid regeneration is evidenced by the vertical distribution of the Si/N molar ratio, considering the value of 1 as typical for diatom growth requirement (Fig. 2c). Values slightly below, but close to 1, are observed in the surface upwelled waters at the equator, whereas the ratio decreases to values lower than 0.5 at the base of the euphotic zone (150 m). At the surface, the ratio increases polewards dramatically to values exceeding 50, resulting from nitrate depletion. 3.3. Biogenic silica distribution across the Equator (83S}83N) along the date line Biogenic silica concentrations were very low (7}80 nmol l\) all along the transect (Fig. 3). The maximum was encountered at the Equator. A two-fold increase of the concentrations was observed in the euphotic zone for stations of the equatorial area (13S}13N), as compared to stations polewards. Vertical distribution of biogenic silica evidenced a maximum at the surface, and often in the subsurface, close to the nitracline, for stations outside the direct in#uence of the upwelling (south of 13S and north of 13N). A slight maximum in biogenic silica concentration was also noticed around 300 m depth (Fig. 3). During the long-term stations at the equator and at 33S, vertical pro"les of BSi were obtained during three successive days. Integrated in the euphotic layer (150 m), BSi concentrations were 6.9, 7.4 and 9.5 mmol m\ at the Equator and 3.4, 4.8, and 3.7 mmol m\, at 33S, giving averages of 8.0 ($1.4) and 4.0 ($0.7) mmol m\. No noticeable variations can be evidenced during the time course of each station, as the maximum standard deviation between the successive experiments (1.4 mmol m\) is close to the detection limit.
Fig. 3. Vertical section of biogenic silica concentrations (nmol-Si l\) along 1803W (negative latitudes are south).
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3.4. Biogenic silica production rates Time course experiments of silicic acid uptake were conducted for 24 h at two stations (33S and the Equator). Sub-samples were taken after 6, 12 and 24 h incubation, and the silicic acid uptake was measured. Results (Fig. 4) clearly show that silicic acid uptake did not proceed at the same rate during day and night. A signi"cant decrease of Si uptake of about 30% was observed during the dark period for both investigated communities. Although we cannot rule out a possible bottle artifact, these results suggest some energy dependence of Si uptake by diatoms. Accordingly, samples collected at 0.1% light depth did not show any Si uptake. Other studies have reported (Brzezinski and Nelson, 1989; Nelson and Brzezinski, 1997) little evidence of systematic day/night di!erences among pro"les of the biogenic silica speci"c production rates for natural phytoplankton population. However diel periodicity in Si uptake was observed by Goering et al. (1973), with
Fig. 4. Time course of biogenic silica production (nmol-Si l\) determined from surface water in situ incubations of 6, 12 and 24 h, in triplicate, (a) at the Equator, and (b) at 33S.
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Fig. 5. Vertical section of biogenic silica production rates (PBSi, in nmol-Si l\ h\), along 1803W (negative latitudes are south).
Fig. 6. Latitudinal (from 83S to 73N) pro"le of daily biogenic silica production, integrated in the photic layer (mmol m\ d\), along 1803W.
maximum rates occurring at noon. It implies that diatom speci"c uptake rate derived from short-term incubations might be over-estimated or under-estimated, depending on whether the incubation has been conducted around noon or not. Daily rates of biogenic silica production (Fig. 5) in surface waters were generally very low, ranging from less than 0.2 to about 36 nmol l\ d\. Vertical pro"les of biogenic silica production show maximum rates in surface or subsurface waters, and a general decrease with depth. Integrated in the upper layer, daily rates of biogenic silica production (Fig. 6) were maximal at the Equator (2.58$0.40 mmol m\ d\) and decreased more or less symmetrically polewards, to 0.3 mmol m\ d\ at 73N and less than 0.1 mmol m\ d\ at 83S. During the long-term station at
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Fig. 7. Biogenic silica biomass (lmol l\) and production (nmol l\ h\) pro"les performed during three successive days at the equator (03S, 1803W).
