Marine Chemistry, 35 (1991) 477-487Elsevier Science Publishers B.Y., Amsterdam

477

The annual production of biogenic silica in theAntarctic Ocean

P. Treguer" and A.J. VanBennekom''a Institute/or MarineStudies, University a/Western Brittany, 6 Avenue Le Gorgeu, 29287Brest­

Cedex, Franceb Netherlands Institute/or Sea Research, NL-1790Ab den Burg, Texel, Netherlands

(Received 10 November 1990; accepted IS March 1991 )

ABSTRACT

Treguer, P. and Van Bennekom, A.J., 1991. The annual production of biogenic silica in the AntarcticOcean. Mar. Chem., 35: 477-487.

The total annual production of biogenic silica (BSi) of the Antarctic Ocean is estimated at about50 tera (T = 101" ) mol Si. This flux is calculated using available direct measurements of integratedsilicic acid uptake rates, indirect estimates from field distribution of orthosilicic acid in austral wintercompared with that in austral summer and/or after conversion of 14C primary production using ap­propriate Si/C mole ratios measured for the four Antarctic subsystems: the Polar Front Zone, thePermanently Open Ocean Zone, the Seasonal Ice Zone, and the continental shelves and coastal zones.We show that most of the total production ofBSi occurs in the surface layers of the Permanently OpenOcean Zone and in the Seasonal Ice Zone, the contribution of the coastal areas being less relevant.Our results fit well with the previously described distributions of the net accumulation rates of opalin Antarctic abyssal and coastal sediments. The mean ratio of net opal accumulation at the sea-bed tothe net production of BSi in the surface layer of the Antarctic Ocean is about 15%.

INTRODUCTION

The importance of biological activity in controlling the abundance, distri­bution and the budget of silicon in the oceans has been emphasized by manyworkers (reviewed by Spencer, 1983). Estimates for the global biogenic silica(BSi) production by marine phytoplankton have grown from 170X 1012

(=tera; T) mol Si year- 1 since Lerman and Lal (1977) to a maximum of540 T mol Si year " ' (Calvert, 1983). Spencer (1983) and Wollast and Mack­enzie (1983) have respectively suggested a global annual production of 413Tmol Si and 429 Tmol Si. Spencer's estimate was calculated using differentratios ofbiogenic silica (BSi) to particulate organic carbon (POe) to convertthe mean annual photosynthetic rates (estimated from the 14C technique)given by Platt and Subba-Rao (1975) for the various domains of the WorldOcean (367 million krn"), as identified by Koblentz-Mishke et al. (1973).

0304-4203/91/$03.50 © 1991 Elsevier Science Publishers B.Y. All rights reserved.

478 P.TREGUER AND AJ. VAN BENNEKOM

This estimate is consistent with a global production of 2575 Tmol of algalcarbon (30.9 Gigatons C), in agreement with Platt and Subba-Rao (1975)and Berger et al. (1989), the siliceous phytoplankton accounting for as muchas 45% of the total.

The contribution of the Antarctic Ocean (38 million krrr', i.e. about 10%of the total oceanic area) to the global production of BSi is largely unknownbut it is usually expected to be very significant for at least two reasons. First,the orthosilicic acid richness of the Antarctic surface waters is unique and apriori creates favourable conditions for the growth of diatoms. In fact, nodirect determination of silicic acid uptake rates were reported in the Antarctic

Fig. 1. The Antarctic Ocean: 1, Polar Front; 2, 40,uM Si contour (at 100 m depth); 3, northernlimit of the pack-ice. The main Antarctic abyssal deposits of opal (shaded area), according toDeMaster (1981) are mainly beneath the Permanently Open Ocean Zone (between the PolarFront and the northern limit of the pack-ice) and beneath the northern part of the Seasonal IceZone (south of the continuous line).

