relative mineralisation of c and si from biogenic particulate matter in the upper water column...

s 63 (2006) 79–90www.elsevier.com/locate/jmarsys

Journal of Marine System

Relative mineralisation of C and Si from biogenic particulatematter in the upper water column during the North East

Atlantic diatom bloom in spring 2001

Louise Brown a,⁎, Richard Sanders a, Graham Savidge b

a National Oceanography Centre Southampton, Empress Dock, European Way, Southampton, Hampshire, SO14 3ZH, UKb 3 Queen's University Marine Laboratory, 12-13 The Strand, Portaferry, Co. Down BT22 1PF Northern Ireland, UK

Received 15 May 2005; accepted 6 March 2006Available online 26 July 2006

Abstract

The standing stocks and production rates of particulate organic carbon (POC) and biogenic silica (bSiO2) were measured in theupper water column at 10 stations in the North East Atlantic during the spring 2001 diatom bloom. The elemental composition ofthe particulate pool was rather homogeneous with depth, suggesting that any material being exported from the photic zone wasgenerally similar in composition to the ambient pool. Pronounced vertical structure was observed in uptake ratios resulting from thestrong light dependence of the carbon fixation and the weak light dependence of biogenic silica production. The integrated C/Simolar ratios of particulate material were found to be generally larger than the corresponding assimilation ratios. We interpret thisdiscrepancy as implying a preferential mineralization of Si relative to C from particulate matter during the earliest stages ofprocessing in the upper water column. The preferential mineralisation of Si relative to C in the early stages of particle processingcontrasts with processes occurring deeper in the water column, where C is typically mineralised preferentially to Si, and particulatematter becomes enriched in bSiO2. In the northern North Atlantic, the balance of mineralisation of Si relative to C from sinkingorganic matter with depth is likely to strongly influence the role of diatoms in export production.© 2006 Elsevier B.V. All rights reserved.

Keywords: Diatom blooms; Biogenic silica; Particulate organic carbon; Euphotic zone; Mineralization; North Atlantic

1. Introduction

The flux of carbon across the thermocline from thesurface layer of the ocean into the deep waters (thebiological carbon pump) is central to the oceanic carbonbudget (Eppley and Peterson, 1979). A key componentof this flux is the export of organic particulate material

⁎ Corresponding author. Present address: School of Geography andGeosciences, University of St. Andrews, Irvine Building, North Street,St. Andrews KY16 9AL, UK. Tel.: +44 1355 463992.

E-mail address: [email protected] (L. Brown).

0924-7963/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jmarsys.2006.03.001

generated by primary producers in the surface ocean tothe ocean's interior (Martin et al., 1987). The loss ofparticulate carbon from the surface waters mustultimately be balanced by the invasion of atmosphericCO2, and thus, the export flux has a potential role inclimate regulation.

The upper limit on carbon export is set by the rate ofnew phytoplankton production; that is, the fraction ofprimary production supported by ‘new’ inorganicnitrogen (nitrate) uptake relative to the uptake of totalnitrogen, which includes recycled, organic forms (Dug-dale and Goering, 1967). The principal source of ‘new’

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80 L. Brown et al. / Journal of Marine Systems 63 (2006) 79–90

dissolved nitrate in the surface layer is the upward flux,either via overturning or diffusive mixing, of deep,nutrient-rich waters.

One important source of nutrients in the deep water isthe mineralization of sinking particulate organic matter.This mineralization of organic matter, either in deep orshallow water, also results in the production of dissolvedinorganic carbon (DIC), such that an upward flux of thiscomponent to the surface layer of the ocean occurs inparallel with the nutrient flux. A net export of carbonfrom the surface waters can only occur over multi-yeartimescales if the elemental ratio of DIC to nutrientsassociated with the upward flux of water is lower thanthe elemental ratio of the particulate material associatedwith the corresponding downward flux of carbon(Christian et al., 1997). This is because the mixing upof nutrients will thus be associated with a stoichiomet-rically inadequate quantity of DIC, and hence, a net air–sea flux of CO2 must occur for some nutrients used innew production. If the opposite is true, and DIC isreleased from sinking organic matter faster thannutrients, then a reduced net export can occur. This isbecause outgassing of DIC will result if upwelled watershave a stoichiometric excess of DIC relative to theamount needed to balance the upwelled nutrients.

