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Annual primary production and export flux in the Southern Ocean from sediment trap data
Marine Chemistry, 35 (1991 ) 597-613 Elsevier Science Publishers B.V., Amsterdam
597
Annual primary production and export flux in the Southern Ocean from sediment trap data
Gerold Wefer and Gerhard Fischer Fachbereich Geowissenschaften, Universitiii Bremen. Klagenfurter StrafJe. D-2800 Bremen 33. FRG (Received I September 1990; accepted 14 February 1991)
ABSTRACT Wefer, G. and Fischer, G., 1991. Annual primary production and export flux in the Southern Ocean from sediment trap data. Mar. Chem., 35: 597-613.
Since 1983 time-series traps have been deployed in the Atlantic sector of the Southern Ocean to measure the flux of organic carbon, biogenic silica and carbonate. The organic carbon flux data are used to calculate primary production rates and organic carbon fluxes at 100 m water depth. From these calculations, annual primary production rates range from about 170 g C m -2 in the coastal area (Bransfield Strait) to almost zero in the Permanent Sea-Ice Zone. High rates (of about 80 g C m- 2 vear:") were calculated for the Polar Front Zone and rather low values (about 20 g C m- 2 year-I) characterize the Maud Rise area. The estimated primary production for the entire Southern Ocean (south of 50° S), using various subsystems with characteristic carbon fluxes, is in the order of I X I09tons year- I: the organic carbon flux out of the photic layer is 0.17 X I09tons vearr '. Our calculation of the Southern Ocean total annual primary production is substantially lowerthan previously reported values.
INTRODUCTION
The flux of organic particles out of the photic layer is a central element of the cycling of carbon and associated biogenic elements in the ocean (Eppley and Peterson, 1979; Suess, 1980; Broecker and Peng, 1982; Sundquist and Broecker, 1985; Berger et al., 1989). Sedimentation of organic carbon can play an important role in changing the CO 2 content of the atmosphere. Photosynthesis and the export of organic carbon from the photic zone as particulate organic carbon or dissolved organic carbon acts as a 'biological pump', which lowers the partial pressure of CO 2 in surface waters (e.g. Berger et al., 1989). The atmospheric CO 2 content orients itself mainly on the pC0 2 of the surface waters of the global ocean. The Southern Ocean (south of 50 S) occupies a position of special interest in this context. In this area, primary production is generally not restricted to 0
0304-4203/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
598
G. WEFER AND G. FISCHER
the availability of nutrients. Therefore, the Southern Ocean has a high potential to extract CO 2 from the atmosphere if primary production and organic flux out of the photic zone increase (Knox and McElroy, 1984; Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984). To estimate the role of the Southern Ocean in the global carbon cycle today, primary production rates and the carbon flux out of the photic zone (export production) have to be known. Because of logistical problems, direct measurements of annual primary production over the whole year are difficult or almost impossible. Depending on the methods and the times of measurements, the published results show a high spatial and temporal variability of primary production (El-Sayed and Taguchi, 1981; Jennings et al., 1984; Smith and Nelson, 1986; Jacques, 1989) and organic carbon fluxes in the Southern Ocean (Wefer et al., 1990). First estimates of primary production and organic carbon sinking out of the euphotic zone are possible using annual organic carbon flux rates derived from sediment trap deployments. Since 1983 we have been deciphering the _10·
10·
20·
40·
,
50·
\ \
\ \ \
60·
70· 80·
Fig. 1. Locations of the mooring sites and minimal (February 1985; stippled line) and maximal (September 1985. dashed line) sea-ice coverage. The trap sites are typical for the following subsystems (zones): KGI-KG3: Coastal Zone; WSl: Permanent Sea-Ice Zone (PSIZ); WS3 and WS4: Permanent Open Ocean and Seasonal Sea-ice Zone (POO and SIZ): PFl: Polar Front Zone CPFZ).