BIOGENIC SILICA PRODUCTION IN ANTARCTIC OCEAN 479

Ocean before the beginning of the 1980s, but Spencer ( 1983) has indirectlyestimated that the annual production of BSi should be about 123 Tmo1 Si(30% of the global production), which is consistent with a photosyntheticproduction of about 274 Tmol C (9% of the total). A second reason for ex­pecting a high production of BSi in the Antarctic surface layers arises fromthe global marine silica budget itself: it can be balanced only if we assume thatmore than two-thirds of the deep removal of opal from the World Ocean (netremoval 6.9::t 1.4 Tmol Si year- I) occurs in the Antarctic Ocean, as demon­strated by DeMaster (1981) and Ledford-Hoffman et a1. (1986). Althoughthose fluxes could have been overestimated (Van Bennekom et a1., 1988), DeMaster's and Ledford-Hoffman's papers are of major interest as they gave, inaddition, indications about the areas of abundant silica deposition (Fig. 1)and thus about the surface areas where we could expect relevant productionof BSi. Most of the net silica deposits (4.2 ±2.1 Tmol Si Year-I) appear tooccur in a circumpolar abyssal zone (19 million km") corresponding to sur­face regions including the Permanently Open Ocean Zone and the northernpart of the Seasonal Ice Zone (Fig. 1). The deposits are interrupted (Fig. 1)in the Drake Passage and in the Scotia Sea, both areas being swept by intensebottom currents. A significant contribution to the opal deposits (0.7::t 0.7Tmo1 Si year :") could come from the Antarctic continental margin (0.9 mil­lion km"). as suggested by Ledford-Hoffman et a1. (1986) from data col­lected in the Ross Sea.

Taking into account available direct and indirect measurements of the pro­duction of BSi, our aim here is to give a realistic estimate of the total amountof BSi annually produced in the Antarctic Ocean.

THE TOTAL PRODUCTION OF BIOGENIC SILICA IN THE ANTARCTIC OCEAN

Methodology

To estimate the total production ofBSi in the Antarctic Ocean wetake intoaccount:

( I) Direct measurements of rates of orthosilicic acid uptake obtained bytracing biological activity using the 30Si stable isotope (Nelson and Gordon,1982; Nelson and Smith, 1986; Nelson et a1., 1991; Queguiner et al., 1991)or the 32Si radioactive isotope (Treguer et al., 1991). Both methods give in­tegrated daily rates of biogenic silica production in the photic layer that arenot corrected for in situ dissolution of particulate silica (Nelson and Gordon,1982). These rates are considered as 'gross production' rates, in analogywiththe terminology in use for the carbon cycle.

(2) Indirect estimates: 14C production (usually representing something in­termediate between gross and net carbon production; Bender et a1., 1987) canbe used to calculate Si production by using appropriate ratios ofBSi to partie-

480 P. TREGUER AND A.J. VAN BENNEKOM

ulate organic carbon (POC). Unlike Spencer (1983), we use only specificratios determined in parallel with 14C production rates in different areas andseasons. Indirect determinations of orthosilicic acid uptake rates from com­parative field distributions in the photic layer (integrated austral winter con­centrations minus integrated austral summer concentrations) are also takeninto consideration.

The use of different techniques to evaluate daily production of BSi in theAntarctic Ocean can introduce significant discrepancies and could bias oursilica budget. For instance, Si and C uptake rates can be directly determinedusing incubations of samples in conditions that are supposed to simulate theenvironmental conditions which prevail in situ. Basically, these direct deter­minations take into account local and daily conditions, eventually missinggrowth peaks depending on the study period, which leads to possible under­estimation of the uptake rates. Also, when the experiments are conducted innon-clean conditions, bias might be introduced, for instance because of inhi­bition of the phytoplankton growth by trace metal contamination (Martin etal., 1989). Any incubation method provides water column stability becauseof the containment of the water in bottles, which could be non-realistic con­ditions for well-mixed layer ecosystems. A priori this could bring us to preferestimates given from orthosilicic acid budgets. None the less, indirect meth­ods have also their own limitations and sources of uncertainties: the referencevalue of the silicic acid concentration during winter is usually not preciselyknown; the subsequent roles of advection and of diffusion during spring andsummer are difficult to calculate with precisions; the variations of the depthsof the wind-mixed layer and of the photic layer during spring and summer arealso largely unknown. Thus to obtain realistic estimates of daily BSi produc­tion in the various subsystems we prefer to use the whole available data sets(i.e. from the direct method as well as from indirect methods) and to discusstheir eventual differences.