the Equator, biogenic silica production measurements, carried out during three successive days, showed little daily #uctuation (Fig. 7). When integrated in the euphotic layer, they varied from 2.14 to 2.91 and 2.70 mmol m\ d\, suggesting steady state. It is interesting to note that there is one order of magnitude di!erence between the production rate of surface waters at the Equator and the most northern and southern stations, whereas there is only a two-fold increase in biogenic silica concentrations. As a result, diatom communities at the equator are characterized by signi"cantly higher speci"c production rates (<, d\). Indeed, the average daily rates in surface waters are 0.43 ($0.23) d\ for stations in the equatorial band (13N}13S), and 0.21 ($0.21) d\ for stations polewards. In the same way, the mean estimated doubling time of the silica biomass (t ) varied from 2.4 ($1.1) days to 6.2 ($5.6) days in the two areas de"ned above. The vertical pro"les of < (not shown) generally paralleled those of PBSi, displaying a sharp decrease with depth in the upper 100 m and very low values below. The speci"c production rate (<), when averaged over the euphotic layer, varied by one order of magnitude during the whole cruise: from 0.03 to 0.29 d\. In the area between 13N and 13S, < averaged 0.24 d\, consistent with typical values reported for coastal upwelling (0.3 d\ o! Baja California, Nelson and Goering, 1978). The mean for stations situated polewards was 0.13 d\, exceeding mean speci"c production rates reported for HNLC polar waters (generally below 0.1 d\, Nelson and Smith, 1986, 1991; Banahan and Goering, 1986), but in the same range as values reported for oligotrophic mid-ocean gyres (Nelson and Brzezinski, 1997). 3.5. Kinetic studies of silicic acid uptake The silicic acid concentration dependence of Si uptake was investigated at two stations (33S and the Equator). Speci"c uptake rates (<, h\) were plotted as a function of the ambient silicic acid
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Fig. 8. Concentration dependence of silicic uptake rate by natural diatom assemblages (a) at 33S and (b) at the Equator, at 1803W. Data points show the measured values of <; the curve represents the Michaelis}Menten hyperbola "tted to the data by the non-linear regression method of Wilkinson (1961).
concentrations (Fig. 8). The dependence of < upon (Si(OH) ) was determined by "tting the Michaelis}Menten equation to the data < ;[Si(OH) ] , <" K #[Si(OH) ] in which < is the speci"c uptake rate, < the speci"c uptake rate at in"nite [Si(OH) ], and K the half-saturation constant (the silicic acid concentration at which <"< /2). These two constants are determined by "tting the data with the non-linear method of Wilkinson (1961). At 33S, the ambient (Si(OH) ) was 1.45 lM. The Si-enrichment experiment gives some evidence of an hyperbolic response of < to increased silicic acid concentration (Fig. 8a), although the "t to the Michaelis}Menten function is only approximate. K and < values are 1.57 ($1.32) lM and 0.028 ($0.006) h\. As diatoms were taking up silicic acid quite rapidly, as indicated by the relatively high < value, the scatter in the data may be attributed to the very low phytoplankton biomass, and particularly the very low biogenic concentrations (30 nmol l\).
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Fig. 9. Ratio of biogenic silica production rates at ambient silicic acid concentrations to biogenic silica production rates at 10 lM of silicic acid concentration above ambient, versus in situ silicic acid concentrations. Experiments were conducted at each station, at the surface and/or at the chl a maximum.
At the Equator (Fig. 8b), ambient (Si(OH) ) was 2.00 lM, and the biogenic silica standing stock was higher (70 nmol l\). Accordingly, the Si-enrichment experiment was more conclusive, providing a much better description of the dependence of < upon silicic acid concentrations. Calculated K and < values are higher, reaching 2.42 ($0.53) lM and 0.052 ($0.004) h\. Finally, systematic indications of the degree to which Si uptake was limited by ambient silicic acid concentrations were provided by comparing Si uptake rate at ambient concentration and after addition of 10 lM silicic acid, at each station, at the surface or at the depth of the chlorophyll maximum (Fig. 9). Considering that the K determined experimentally at 33S and at the Equator (1.57 and 2.42 lM) are representative for diatom assemblages of the equatorial area, < could theoretically be reached by adding 10 times the K concentration (i.e. 15 to 24 lM of Si(OH) ). The addition of 10 lM of silicic acid, as applied in our experiments, would thus only enhance Si uptake rate up to approximately 85% of < . Therefore, the < /< l+ ratio, experimentally determined at all stations (Fig. 9), probably underestimates the degree to which Si uptake by the diatoms is limited by ambient silicic acid concentration. However, a signi"cant increase of Si uptake was observed at all stations after addition of 10 lM Si(OH) . In situ production rates were restricted to between 13 and 76% of < l+ . A stronger Si limitation was observed for lower ambient concentrations (Fig. 9). 4. Discussion 4.1. Production and export 4.1.1. Biogenic silica stocks and production rates At the Equator (03N, 1803E). The geographical limits of the area in#uenced by the equatorial upwelling vary more or less longitudinally depending upon the oscillation of the system from warm (El Nin o) to cold conditions (La Nin a). Chemical and physical conditions encountered during the EBENE cruise at the Equator (1803W) show a very similar pattern to those recorded during the FLUPAC cruise (Le Borgne et al., 1995; Eldin et al., 1997), at 1633W (station 76). Although our cruise track crossed the Equator more to the west, situations are highly comparable because