599
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
TABLE 1 Locations and details of trap deployments Location
Mooring name
Bransfield Strai t 62' 15.4'S. 57'31.7'W 62'20.1 '5. 57'28.3'W 62°22.0'5. 57 Northern Weddell Sea 62°26.5'5. 34°45.5'W Maud Rise 64° 55.0'S. 02 0 3 0 . 0 ' W 64° 54.1 'So02' 33.8' W 64° 55.5'5.02 0 3 5 . 5 ' W Polar Front 50 0 0 9 . 0 ' S , 0 5 ' 4 6 . 4 ' E Q49.9'W
Water depth (m)
Trap depth
KGI
1952
KG2 KG3
1650 1992
494 1588 693 687
4.12.84-13.11.85 26.11.85-07.05.86
W51
3880
863
25.0 J.85-J 9.03.86
WS2 WS3 WS4
5000 5053 5044
4456 360 352
20.01.87-20.11.87 16.01.88-04.02.89 03.03.89-26.02.90
PFI
3750
700
15.01.87-12.03.88
Deployment time
(m) 1.12.83-25.11.84
seasonal sedimentation in the Southern Ocean with time-series sediment traps deployed in the area of the Bransfield Strait (KG1, KG2 and KG3), the northern Weddell Sea (WSl), west of Maud Rise (WS2, WS3 and WS4) and in the Polar Front (PFl) area (Fig. 1, Table 1). The flux data in part were reported elsewhere (Fischer et al., 1988; Wefer et al., 1988, 1990). Here we report seasonal flux data from two deployments west ofthe Maud Rise (WS3 and WS4) and annual flux rates from the Polar Front area (PF1). With these data, we calculated primary production and export carbon fluxes (water depth of 100 m) using empirical relationships between primary production and organic flux rates (Suess, 1980; Betzer et al., 1984; Pace et al., 1987; Berger et al., 1987) and the organic carbon flux-water depth relationship of Martin et al. (1987). The result is that annual primary production varies between 0.7 g C m -2 in the open ocean area and more than 170 g C m -2 in the coastal zone. The organic carbon flux at 100 m ranges from almost zero in the Northern Weddell Sea to 30 g C m -2 year- I in the Bransfield Strait. Total primary production and export production for the entire Southern Ocean were estimated using different Antarctic subsystems (Treguer and van Bennekom, 1991) with characteristic carbon fluxes derived from our trap experiments. The total primary production for the entire Southern Ocean is about 1X 109 tons year- I, and the organic carbon flux to 100 m water depth (export production) is less than one-fifth. FIELDWORK AND LABORATORY ANALYSIS
Table 1 shows the locations and deployment times of the sediment trap experiments. Before deployment, HgCl 2 was added to the seawater of the col-
600
G. WEFER AND G. FISCHER
lecting bottles to minimize bacterial degradation; some dissolved HgCh was still present after recovery. On recovery, a very small amount of the trap material was removed for the preparation of scanning electron microscopy (SEM) slides and for inspection on board using a transmission light microscope. Subsequently, the trap material was wet-sieved with a 1 mm screen to remove larger swimmers and split into aliquots by a rotary liquid splitter. Generally we used a quarter-split from each sample for this study. The remainder was retained for detailed analysis of the organic chemistry and the micropalaeontological constituents. We obtained total fluxes from quarter-splits (smaller than I mm) which were desalted by means of repeated short washings, freeze-dried at 30°C and weighed. A portion of this material was decalcified with 2 N HCI, dried and measured in a Heraeus CHN Analyser for organic carbon. No filtration step was involved. Carbonate contents were measured with the CHN Analyser using samples which had not been treated with HCI before analysis. Carbonate is calculated as follows: (ClOtal-Corg) X 8.33.