To transform daily production into annual production we need to know theduration of the productive period, which is variable for different latitudesand ecosystems.

The/our majorecosystems a/the Antarctic Ocean

As far as the concentration of orthosilicic acid in the surface layer is con­cerned, four main provinces can be distinguished in the Antarctic Ocean (Fig.1) .

( 1) The Polar Front Zone (PFZ; about 3 million km") where the orthosil­icic acid concentration is usually lower than 10,uM Si (i.e. at the limiting levelfor the growth of Antarctic diatoms; Jacques, 1983), unlike that of nitrateand phosphate which, even on the northern side of the Polar Front, can reach

BIOGENICSILICAPRODUCTION IN ANTARCTICOCEAN 481

values comparable with those measured near the continent (Van Bennekomet al., 1988; Jones et al., 1990).

(2) The Permanently Open Ocean Zone (POOZ; 14 million km2): usually

a well-mixed water layer with low primary production during summer, as de­scribed by various workers (reviewed by Jacques and Treguer, 1986; Treguerand Jacques, 1986). This zone is characterized either by a dramatic gradientof orthosilicic acid ( =silicate), for instance in the Indian sector, or by a sili­cate front, for instance in the Weddell-Scotia Confluence (Van Bennekom etal.,1990).

(3) The Seasonal Ice Zone (SIZ, i.e. the area influenced by the pack-ice;16 million km") where orthosilicic acid concentrations are usually higher than60,uM Si during winter; during ice-melting this area is a stratified system andbecomes an open zone as far as the ice retreats.

(4) The continental shelves and coastal regions (0.9 million km"), wherethe orthosilicic concentrations usually reach high values during winter butwhere significant depletion of silicate has been found during summer (Nelsonand Smith, 1986; Nelson et al., 1991).

Seasonal variations and annualproduction ofbiogenic silica in thefourecosystems

Table 1 takes seasonal variations into account and gives estimates for theannual BSi production (mean and range) for the four provinces.

TABLE 1

Production of biogenic silica in the Antarctic Ocean

Domain Surface BSi-P day-I BSi-P year" '(million krrr') (mmol Si m- 2 ) (TmoISi)

Range Average Range Average

Polar Front Zone 3 3 2(PFZ)

Permanently OpenOcean Zone (POOZ) 14 8 23

4-19 12-56Seasonal Ice Zone 16 7 20(SIZ= MIZ+ice-edge) 2-14 6-40

Continental shelves/coastal areas 0.9 14.5 2.5

7-93 1- 15Total 34 47.5

22-114

482 P. TREGUER AND A.J. VAN BENNEKOM

(1) ThePFZIn this less documented zone we have only the measurements made during

autumn 1987 in the Scotia-Weddell Seas and Drake Passage (P. Treguer, un­published report, 1987). Carbon primary production is 10.7 mmol C m- 2

day-I and the mean molar ratio BSi/POC in living phytoplankton is 0.3(Treguer et al., 1990); biogenic silica production (BSi-P) can therefore beestimated as 3 mmol Si m-2 day-I. On an annual basis, this value is probablyunderevaluated because field data usually showed enhanced biomass in thisarea in spring and summer (Jones et al., 1990) unlike the situation we ob­served. The total annual BSi-P (from October to April) for this zone is 2Tmol Si, and it should not exceed 10 Tmol Si.