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FLUPAC occurred during a warming event (El Nin o), and the warm pool was displaced eastward (Fig. 1). The two sampling areas were situated at the edge of the cool tongue, with a sea surface temperature close to 28.53C. Silicic acid and nitrate concentrations were about 2 lM, thus with a Si/N ratio close to 1. For both cruises, surface biogenic silica concentration and production rates were highly comparable, with values close to 80 nmol l\ and 2 mmol m\ d\ (Blain et al., 1997). Accordingly, similar speci"c rates of biogenic production were 0.5 d\ for surface waters. Although exceptional situations can occur and have been met in frontal structures (Yoder, 1994), our results indicate comparable biogenic silica concentrations and production rates for similar physical and chemical conditions. This low variability reinforces the idea that, because of the quasi stationary equatorial upwelling (driven by trades winds) associated with the surface divergence, the equatorial upwelling area works like a chemostat (Frost and Franzen, 1992). Interesting comparison can also be made with recent estimates calculated indirectly from silicic acid supply rate to the euphotic zone (Dugdale and Wilkerson, 1998). The authors estimated a Si uptake rate of 2.36 mmol m\ d\, i.e. similar to the rates measured directly. However, the reported biogenic silica concentration (43 nmol l\) was substantially lower, which led to higher speci"c uptake rates (0.8 d\) compared to our results. This disagreement could be explained by the uncertainties resulting from their indirect estimate of the silica biomass. Dugdale and Wilkerson (1998) evaluated the biogenic silica biomass from an estimate of diatom chlorophyll contribution to total chl a (12%) by converting the diatom}chlorophyll to biogenic silica, considering that 1 lg l\ Chl-a"1 mmol m\ PON, and then using a 1 : 1 molar ratio for diatom N/Si. This was a stopgap measure in the absence of available data, but it is not surprising to get a two-fold di!erence given the variations that can be observed in Chl-a/PON or N/Si ratios. Hutchins and Bruland (1998), for example, found diatom N/Si ratios two to three times lower in Fe-limited medium than in Fe-enriched incubations. Stations poleward (north of 13N and south of 13S). At stations away from the direct in#uence of the equatorial upwelling, conditions encountered were typical of oligotrophic ecosystems. Integrated production rates were very low (range 0.1}0.7 mmol-Si m\ d\), averaging 0.4 mmol-Si m\ d\. These values are close to the lowest rates reported to date from other low silicic acid environments, like the western Sargasso Sea (range 0.2}1.5 mmol-Si m\ d\, Brzezinski and Nelson, 1996), the BATS site (range 0.1}0.9 mmol-Si m\ d\, Nelson and Brzezinski, 1997) or the western equatorial Paci"c (mean 1.5 mmol-Si m\ d\, Blain et al., 1997). These low silica production rates may partly be the result of the low availability of the major nutrients, as compared to silicic acid, and as shown by the Si/N ratios '1. 4.1.2. Contribution of diatoms to the new production of the equatorial Pacixc The contribution of diatoms to the new production was estimated indirectly from the comparison of biogenic silica production rates to new production measurements reported in the same area (Table 1). In the equatorial area (13S}13N), biogenic silica production rates range between 0.61 and 2.91 mmol-Si m\ d\ (average: 1.78 mmol-Si m\ d\) and can be compared with direct estimation of total new production based upon NO uptake rate measurements. Such a comparison is consistent as, owing to the low ambient ammonium concentration ((0.1 lM), N requirements of diatoms were probably fully met by NO uptake (Dortch, 1990). Furthermore, a strong correlation between NO uptake rates and diatom chlorophyll biomass was found by Landry et al. (1997) on
17 13.5}20 4.8
Ra/nitrate distribution C and upwelled nitrate N uptake, diel periodicity El Nino Post El Nino 2D advective balance between upw and meridional divergence of NO Th/scavenging model Th/Th
02}03/1992 -1992
02}03/1992
16.6 19.1 9}20
C, N, sed. trap Si
10/94 10}11/1996
8.2
N
Th, one-D model
Th/sed. trap
02}03/1992 08}09/1992 02}03/1992 08}09/1992 03}04/1992 10/1992 04/88
18.5 28
15
C;f ratio (0.5)
08}09/1992
New production (mmol-C m\ d\)
Method
Period
12.1 (155 m)/3.7 (320 m)
1.9 (120 m)/0.54 (200 m) 2.4 (120 m)/0.71 (200 m)
4}5 (100 m) 0.6}1.3 (100 m)/0.14}0.5 (850 m) 1.5}5.0 (100 m)/0.5}1.1 (850 m) 4.6}8.3 (150 m)
Export #ux (mmol-C m\ d\)
A constant molar Red"eld ratio of 6.6 was used to convert nitrogen uptake data into carbon uptake.