Preferential mineralisation of particulate organicnitrogen (PON) and particulate organic phosphorus(POP) relative to particulate organic carbon (POC) isobserved almost ubiquitously throughout the ocean(Copin-Montegut and Copin-Montegut, 1983; Priddle etal., 1995; Shaffer, 1996; Loh and Bauer, 2000). It likelyreflects the complex suite of physico-chemical andbacterially mediated reactions involved in the reminer-alisation of organic material in the oceans (Hurd, 1973;Tezuka, 1990; Anderson, 1992; Sterner, 1992; Bidle andAzam, 1999). It results in increases in the C/N and C/Pratios of sinking particles with depth (Martin et al.,1987; Knauer et al., 1979), a vertical decoupling of theconcentrations of inorganic nutrients and DIC and anincrease in the pool of dissolved nutrients in subsurfacewaters relative to DIC (Christian et al., 1997). Withinthe Pacific subtropical gyre, it has been estimated thatthis decoupling of the nutrient and carbon cyclessupports 17–27% of total air–sea CO2 drawdown intothe surface layer of the ocean (Christian et al., 1997).Preferential mineralisation of POP relative to POC hasalso been documented in areas where phytoplanktonblooms result in rapid export of large quantities ofparticulate matter to the deep ocean (Paytan et al., 2003).

Unlike nitrogen and phosphorus, which are funda-mental to the metabolism of all primary producers, athird macronutrient, silicic acid, is utilised primarily by

a particular class of phytoplankton, the diatoms.Diatoms are considered to be important primaryproducers and exporters of carbon (Treguer et al.,1995; Nelson et al., 1995), and the availability of silicicacid acts as a major control on diatom production(Brzezinski and Nelson, 1996; Boyd et al., 1999; Francket al., 2000; Leynaert et al., 2001). In further contrast,sediment trap data shows better preservation of bSiO2

relative to POC in deep waters of the Southern Ocean(Nelson et al., 2002) and oligotrophic low-latitudeAtlantic and Pacific Oceans (Martin et al., 1991;Ragueneau et al., 2002). However, on shorter time-scales, the rate of bSiO2 mineralisation in the upperocean varies both seasonally and spatially, with rapidremineralisation in upwelling areas (Nelson and Goer-ing, 1977) and subtropical gyres (Brzezinski andNelson, 1995; Nelson and Brzezinski, 1997), and duringperiods of high productivity in temperate and high-latitude waters (Nelson et al., 1991; Brzezinski et al.,2001, 2003).

The North East Atlantic is of potential importance inglobal biogeochemical cycling, due to the predictedhigh export production (>100g C m−3 year−1; Falk-owski et al., 1998) of the region. Export productionoccurs mainly as a result of the spring bloom (Ducklowand Harris, 1993), fuelled by nutrients replenished bywinter overturning and initiated by thermal stratificationof the water column. In the North Atlantic, diatoms playa major role in export production (Dugdale et al., 1995),with non-siliceous phytoplankton including flagellates,coccolithophores and mixotrophic ciliates (Savidge etal., 1995) dominating a recycled production regime oncesilicic acid depleted. The zooplankton community in theopen northeastern North Atlantic is dominated by thecopepod Calanus finmarchics (>90% zooplanktonbiomass) (Gislason and Astthorson, 1995). Populationmaxima occur in two peaks; typically following thespring bloom, in (May–June) and another in latesummer (July–September). Like phytoplankton, thehighest abundances are in the coastal and shelf waters,decreasing in the open waters.

In the present study, we examine the stoichiometry ofC and Si assimilation and early mineralisation within thephotic zone in the North East Atlantic during the highlyproductive spring bloom. This approach enables us toexamine rates and controls of fluxes of biogenicmaterials between different pools (e.g., Reiners, 1986;Hassett et al., 1997). Here, we compare the C/Si ratios ofnewly formed biogenic particles (i.e., the elementalratios of assimilation corrected for algal respiration andshort-term losses of silicon) with the ratios in theparticulate pool (which we hypothesise represents the

81L. Brown et al. / Journal of Marine Systems 63 (2006) 79–90

material produced as above modified by grazing,bacterial processing and solubilisation) as a means ofassessing the relative mineralization of C and Si in thephotic zone during the earliest stages of particlediagenesis. We then interpret our results in terms oftheir implications for the role of diatoms in exportproduction in the northern North East Atlantic duringthe spring bloom.