(2) ThePOOZSpring/summer: During October 1978, Nelson and Gordon (1982), at two

stations in the Pacific sector, measured BSi-P of 3.6 and 6.8 mmol Si m- 2

day"; an average of 5.2 mmol Si m- 2 day-I. During spring 1988-summer1989, for two stations situated in the Scotia Sea, Treguer et al. (1991) alsousing direct determinations (incubations in clean conditions), measured BSi­P values of 6.9 (mid-December 1988) and 18.8 mmol Si m- 2 day:" (begin­ning of January 1989); an average of 12.9 mmol Si m- 2 day-I. From orthos­ilicic acid depletion in surface waters of the Atlantic sector, Van Bennekomet al. (1988) calculated a mean BSi-P of 13.3 mmol Si m-2 day" ' for 4months, which is reasonably consistent with Treguer et al.'s determinations.

Summer/autumn: During March-April 1987, P. Treguer (unpublished re­port, 1987) measured 14C primary production values of9.7, 25.2 and 20 mmolC m- 2 day-I at three stations located in the Atlantic sector; the average is18.3 mmol C m- 2 day-I. BSi/POC in living phytoplankton was 0.3; thus thecorresponding average value for BSi-Pis 5.5 mmol Si m- 2 day- I.

In the POOZ, the weighted average from October to April is 7.9 mmol Sim-2 day-I (range 3.6-219 mmol Si m-2 day- I) and the annual productionshould reach 23 Tmol Si (Table 1).

Holm-Hansen et al. (1977) gave an average primary productivity of 11.2mmol C m- 2 day-I, calculated from J4C data in the Indian and Pacific sec­tors during the productive period (120 days year- I). This value is at presentconsidered to be representative of the background daily production of theareas not influenced by ice-melting, i.e. the open ocean zones (Nelson andSmith, 1986;Smith et al., 1988). Using our own estimate for BSi-P,this wouldgive a mean Si/C molar ratio as high as 0.7. Although it is not inconsistentwith the present data range for this area during blooms of diatom-dominatedpopulations (004-0.8; Treguer et al., 1990, 1991), on an annual basis thisratio seems rather high as it leaves no allowance for other non-siliceous phy-

BIOGENIC SILICA PRODUCTION IN ANTARCTIC OCEAN 483

toplankton species. This means that our mean daily value could have beenoverestimated (for instance, as a result of abnormally high BSi-P during latesummer and autumn for the studied years).

(3) The SIZ (including the marginal ice zone, MIZ, and the ice-edge) springand summer: during November-December 1988, after a bloom period, at ninestations situated in the MIZ of the Weddell Sea, using a direct method, Tre­guer et a1. (1991 ) measured silicic acid uptake rates of 4.4, 10.7, 4.8, 4.7, 5.4,7.6, 12.3,9.1 and 1.8 mmol Si m- 2 day-I. The average is 7 mmol Si m- 2

day-I (range 1.8-12.3 mmol Si m- 2 day-I). From indirect measurements(integrated austral winter concentrations in the wind-mixed layer minus in­tegrated austral summer concentrations), over a period of 60-90 days, Jen­nings et a1. (1984) calculated that the maximum production ofbiogenic silicain the SIZ of the Weddell Sea ranges between 9.5 and 14.2 mmol Si m- 2 day",when the effects of silicic acid remineralization are included. Jennings' dailyaverage should a priori have been lower than that given by the direct methodbecause it provides an estimate closer to a net production than to a gross pro­duction. The difference between the two averages could come from (1)underestimation of the rate determined by using the direct method, which isbased on measurements performed during a non-peak period, characterizedby a dramatic decrease of phytoplankton standing stock after intense grazingof diatoms by krill and copepods (Treguer et a1., 1991) and (2) regional andinterannual variations, which could create environmental conditions more orless favourable to growth of siliceous phytoplankton. Thus a more realisticestimate for BSi-P could be 10 mmol Si m -2 day- I.