1353W 23N-63S 1503W, 03 1803W 03S, 1803W
1403W 93N}123S 1403W 123N-123S 1403W
903W}1803 103N}103S 1403W 903W}1803 53N}53S 1403W, 53N}53S 23N}23S 1503W 13S}13N
Location
Rodier et al. (1997) This study
Pen a et al. (1992)
Bacon et al. (1996)
Murray et al. (1996)
Buesseler et al. (1995) Luo et al. (1995)
Carr et al. (1995)
McCarthy et al. (1996)
Chavez and Barber (1987) Ku et al. (1995) Chavez et al. (1996)
Ref.
Table 1 Summary of reported rates of new production and exported #ux of organic matter from the surface layer of the equatorial Paci"c
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EqPac cruises during El Nin o (January}February 1992), and during normal upwelling conditions (August}September 1992). Our results based upon Si-uptake are consistent with rates of new production reported by McCarthy et al. (1996) and Pen a et al. (1992) from NO uptake (0.72}2.51 mmol-N m\ d\) in the same area. This comparison gives a mean Si/N ratio of 1.1, which is consistent with existing Si/N data for diatoms in culture. These ratios were shown to vary between 0.7 and 2, depending on ambient Fe concentrations (Takeda, 1998). The primary production due to diatoms in the equatorial area (13N}13S) can be estimated from the biogenic silica production and a Si/C diatom stoichiometry ranging from 0.13 (Brzezinski, 1985) to 0.29 (Takeda, 1998). Such a range is used to take into account a possible variation of the degree of silici"cation in response to a potential iron de"ciency (Hutchins and Bruland, 1998). Results of these calculations give a primary production due to diatoms that ranged between 2.10 and 22.38 mmol-C m\ d\ (average: 12.2 mmol-C m\ day\). These values compare well with carbon new production estimates reported from the same area (Table 1), ranging from 4.8 to 28 mmol-C m\ d\. Altogether, these calculations based on direct measurement of Si uptake suggest that new production in the equatorial region (13N}13S) could mostly be sustained by diatoms. 4.1.3. Biogenic silica export yuxes (03S, 1803W) It is generally admitted that the diatom-based production is exported to deeper layers (Dugdale and Goering, 1967; Eppley and Peterson, 1979), either directly (aggregation, sedimentation) or indirectly (after mesozooplankton grazing). An indirect estimate of biogenic silica loss in the surface layer can be derived from biogenic silica production rates measured during three successive days at the equator (Fig. 7), where steady-state conditions were met. Steady state was deduced from the stability of physical (Fig. 10), chemical and biological parameters (Le Borgne et al., 1998). Under such conditions, the biogenic silica produced in the euphotic layer (the gross production, equal to 2.6 mmol m\ d\) is balanced by the biogenic silica loss (i.e. export#recycled production) of the system. An indirect estimate of the contribution of diatom export production to the total carbon export production can be derived from the comparison between the diatom-C loss and carbon export production #uxes reported by others (Table 1) for the same area. The former was estimated from the mean biogenic silica production rate measured at steady state, during the 3-days station at the equator (03S, 1803W), and converted to carbon using Si/C ratios. It ranges between 9 and 20 mmol-C m\ d\. Carbon export production from the euphotic zone was estimated either from
Fig. 10. Evolution of salinity and temperature at 20 m depth, at a station located at 03N, 1803E, during three successive days.