2. Methods

2.1. Sampling

The assimilation rates of inorganic C and Si intoparticulate matter and the concentrations of POC andbSiO2 in the photic zone were determined at 10 stationsin the North East Atlantic during RRS Discovery cruiseD253 (Faeroes–Iceland–Scotland Hydrographic and

Fig. 1. Map of the FISHES study area and cruise track. The 10 stationmeasurements are labelled.

Environmental Survey) in May 2001 (Fig. 1). At eachstation, seawater samples for assimilation rate andparticulate concentration measurements were collectedat seven depths (97%, 45%, 17.6%, 8.0%, 2.9%, 1.3%and 0.1% of the surface photosynthetically activeradiation [PAR]).

Samples were collected on a pre-dawn CTD castusing 20L Go-Flo bottles mounted on a rosette samplerand decanted into acid-cleaned 4-L polycarbonatebottles. Subsamples for carbon and nutrient assimila-tion studies were prepared for incubation immediatelyafter collection under minimal light conditions; thosefor particulate composition analysis were kept refrig-erated (4°C) in the dark until filtration. Detailedanalyses of C and Si assimilation rates and thefluorescence characteristics of the plankton communitystructure are presented elsewhere (Brown et al., 2003;Moore et al., 2005).

s sampled for particle composition and carbon and nutrient uptake


2.2. Nutrient assimilation

The methodologies used to measure C and Siassimilation are described fully in Brown et al. (2003).Net primary production was determined using the 14Cincubation technique outlined in Parsons et al. (1984),and bSiO2 production by the 32Si radiotracer method ofBrzezinski and Phillips (1997). All incubations were runfrom dawn until midday; approximately a 6-h period.This time period was chosen primarily to minimiseeffects of silicic acid depletion over the course of theexperiment. Each experiment also included a sampleincubated in a foil-covered bottle, to obtain a blankcorrection for the photosynthetic C assimilation, and todetermine rates of assimilation in dark conditions forbSiO2. No correction for abiotic bSiO2 assimilation wasmade as previous work has shown this to be inconse-quential (Leynaert et al., 2001).

As expected, primary production was minimal in darkconditions; however, significant dark production (typi-cally 80% of production measured in daylight condition)of biogenic silica was observed. Daily primary produc-tion rates were calculated by scaling the values obtained(corrected for minor [<5% total uptake] inorganicproduction measured in the dark bottles) in the 6-h incubations to a full dawn-to-dusk time period. A 24-h bSiO2 production rate was obtained by calculatingdawn-to-dusk rates from the 6-h incubation as forprimary production, and adding a night-time valuecalculated by scaling the bSiO2 production observed inthe dark incubation bottles to the period of darkness.

Dissolved nutrient concentrations, required forcalculation of bSiO2 assimilation rates, were determinedusing standard colorimetric methods on a SkalarSanplus autoanalyser, following Sanders and Jickells(2000). All carbon and nutrient assimilation experi-ments were run in triplicate; the mean precision of allmeasurements above detection limit was 9.2% for C,13.7% for Si and 13.2% for P. Dissolved inorganicnutrient samples were run in duplicate and included bothinternal and external standard reference materials; themean precision for all nutrients was <3%.

2.3. Particulate matter composition

Biogenic silica concentrations were determined byfiltering a 500-mL subsample onto a 0.8-μm poly-carbonate filter under gentle vacuum, followed by a 2-h digestion with 0.2M NaOH at 80°C (Ragueneau andTreguer, 1994). Silicic acid concentrations of thedigested samples were determined using standard auto-analyser methods as described above. A further 500-mL

subsample was filtered using gentle vacuum throughGF/F (nominally 0.7μm pore size) filter and immedi-ately frozen at −20°C for POC analysis, using a CarloErba CHN analyser on return to shore. Precision ofthe analyses was 11% for POC and 3.3% for bSiO2.

3. Results and discussion

3.1. The Iceland basin spring bloom: nutrient andbiological background

The 0.1% PAR depth during the cruise was between29 and 99m, and the range of the surface mixed layerdepth (estimated as the depth at which a >0.2°C changeoccurs in the CTD temperature profile) was between 5and 75m. The surface mixed layer depth exceeded thatof the photic zone at 3 of the 10 stations. Neitherparameter displayed a robust correlation with either timeor latitude. Surface chlorophyll, silicic acid andphosphate concentrations ranged from 0.53 to 7.7μgL−1, 0.3 to 3.4μmol L−1 and 0.03 to 0.77μmol L−1

respectively, representing a range of conditions from theearly stages of diatom spring bloom developmentthrough to silicic acid exhaustion and bloom collapse.