In the Maud Rise area, Van Bennekom et a1. (1988) have calculated, usingan indirect method, an average value of 16.7 mmol Si m- 2 day-I for grossBSi-P over a 5-month period. Although this area is formally within the SIZ,its production could be higher because the water remains open in this regionfor a long time compared with the usual situation for the rest of the SIZ. Al­though local production in polynyas can be important their contribution tothe total should not be relevant; thus it is not taken into account for our meanestimate.

Summer/autumn: During March-April 1987, P. Treguer (unpublished re­port, 1987) measured primary 14C primary production of 15.8,20.0 and 16.1mmol C m -2 day- I at three stations located in the Weddell-Scotia Sea. Theaverage is 17.3 mmol C m- 2 day- '; with BSijPOC=0.3 in living phytoplank­ton (Treguer et a1., 1990) the average BSi-P is 5.2 mmol Si m- 2 day",

Thus our annual weighted average for BSi-P in the SIZ from November toApril is 6.8 mmol Si m -2 day" ' (range 2-17 mmol Si m -2 day-I). The totalannual production of biogenic silica in the SIZ of the Southern Ocean aver­ages 20 Tmol Si (range 6-49 Tmol Si), about 50% being produced within thetwo spring months. Using a simple model of ice-edge bloom genesis, Smith eta1. (1988) have calculated that the primary production ofthe SIZ should standat about 28 Tmol C (range 22-34 Tmol C) for a period of 5 months, which

484 P.TREGUER AND A.J. VAN BENNEKOM

gives a mean rate of 21.4 mmol C m -2 day-I. This leads to a mean SijCmolar production ratio of 0.3, which is reasonable for this area (Si/C valueshave been reviewed by Treguer et al., 1990).

(4) Continentalshelves/coastal areasSpring: no direct determinations were reported during this season.Summer: during summer 1983, Nelson and Smith (1986) measured the

BSi-P in the southwestern Ross Sea (eight stations): the average is 38 mmolSi m ? day : ', with data ranging from 7 to 93 mmol Si m- 2 day" '. Nelson etal. (1991) showed comparable values for a diatom-dominated ice-edge bloom(average 32.9 mmol Si m -2 day-I for seven stations, range 13.9-55.3 mmolSi m -2 day-I) during summer 1990. None the less, lower values were foundfor the Phaeocystis-dominated blooms in continental shelf waters (average15.1 mmol Si m -2 day '! for six stations, range 7.5-31.3 mmol Si m -2 day-I;Nelson et al., 1991).

Autumn: At a coastal and turbid water station in the Bransfield Strait, P.Treguer (unpublished report, 1987) measured a production of 3.3 mmol Cm- 2 day-I; with BSijPOC=0.3 the estimate for BSi-P is 1 mmol Si m- 2

day-I. This value is given to show that because light limitation, waters closeto the coast (the station was located 0.3 mile from the shore of GreenwichIsland, South Shetland)' could present a dramatic reduction of the primaryproduction.

Knowing that the main bloom lasted about 2 months (Smith and Nelson,1986) and that the remaining production should be comparable with that ofthe POOZ, from November to April, our mean weighted estimate for BSi Pis14.5 mmol Si m -2 day- I i.e, equivalent to 2.4 Tmol Si year- I for the conti­nental shelves and coastal areas.

The total mean annual BSi-P for the Antarctic Ocean is about 47 Tmol Si.Because of the lack of data within the PFZ this value could be an underesti­mate. Nevertheless, the mean total BSi-P should be less than about 60 TmolSi per year. The production of BSi in the Southern Ocean (Antarc­tic +subantarctic regions) can also be calculated, using, in addition, the datameasured within the subantarctic Pacific sector by Nelson and Gordon (1982)during spring 1983 at four stations: 2.5, 0.9, 3.1 and 1.4 mmol Si m- 2 day" '.This gives a production of 15.6 Tmol Si year- l for 7 months in the subantarc­tic zone. An estimate for the total annual gross production of the SouthernOcean is therefore 47+ 16=63 Tmol Si.