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Th isotopes distribution or from particles interceptor traps (PIT). If we consider C export #uxes from the upper layer (100}150 m), they range altogether between 0.6 and 12.1 mmol-C m\ d\, with most of the data in the lower range (Table 1). This comparison shows that the minimum value calculated for the diatom-C loss is close to or higher than the maximum value of particulate organic carbon exported #ux. This suggests, on the one hand, that diatoms are a major contributor to the export #ux of organic matter from the surface to deeper layers, and, on the other hand, that a signi"cant fraction of the diatom biogenic silica is redissolved in the euphotic layer. 4.1.4. Dissolution of biogenic silica in the surface layer Direct measurements of biogenic silica dissolution rates are seldom made in low chlorophyll environment because of the lack of su$ciently sensitive methods. However, the few existing data (Nelson and Goering, 1977; Nelson et al., 1981; Nelson and Gordon, 1982; Brzezinski and Nelson, 1989) cover a fairly wide range of marine systems. The overall mean resulting from these studies indicates that, in surface waters, dissolution represents 58% of the biogenic silica production (Nelson et al., 1995). Temperature, physiological state of diatom cells (Kamatani, 1982), and bacterial activity (Bidle and Azam, 1999) have been shown to enhance the processes of biogenic silica dissolution. The speci"c value of 0.006 h\ or 0.14 d\ reported by Brzezinski and Nelson (1989) from the relatively warm waters (203C) of a Gulf Stream core ring is the one that could best compare with expected values for the equatorial Paci"c. On this basis, we calculate that biogenic silica dissolution could represent, in the upper 150 m, 33% (0.006 h\/0.018 h\) of the average speci"c production in the 13N}13S area, and 75% for stations polewards (0.006 h\/0.008 h\). In the equatorial Paci"c, biogenic silica dissolution rate can also be estimated from the budget between production rate and downward #ux of biogenic silica measured at 300 m depth with drifting sediment traps (Blain et al., 1997) in October 1994 (FLUPAC cruise). This percentage reaches 84%. It is very high compared to rates calculated above for the upper layer (150 m), but we must note that it is measured at 300 m depth, and dissolution at intervening depths might be high, as evidenced by Bacon et al. (1996). These results suggest strongly that, as in oligotrophic gyres, biogenic silica dissolution is of quantitative signi"cance and must be taken into account in the silicic acid cycle of the equatorial Paci"c. 4.2. Diatom limitation 4.2.1. In situ diatom growth limitation At steady state, the diatom growth rate, calculated from the measurement of Si uptake rate, is equal to the division rate of the diatom assemblage. Steady-state uptake rate is measured when the instantaneous conditions (nutrient concentration, temperature, salinity, light, etc.) are the same as the preconditioning one (Morel, 1987). The equatorial Paci"c has many non-steady-state characteristics. Westward propagating tropical instability waves, and eastward propagating Kelvin waves, are able to induce variability in biogeochemical processes on time scales of days. Their signatures can be seen in modi"cations of sea surface characteristics. However, no such events were observed at the equator over the course of the study (5}7 November 96), as shown by the
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temperature and salinity evolution (Fig. 10) or as described in more detail in the cruise report (Le Borgne et al., 1998). In this context, the Si uptake rate was measured at steady state, and the deduced diatom growth rate (k ) should be equal to the division rate of the diatom assemblage. In the equatorial zone (13S}13N), mean uptake rates measured under simulated in situ conditions at the surface imply a mean growth rate (k ) of 0.35 d\, yielding 0.5 doubling of biogenic silica per day. These growth rates are substantially lower than the 2.5 doublings of biomass per day predicted by the temperature-dependent equation of Eppley (1972), i.e., in non-limiting light and nutrient conditions, and suggest that diatom growth is limited. 4.2.2. Direct evidence of Si limitation The positive response of Si uptake rates by natural diatom communities of the central equatorial Paci"c after silicic acid enrichments, provides the "rst direct evidence of a widespread silicic acid limitation of biogenic silica production rates in this area. In both areas, half saturation constants of the diatom population are close to the ambient silicic acid concentration. At 33S, the measured K is 1.57 ($1.32) lM when ambient concentration is 1.45 lM, whereas at the Equator, the K is 2.42 ($0.53) lM in ambient concentration of 2 lM. These half saturation constants of Si uptake are in the range of values reported for other natural diatoms assemblages. Previously measured K values for silicic acid uptake vary widely, at least by one order of magnitude, from 0.53 lM (in a Gulf Stream warm-core ring, where ambient silicic acid concentration was below the detection limit, Nelson and Brzezinski, 1990) to 5.3 lM (in the Mississippi river plume, Nelson and Dortch, 1996). Higher K values are generally characteristic of silicic-acid-enriched environments. One example is the Ross Sea, where a K of 4.6 lM has been measured by Nelson and TreH guer (1992), in waters where silicic acid concentration seldom decrease below 5 lM. In situ measured K do not refer to a speci"c species, but rather to an assemblage