The progression of the diatom bloom was assessedby considering the mean silicic acid concentrationwithin the photic zone, reaffirming previous studies(Egge and Aksnes, 1992), which demonstrated diatomdominance of primary production at silicic acidconcentrations above 2μmol L−1. Excess surface nitrate(4–10μmol L−1) was present at 9 out of 10 stations,including those where total silicic acid depletion wasobserved, implying further production potential by non-siliceous phytoplankton.

Primary production and bSiO2 assimilation ratesduring the FISHES cruise are detailed fully elsewhere(Brown et al., 2003). Primary production rangedbetween 0.49 and 3.2g C m−2 day−1, with five stationsidentified as active spring bloom sites. At most sites,bSiO2 assimilation was between 5 and 20mmol Si m−2

day−1, in the range of non-bloom bSiO2 assimilationobserved in the Southern Ocean (Brzezinski et al., 2001;Gall et al., 2001); substantially higher rates of 78 and167mmol Si m−2 day−1 were recorded at two stations.

The phytoplankton taxonomy and ecosystem observedduring the FISHES study have been described in detailelsewhere (Moore et al., 2005). Briefly, the basin isdivided into five representative areas. Southeast ofIceland, diatoms, predominantly Nitzschia sp. are abun-dant. Diatoms also dominate the region of the IcelandFaeroes front, but here, a broader variety of species arepresent, including Chaetoceros sp., Thalassosira gravida


and Astellioneriopsis glacialis. North of the Iceland–Faeroes Front and in the Rockall Trough region,flagellates are present in large numbers, but withsignificant contributions from cryptomonads and cocco-liths, respectively. In the central Iceland Basin, a mixedcommunity of flagellates and ciliates is present.

3.2. Spatial and vertical variation in C/Si

C/Si molar ratios were calculated for the assimilation(C/Siassimilation) rates and for the particulate standingstock (C/Siparticulate) (Fig. 2). The C/Si assimilation ratioswere calculated on the basis of the 24-h assimilation ratesestimated as described in Section 2.2. An overall valuefor each station was calculated by vertically integratingthe data to the depth of 0.1% surface PAR.

Little variation in C/Siparticulate with depth wasobserved at individual stations (Fig. 2), suggesting thatthe vertical mixing of particles and nutrients in thesurface mixed layer was sufficiently rapid to prevent anydepth variability in mineralisation or fixation ratiosintroducing vertical structure to elemental composition.In contrast, C/Siassimilation (Fig. 2) always decreasedfrom the surface to the base of the photic zone, as a

Fig. 2. Vertical profiles of assimilation (a) and particulate composition(b) for C/Si. The vertical line indicates the mean of the depth-integrated C/Si values for all stations. The errors bars are the analyticalerror as described in the text.

consequence of the strong light dependence of photo-synthetic carbon uptake and the light-independentuptake of silicic acid (Azam and Chisholm, 1976;Brzezinski et al., 1997).

The station-averaged C/Siparticulate ranged from 3.3 to108.9 (mean 21.2±34.4). The highest value wasobserved at station 14029, which was atypical in thatnitrate was fully depleted, yet excess silicic acid waspresent. This suggests that primary production at that sitehad been dominated by non-siliceous phytoplankton,consistent with the high elemental C/Si ratio. The secondhighest value (station 14078) was 51.5, half that at14029. C/Siassimilation varied from 1.6 to 22.6, again withthe highest value at station 14029 being substantiallygreater than the second highest value (station 13984,16.0). The mean value for C/Siassimilation was 8.2±7.4,lower than the mean C/Siparticulate; thus, the elementalcomposition of particulate matter in the photic zone didnot reflect the stoichiometry of the processes involved inits synthesis, but instead was comparatively carbon rich.

3.3. Vertical structure of particulate matter composition

Our observation that the particulate material in thephotic zone is systematically carbon rich relative to theprocesses involved in creating it could either be causedby an export of nutrient-rich particulate matter from thephotic zone or a preferential mineralisation of bSiO2

relative to carbon from particulate matter in the photiczone.