Every entry in Table 1 could be subject to considerable variations arisingfirst from the small number of direct (or indirect) BSi-P measurements (in­cluding only 53 integrated daily rates, for the whole Antarctic Ocean and forthe different seasons), and second from the assumptions made in calculatingannually and spatially integrated rate estimates. Nevertheless, the generalpattern that emerges is clear and interesting.

BIOGENIC SILICA PRODUCTION IN ANTARCTIC OCEAN 485

( 1) Although the Antarctic continental shelf and coastal regions are as pro­ductive as coastal upwelling ecosystems (Nelson et al., 1981), because of theirsmall extent, these regions contribute less than 10% to the total.

(2) The contribution of the POOZ to the total is comparable with that ofthe SIZ. Both regions are of prime interest for the study of the silica cycle inthe Antarctic Ocean.

(3) The total primary production of the Antarctic Ocean (open oceanzone + ice-edge zone, but not including the PFZ) has been recently re-esti­mated by Smith (1991 ) at about 105 Tmol C year- I, which compared withour annual estimate for BSi give a mean SijC ratio of 0.43 for the Antarcticphytoplankton (siliceous+non-siliceous). In fact, field data show that theSijC ratios from production and standing stock data range from 0.05 for non­siliceous phytoplankton to one for diatom-dominated populations (Smith andNelson, 1986; Treguer et al., 1990, 1991); this tends to demonstrate that ourestimate for the total annual BSi-P is reasonable.

(4) Table 1 shows that the distribution pattern of the production of bio­genic silica by diatoms in the various subsystems of the Antarctic Ocean issimilar to that of the net accumulation rates of opal at the sea-bed (Fig. 1),as shown, for instance, by DeMaster ( 1981 ). Assuming with Nelson and Gor­don (1982) that about 30% of the gross BSi-P dissolves in the photic layer,the net BSi-P is about 32 Tmol Si. The net accumulation rate (about 4.9 TmolSi, according to Ledford-Hoffman et al., (1986) ) could represent 15%of thenet production in the surface waters of the Antarctic Ocean.

(5) As far as the annual BSi-P is concerned, our mean estimate is only 40%of that of Spencer (1983), who assumed that all primary production wouldbe in the form of diatoms in the Antarctic ecosystems.

Consequences for theglobal annual production ojBSi

This study finally suggests that previous estimates for global annual BSi-Pshould be re-examined. On the one hand, from this new estimate for BSi-P inthe Antarctic Ocean, one of the three major silica productive zones (see Spen­cer, 1983, Table 4.1, p. 129), a significant decrease for the annual global ma­rine silica production could be expected. But, on the other hand, since Spen­cer's work the status of oligotrophic areas has been changing. Although Berger'sproductivity map (1989) gave much smaller oligotrophic areas than before,indications are also appearing (e.g. Jenkins, 1982; Martin et al., 1987; Jen­kins and Goldman, 1988) that the production of biogenic matter (includingsilica) for these oligotrophic areas had been underestimated and that the totalcarbon production of the World Ocean could be considerably higher than the30 Gigatons used by Spencer. Platt et al. ( 1989) also pointed out the impor­tance of transient events in generating increasing upward flux of nitrate, whichalso brings orthosilicic acid into the surface layer, as was demonstrated for

486 P.TREGUER AND A.J. VANBENNEKOM

the Atlantic Ocean by Brzezinski and Nelson (1989) in warm-core rings ofthe Gulf Stream. From these data, Brzezinski and Nelson extrapolated thatthe BSi-P in open ocean areas should range between 0.4 and 0.8 mol Si m- 2

year" '. Finally, the contribution of these areas to the total could be as high as50-100 Tmol Si year-I (D.M. Nelson, personal communication, 1990). Ifthis is true, an increased contribution of the oligotrophic provinces mightlargely make up for our lower estimate for the Antarctic province.

ACKNOWLEDGEMENTS

This study was supported by the French Ministry of Research and Tech­nology (Polar Network) and the French-Dutch Co-operation Office.

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