of di!erent diatoms and could re#ect some adaptation of the diatom assemblage to in situ conditions. These results are in good agreement and support the Dugdale and Wilkerson (1998) modi"ed Si(OH) pump model, which states that the diatom population is regulating at about half the maximal uptake, with a substrate concentration near the half saturation constant, K . The < values reported from the two Si uptake kinetic studies (0.028 h\ at 33S and 0.052 h\ at the Equator) are comparable to those previously reported for natural population of the oligotrophic waters of the Sargasso Sea and the Gulf Stream warm-core ring (Brzezinski and Nelson, 1996; Nelson and Brzezinski, 1990), but lower than in the Peru upwelling region, where Goering et al. (1973) reported a < as high as 0.075 h\. Furthermore, the ratio < /< provides an indication of the degree to which the diatom Si uptake is limited by ambient silicic acid concentrations. That limitation is evaluated at 54% of the maximal speci"c uptake rate at 33S and 42% at the Equator. This direct evidence of Si uptake rate limitation does not necessarily imply that diatom growth rate is limited by Si. It is known that diatoms can maintain division rates close to k , even when the rate of Si uptake is signi"cantly less than < , by decreasing the Si content of the frustule. Conversely, physiological adaptation can take place in response to nutrient abundance (increase in cellular quota), in the course of the experiment, while division rate remains approximately constant. In other words, diatoms are able to maintain a growth rate signi"cantly lower than k , even when the rate of Si uptake is close to < , by increasing the Si content of their frustule. Then the diatom division rate (0.8 d\, or 1.15 doubling of the biomass per day) that we can predict from < is certainly an over-estimation of
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the maximal growth rate that diatoms could reach in their environment if ambient silicic acid was not limiting. However, this predicted and over-estimated daily growth is still signi"cantly lower than the 2.5 doublings of biomass per day predicted by the temperature-dependent equation of Eppley (1972), i.e., under non-limiting light and nutrient conditions. Comparing doubling times reported for equatorial Paci"c diatoms and those expected from temperature (Eppley, 1972) could appear bold. Eppley's curve is the upper limit of a scatter plot that has tremendous variations, and it does not exclude the possibility that many phytoplankton assemblages may not be physiologically capable of reaching this maximum. Nevertheless, Latasa et al. (1997) have measured, from dilution experiments in the central equatorial Paci"c, diatom growth rates of 1.7 d\. This result implies 2.4 doublings per day of the diatom population. Even if Latasa et al. (1997) did not rule out a possible containment or contamination artifact in their experiments to explain such high growth rates, this result shows that some diatom assemblages of the central equatorial Paci"c are physiologically capable of reaching the maximum predicted by Eppley's equation. As far as silicic acid limitation in the central equatorial Paci"c is concerned, we can conclude that ambient silicic acid is limiting biogenic silica production, but the silicic acid limitation of diatom growth is an hypothesis that remains to be tested. However, even if diatom growth was limited by ambient silicic acid, such Si limitation would certainly not be su$cient in itself to explain the low diatom growth rates observed in situ. 4.2.3. Co-limitation These results suggest that diatoms in the central equatorial Paci"c might be co-limited simultaneously by some other resource than silicic acid. In this area of the Paci"c, when looking for additional potentially co-limiting factor, we evidently think about iron, because iron limitation has been extensively reported as a controlling factor of phytoplankton growth. For instance, the Ironex II experiment (Coale et al., 1996b) resulted in a 85-fold increase in diatom biomass within a 10 km patch of surface water over 10 days, corresponding to a growth rate of about 1 db d\ (Coale et al., 1996a), which is also signi"cantly lower than the maximum expected from Eppley's formula (i.e. 2.5 db d\). If, with Fe addition alone, or Si addition alone, diatom growth does not even approach the upper limit imposed by the temperature dependence relation, then one explanation could be that Fe and Si limitations interact, rather than just being opposing explanations for the HNLC character of the system. Although little is known on the synergy between iron and silicic acid limitation, processlevel experiments performed at non-limiting silicic acid concentrations (Hutchins and Bruland, 1998; Takeda, 1998) have shown that Fe de"ciency could induce increases in cellular silica or a decrease in the cellular N content of diatoms. It is not clear, however, what the relationship between Fe limitation and cellular Si content would be once silicic acid is depleted to levels that limit the rate of Si uptake. All experiments to date that show increases in cellular Si under Fe limitation have been preformed at non-limiting silicic acid concentrations, and once Si uptake is limited by silicic acid, it would probably be more di$cult for diatom cells to produce frustules with elevated Si content. We could envision, as suggested in Ragueneau et al. (in press), that when both Si and Fe are depleted, diatom Si uptake rate is limited by silicic acid concentrations, but NO uptake rate or photosynthesis are limited by Fe.