We examined the former possibility by looking indetail at the vertical variability in C/Si ratios at individualstations, based on the assumption that particles at thebase of the euphotic zone will be representative of theexported particulate matter. C/Siparticulate at the deepestsampling depth was lower than that of the surfacematerial (i.e., consistent with an export of nutrient-richparticulate matter) at only two stations, 14005 and14060. Overall then, it seems likely that the generallyhigh C/Siparticulate relative to processes involved in itssynthesis is most likely to have been caused by apreferential mineralisation of Si relative to C in the upperwater column. At the anomalous stations where the deeppool is Si rich relative to the surface pool, it is possiblethat the low C/Si ratio of the deep particles relative to thesurface pool was caused by the strong light dependencyof carbon assimilation compared to the short-term lightindependence of silicon assimilation. Both stationsshow deep subsurface maxima in the concentration ofthe diatom-diagnostic pigment fucoxanthin, whichcorrespond with gradients in silicic acid concentration,but are not mirrored in other photosynthetic pigments

Fig. 3. (a) Depth-integrated values of C/Si particles (filled symbols)and C/Si assimilation (open symbols) for each of the FISHES stations,shown on a logarithmic scale for clarity. Error bars are the station meanof the analytical error on each measurement. (b) The ratio of mixedlayer depth to euphotic zone depth.


(Brown et al., 2003). It has previously been shown thatnutrient-depleted diatoms sink faster than nutrientreplete cells (Bienfang et al., 1982, Brzezinski andNelson, 1988), and that nutrient-depleted diatoms can‘mine’ deep silicic acid reserves (Allen et al., 2005).Thus, it would seem at these sites that nutrientdepletion-induced vertical variation in the phytoplank-ton community structure, combined with the rapiduptake of silicic acid by Si-depleted diatoms at depth iscapable of affecting the particulate composition in theshort term.

Thus, it is possible that, even at these stations, thehigh C/Siparticulate relative to C/Siassimilation is caused by apreferential mineralization of nutrients relative to carbonfrom particulate matter rather than an export of nutrient-rich particulate matter.

3.4. Preferential remineralisation in the FISHES dataset

The relative mineralisation rates of POC and bSiO2 ateach station are investigated by comparing the photiczone averaged C/Siassimilation ratio to the C/Siparticulateratio (Fig. 3) for each station. There is substantialvariation between stations in both C/Siassimilation and C/Siparticulate (Fig. 3a); however, there is a reasonablygood, although non-linear, correspondence between C/Siparticulate and C/Siassimilation at individual stations. Thisvariability is assessed by examining the ratio of C/Siparticulate to C/Siassimilation at each station. The meanvalue of C/Siparticulate:C/Siassimilation is 3.7±2.8; thus, ingeneral, the particulate matter is about four times as richin carbon relative to silicon as would be expected fromthe elemental uptake ratios.

C/Siparticulate is higher than C/Siassimilation at 7 out of10 stations, suggesting bSiO2 is preferentially miner-alised relative to POC in the surface waters at thesestations. Preferential mineralisation of bSiO2 relative toPOC, as suggested by the FISHES data set, is apparentlyin conflict with much of the existing data on POC andbSiO2 fluxes in the surface ocean (e.g., Wong et al.,1999; Buesseler et al., 2001; Shipe and Brzezinski,2001; Nelson et al., 2002; Queguiner and Brzezinski,2002). All these studies suggest a preferential mineral-ization of POC relative to bSiO2 in the upper watercolumn. However, most of these data originate fromsediment trap samples taken deeper in the water columnthan our euphotic zone samples or are averaged overannual cycles, in contrast to the much shorter timescaleand surface ocean focus of the FISHES observations.They are also often in HNLC waters where silicic acidexhaustion is less likely to occur, and where Fe-

limitation may affect Si/C assimilation ratios (Hutchinsand Bruland, 1998).

Although typically considered less labile than POC,rapid recycling of bSiO2 has been observed in lownutrient waters of the tropical Atlantic (Nelson andBrzezinski, 1997) and in highly productive upwellingareas (Nelson and Goering, 1977). Further, Wong et al.(1999) observed that bSiO2 was less efficientlytransferred from the surface ocean during rapid sinkingevents in the North Pacific, and globally, absolute ratesof bSiO2 recycling were fastest during periods of rapiddiatom production, such as experienced in the NorthEast Atlantic diatom spring bloom (Brzezinski et al.,2003; Beucher et al., 2004). It therefore seemsreasonable to conclude that rapid recycling of bSiO2

relative to POC in the upper water column is a generalfeature of the spring diatom bloom in this region.

3.5. Influence of the relationship between the mixedlayer and photic depths on C/Si ratios

Comparison of the mean C/Si ratios of assimilationand of the particulate pool across the whole basin


suggests frequent preferential mineralisation of bSiO2

relative to POC. However, C/Siassimilation is larger thanC/Siparticulate, suggesting preferential mineralisation ofPOC relative to bSiO2 at three stations (13971, 13984and 14010). The cause of these anomalies must beexamined before the apparent preferential mineralisa-tion of bSiO2 over POC during the North Atlantic springbloom can be accepted as a general pattern.

We now consider the possibility that the anomaliesare a result of overestimation of C/Siparticulate or anunderestimation of C/Siassimilation. Particulate C/Si ratiosvary little with depth (Fig. 2), and thus, it is likely thatthe integrated POM data are representative of particulateratios throughout the upper water column. However,with regard to carbon and nutrient assimilation, carbonfixation by photosynthesis is a highly light-dependentprocess, unlike the biological uptake of silicic acid(Raven, 1983; Martin-Jezequel et al., 2000).

Since our measurements were limited to the depth of0.1% PAR, it is possible that, at stations where the depthof the surface mixed layer exceeded that of the photiczone, additional Si (but not C) assimilation may havebeen occurring at depths within the mixed layer butbelow the limit of photosynthesis. We now assesswhether these conditions, which would result in anundersampling of deeper waters and, hence, anoverestimate of the true mixed layer C/Siassimilation,may explain the anomalous data obtained from somestations.

The hypothesis is addressed by examining therelationship between the mixed layer depth and photiczone depth at the stations sampled (Fig. 3b). At the threeanomalous stations (13971, 13984, 14010), the surfacemixed layer was substantially undersampled because itwas much deeper than the depth of the photic zone,which was used to define the sampling depth. Thus, atthese sites, the biogenic silica production is likely to havebeen underestimated, leading to an overestimate of theC/Siassimilation ratio. Whether this effect is sufficientlylarge to make a more realistic estimate of C/Siassimilation

lower than the corresponding value of C/Siparticulate and,hence, imply a preferential mineralization of nutrientrelative to carbon from particulate matter is nowaddressed.

To examine this possibility, we extended the depth ofintegration of assimilation or particulate concentrationto the depth of the surface mixed layer rather than thephotic zone at the anomalous stations, using the 0.1%PAR measurements to represent the sub-photic zonewaters. This recalculation yields C/Siassimilation values12–20% less than the photic zone-only ratios. Thevalues are still greater than the corresponding C/

Siparticulate ratios, suggesting at these sites C/Siparticulatemay indeed be larger than C/Siassimilation, and thatpreferential mineralization of carbon over nutrients maybe occurring. However, many of the North AtlanticbSiO2 production profiles show large increases at thebase of the thermocline, possibly due to uptakesupported by silicic acid mixed across the boundary(Brown et al., 2003). The above calculations maytherefore still be underestimates of C/Siassimilation, andthe possibility that our calculations of C/Siassimilation

might exceed C/Siparticulate had measurements had beenmade across the whole surface mixed layer cannot beexcluded.

Overall, the majority of sites display behaviourconsistent with greater nutrient mineralisation relativeto POC; hence, we conclude that preferential miner-alisation of bSiO2 relative to POC in the photic zoneduring the NE Atlantic spring diatom bloom is afrequent feature of this event, particularly when themixed layer is shallow relative to the depth of the photiczone.

3.6. Qualification of steady-state assumption

3.6.1. Experimental conditionsOur comparison of uptake stoichiometry in the

bottles relative to the elemental particulate stoichiom-etry is intended to shed light on the processes whichmodify the pool of material created by phytoplanktonuptake after initial respiration and silicic acid release andtransform it into the pool of material in the watercolumn. The underlying assumption in our analysis isthat the uptake rates within the bottle reflect all of thealgal-mediated cycling and none of the longer termdiagenetic processes, and that the particles are thentransformed into the particle field present in the watercolumn by processes which do not occur in vitro.

Clearly, we cannot fully distinguish between thoseprocesses mediated by algal physiology from processesacting on the particles mediated by bacterial processes,grazing and solubilisation on the basis of time. Thus, ourassumption is not strictly valid; some of these processeswill occur in the incubations, and some of the particlessampled will have been only recently synthesised. Thus,the measured C/Si is partly controlled by (a) the amountof processing in the assimilation experiments and (b) theproportion of living cells making up the particulate pool.However, on the whole, the assimilation rates measuredin the bottle incubation will be more representative ofshort-term particle synthesis, and the particulate poolwill be more representative of the effects of longerterm processing on the particle pool. Thus, that the


assumption does not strictly hold true does notinvalidate our conclusion; however, it does imply thatthe difference in stoichiometry between the fresh pooland the bulk POM pool may be smaller than our analysissuggests.