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Additional data on those interactions would be of great value. But already existing models and studies of nutrient limitation should be reconsidered. They generally assume a single-controlling nutrient (Hecky and Kilham, 1988; Lancelot et al., 1997; Dugdale and Wilkerson, 1998), which is determined as the most limiting nutrient. The latter is "xed by the comparison between the ambient concentrations and the Michaelis}Menten kinetic parameters, characterizing the uptake of each nutrient. All nutrient uptake rates are then set proportional to that nutrient (Liebig model). O'Neill et al. (1989) show that although the Liebig model remains useful when one nutrient limitation predominates, it fails to converge to reasonable numbers when several nutrients are reaching sub-optimal concentrations. He studied and compared the behaviour of di!erent models: additive, multiplicative, and others. Among them, the additive model appears to have the best properties to serve as a general description for multiple nutrient limitation. In the equatorial Paci"c, conditions might be those reported by O'Neill et al. (1989) as ambient silicic acid and iron concentrations have both been reported as being at sub-optimal levels. Future studies should take into account simultaneous limitation by di!erent nutrients reaching sub-optimal concentrations, and new models (such the one proposed by O'Neill et al., 1989) should be tested. The development of a high sensitivity method to measure in situ silica dissolution rates and process-level studies to better understand the links between the silica and carbon cycles are necessary. Acknowledgements We especially thank Robert Le Borgne for his leadership as chief scientist, the captain and crew of the R.V. Atalante, and Annick Masson for technical assistance in sample analysis. This research was supported by CNRS-INSU and ORSTOM. References Bacon, M.P., Cochran, J.K., Hirschberg, D., Hammar, T.R., Fleer, A.P., 1996. Export #ux of carbon at the Equator during the Eqpac time-series cruises estimated from Th measurments. Deep-Sea Research II 43 (4}6), 1133}1154. Banahan, S., Goering, J.J., 1986. The production of biogenic silica and its accumulation on the southeastern Bering Sea shelf. Continental Shelf Research 5, 199}213. Bidle, K.D., Azam, F., 1999. Accelerated dissolution of diatom silica by marine bacterial assemblages. Nature 397, 508}512. Blain, S., Leynaert, A., TreH guer, P., ChreH tiennot-Dinet, M.J., Rodier, M., 1997. Biomass, growth rates and limitation of equatorial Paci"c diatoms. Deep-Sea Research I 44, 1255}1275. Brzezinski, A.M., 1985. The Si : C : N ratio of marine diatoms: interspeci"c variability and the e!ect of some environmental variables. Journal of Phycology 21, 347}357. Brzezinski, M.A., Nelson, D.M., 1989. Seasonal changes in the silicon cycle within a Gulf Strean warm-core ring. Deep-Sea Research I 36, 1009}1030. Brzezinski, M.A., Nelson, D.M., 1996. Chronic substrate limitation of silicic acid uptake rates in the western Sargasso Sea. Deep-Sea Research II 43, 437}453. Brzezinski, M.A., Phillips, D.R., Chavez, F.P., Friederich, G.E., Dugdale, R.C., 1997. Silica production in the Monterey, California, upwelling system. Limnology and Oceanography 42 (8), 1694}1705. Buesseler, K.O., Andrews, J.A., Hartman, M.C., Belastock, R., Chai, F., 1995. Regional estimates of the export #ux of particulate organic carbon derived from thorium-234 during the JGOFS Eqpac program. Deep-Sea Research II 42 (2}3), 777}804.
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