3.6.2. Changes in plankton community structureRather than being a consequence of preferential

mineralisation of biogenic material, the generally highervalues of C/Siparticulate compared to C/Siassimilation maybe caused by temporal changes in one or bothparameters over the duration of the bloom; i.e., theassumption of steady state within the surface layer poolmay be invalid. Measurements of particulate poolcomposition and assimilation rates represent processesoccurring on different timescales; particle compositionmeasurements reflect nutrient assimilation and miner-alisation processes integrated over several days, whilstassimilation rate measurements are determined within atimeframe of hours associated with the experimentalincubation.

Thus, the measured C/Si assimilation ratio couldincrease in response to silicic acid limitation, either bydiatoms altering their metabolism or by replacement ofthe diatom population by non-silicifying phytoplankton,faster than the associated changes in the C/Si ratios ofthe particulate pool. These would then lag the change inuptake ratios given the longer residence time of materialin this pool compared to the duration of the incubations.Should this be the case, our analysis would conclude,erroneously, from observations of C/Siparticulate beingsystematically larger than C/Siassimilation that a preferen-tial mineralization of silicon relative to carbon must beoccurring.

The possibility that C/Siassimilation and C/Siparticulatechange systematically within the data set as a function ofdiatom bloom development is considered by plotting

Fig. 4. The depth-integrated values of C/Si in particulate (open triangles) andlayer silicic acid concentration (solid diamonds, bold line). The latter term ha2003), with stations of mean Si(OH)4<2μmol L−1 designated as ‘post-bloom

these parameters against mean mixed layer silicic acidconcentration (Fig. 4) as an indicator of diatom bloomprogression, following Brown et al. (2003). Neitheruptake ratios nor the elemental composition of theparticulate pool decline systematically as the bloomsubsides. Hence, our conclusion regarding the frequentpreferential mineralisation of bSiO2 relative to POCfrom particulate matter appears robust.

Our general conclusion is also supported by simplecalculations of the residence time of material in theparticulate pool (calculated as pool size/assimilationrate), which yield values of 10±10 days for POC and3.8±3 days for bSiO2. These residence times arerelatively short compared to the typical 3–4 weeksduration of the North Atlantic spring phytoplanktonbloom (Sieracki et al., 1993; Savidge et al., 1995; Buryet al., 2001). This suggests that POC is contained withinthe bulk pool for approximately two to three times aslong as bSiO2, consistent with it being less susceptibleto mineralisation.

3.7. Role of grazing in modification of C/Si

Thus far, we have considered mainly the physical–chemical aspects of cycling of particulate organic mate-rial in the water column. However, in addition to thesepassive mechanisms, the active grazing and processingof POM by zooplankton may selectively modify par-ticles. During the North Atlantic spring bloom, there isevidence to demonstrate that no significant grazingoccurs during the spring bloom while it is composed oflarge (>20μm) phytoplankton, typically Nitzschia sp.diatoms, but that when the bloom is in decline andcomposed predominantly of <20 μm cells, grazing bymicrozooplankton can account for up to 100% ofprimary production (Gifford et al., 1995). Grazing ofthe large phytoplankton fraction by mesozooplankton

assimilated (open squares) material, ordered by decreasing mean mixeds been used as an indicator of diatom bloom progression (Brown et al.,.’ The errors are the standard deviation of the value at all stations.


may occur; however, reports indicate that consumptionby C. finmarchicus, the dominant mesozooplankton,accounts for at most a few percent of primary production(Morales et al., 1993; Gifford et al., 1995) and may havea preference for non-diatom food sources (Nejstgaard etal., 2001; Van Niewerburgh et al., 2004).

Like most phytoplankton, the zooplankton whichconsume them have no or little requirement for silicon.Consumption of diatoms would therefore likely lead toassimilation of carbon and nutrients in the zooplanktonand higher trophic levels, whilst silicon would beexcreted. Such a mechanism could lead to an increase inC/Si ratios in particulate material relative to the C/Siratio of assimilated material. This was not generallyobserved in the study, suggesting grazing is not animportant factor in particle transformation during theNorth Atlantic spring bloom. At stations 13971, 13984,and 14010 C/Siparticulate was lower than C/Siassimilation.These stations were significantly different from eachother in terms of diatom bloom development (Brown etal., 2003) and are also likely to have been differentiallyimpacted by grazing. Thus, although we cannot excludethe possibility that grazing may increase C/Siparticulatevalues, it would seem not to have a very significantimpact.

3.8. Effects of preferential mineralisation on ourunderstanding of carbon export

Results from the FISHES data set indicate preferen-tial mineralisation of bSiO2 relative to POC in thesurface mixed layer at most stations during the springbloom in the North Atlantic. The lability of POP andPON relative to POC is widely recognised, and bothdissolved organic and regenerated inorganic nutrientsare important in generating further production (e.g.,Jackson and Williams, 1985; Smith et al., 1986; Vidal etal., 1999). However, whether the observed preferentialmineralisation of bSiO2 relative to POC in the upperwater column is sufficient to make diatom productivity anet sink for inorganic carbon in the northern NorthAtlantic, where most particle mineralisation occursabove the depth of deepest winter mixing (ca. 800m;Koeve, 2001), depends on processes deeper in the watercolumn.

Specifically, it will be a function of the variation inthe stoichiometry of carbon and silicon regenerationwith depth and the extent of the deep winter mixingwhich regenerates the surface nutrient pool. If bSiO2 ispreferentially released from particulate matter relative toPOC throughout the depth to which winter mixingpenetrates (ca. 800m; Koeve, 2001), diatom production

is likely to be a carbon sink over multi-year timescales,and the selective regeneration of carbon relative tosilicon below this depth will be of reduced importance.If, however, a preferential mineralisation of POCrelative to bSiO2 begins at the base of the summermixed layer (<100m), then the water mixed up into thephotic zone during overturning will not contain anexcess of dissolved Si relative to C, and the significanceof diatoms in long-term export production may bereduced.

Evidence from deep sediment traps in the NorthAtlantic shows clearly the better preservation of bSiO2

relative to particulate organic carbon (Honjo andManganini, 1993; Jickells et al., 1996) at depths belowabout 1000m. Thus, if our observations of significantpreferential mineralization of Si relative to C fromparticulate matter in the upper water column are typical,then there must exist a horizon between 100 and 1000mwhere a switch between preferential mineralization ofbSiO2 relative to POC (above 100m) to a preferentialpreservation of bSiO2 relative to POC (below 1000m)occurs. Further measurements of the relative magnitudeof C and Si mineralisation in the upper ocean, especiallyin the 100–1000-m depth range are needed to fullyestablish the role of diatoms in the biological carbonpump.

4. Conclusions

We suggest that bSiO2 is frequently preferentiallymineralised relative to POC from particulate matter inthe photic zone during the NE Atlantic spring diatombloom. The strong coherence between C/Si assimilationand mineralisation ratios, and the short residence time ofparticulate material in the surface mixed layer poolrelative to the length of the spring bloom support ourinherent assumption of steady state over shorttimescales.

High variability in the C/Si ratios of assimilation andof particulate matter in the photic zone was observedbetween stations. Within stations, particulate composi-tion ratios were relatively invariant with depth, whereasthe assimilation ratios often showed a decreasing C/Siwith depth. This is likely due to a decoupling of thelight-dependent primary production from the non-light-dependent assimilation of Si in the short term.

The mean elemental composition of the particulatematerial (C/Si=21.2) was carbon rich relative to thecorresponding assimilation ratio, implying a preferentialmineralisation of Si relative to C in the photic zone.Carbon is contained in the particulate pool for two tothree times longer than silicon, consistent with it being


more refractory and likely to be exported from thesummer surface mixed layer. The rapid recycling ofbSiO2 relative to POC, and in particular its magnituderelative to a preferential mineralisation of POC whichoccurs deeper in the water column may have importantimplications regarding the role of diatoms in exportproductivity. More estimates of silicon assimilation anddepth variability in C and Si cycling are required tofurther address this point.

Acknowledgements

We thank the captain, crew and scientific team ofRRS Discovery cruise D253 for their invaluableassistance on the cruise. This work was supportedthrough UK Natural Environment Research Councilsmall grant NER/B/S/2000/00815, awarded to Queen'sUniversity Belfast.

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relative mineralisation of c and si from biogenic particulate matter in the upper water column...

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