Biogenic opal was determined using a modified sequential leaching technique (DeMaster, 1981; MUller and Schneider, personal communication, 1990). The sample was extracted with 1 M NaOH at 85°C under stirring in a stainless steel vessel. The increase in dissolved silica is simultaneously recorded by continuous flow analysis (Molybdate Blue method). A typical extraction curve for sediment trap particles is characterized by a rapid initial silica increase, as a result of the dissolution of opal particles, followed by a much slower increase representing the dissolution of silicate minerals (e.g. clay minerals). The y-intercept value obtained by linear extrapolation to an extraction time of zero gives the opal content of the sample. To calculate primary production from annual organic carbon flux data we used the equation of Berger et al. (1988):
J= 17XPP /z+ PP /100
(1)
which becomes
PP=J/ (17 /z+ 1/100)
(2)
where J is the flux at any depth, PP is the primary production (in g C m- 2 year- I) and z is the water depth (in metres). It must be emphasized that this equation and also other carbon flux-primary production relationships originate largely from the extrapolations of short-time productivity and particulate organic carbon (POC) flux measurements to annual averages (von Bodungen, 1989). The productivity-POC flux equations further assume that the proportions of new production of primary production (fratio) are relatively constant in all regions of the World Ocean. This is not true, as was demon-
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
601
strated by Eppley and Peterson ( 1979). Especially for the pulsed productivity in high latitudes a nonlinearity between the export flux and primary productivity can be expected (Berger et al., 1987). In the southeastern Weddell Sea, highly variable POC and particulate organic nitrogen (PON) flux rates may be due to changing mixing depths and, hence, to a seasonally variable .fratio (von Bodungen, 1989). Nevertheless, the average POC flux measured may correspond to the average primary productivity. To calculate the organic carbon flux out of the euphotic zone (below 100 m), the equation of Martin et al. (1987) is applied: F= FIQO X (zjlOO)b
(3)
which becomes F lQo= F j ( zj 100 )b
(4)
where b (log-log slope) is -0.858 (Martin et al., 1987), Fis the flux at any depth, F lOo is the flux at 100 m and z is the depth in metres. RESULTS AND DISCUSSION
Annual sedimentation patterns Data on deployments and annual flux rates (per m") of total mass, organic carbon, carbonate and biogenic silica for the two Maud Rise deployments (WS3 and WS4) and the annual flux rates for the Polar Front site (PF1) are given in Table 2a-2c. The seasonal patterns of total mass and organic carbon sedimentation for the two Maud Rise sites are shown in Fig. 2. Because of a malfunction of the trap at the Polar Front site, only annual flux rates are available. The annual patterns of the Bransfield Strait and Northern Weddell Sea sites have been given by Fischer et al. (1988) and Wefer et al. (1988, 1990). The main results are briefly reported here. In the Bransfield Strait (trap sites KG1-KG3; Fig. 1) particle flux is restricted to a surprisingly short period in austral summer (Wefer et al., 1988, 1990). During austral summer (December and January), total flux can be more than 1.5 g m -2 day-I, whereas during all other months flux was about 10-100 times lower. The flux of the most productive months or weeks can reach 97% of the total. Under conditions of high export production, faecal pellet transport is dominant. It is assumed that krill swarms feed on phytoplankton blooms and incorporate the small particles into their pellets. These pellets settle to the traps within days. For the other months of the year, downward transport mainly by settling of discrete small particles is assumed. The 3 year record also shows a large interannual variability. Total annual fluxes varied between 12 and 120 g m -2 (Table 3; Wefer et al., 1990). At the northern Weddell Sea site (trap site WS1; Fig. 1), the annual particle
602
G. WEFER AND G. FISCHER
TABLE 2 Flux rates of total mass and individual biogenic components for traps WS3 (a), WS4 (b) and PF1 (c) (a) WS3 Sample no. I
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Sampling interval
16.01.88-01.02.88 01.02.88-17.02.88 17.02.88-04.03.88 04.03.88-20.03.88 20.03.88-05.04.88 05.04.88-21.04.88 21.04.88-06.05.88 06.05.88-23.05.88 23.05.88-09.06.88 09.06.88-11.07.88 11.07.88-12.08.88 12.08.88-13.09.88 13.09.88-15.10.88 15.10.88-31.10.88 31.10.88-16.11.88 16.11.88-02.12.88 02.12.88-18.12.88 18.12.88-03.01.89 03.01.89-19.01.89 19.01.89-04.02.89
Duration (days) 16 16 16 16 16 16 16 16 16 32 32 32 32 16 16 16 16 16 16 16
g m _2 (384 days)-I Annual estimate (g m -2)
Flux rates (mg m -2 day") Total
Opal
Corg
CaC0 3
96.5 424.0 647.5 332.5 202.3 121.0 36.8 113.5 12.8 15.9 13.3 5.6 4.4 15.3
5.1 30.0 41.8 21.9 9.4 5.5 2.4 5.8 1.0
43.8 24.2 53.5 15.6 15.4 9.1 2.1 7.3 1.9 1.4 1.6 0.5 0.9 0.0
0.5 1.8 93.0 17.0 15.8
38.8 298.9 472.6 215.5 171.1 89.3 22.8 97.2 7.8 8.8 7.7 3.0 1.4 0.5 n.d. n.d. n.d. 76.9 15.5 10.9
35.51 33.70
24.95 23.75
1.3
1.7
1.2 0.8 0.7 6.1 n.d. n.d. n.d. 4.8 3.0 1.4 2.36 2.28
n.d,
n.d. n.d. 0.4 1.2 0.3 2.94 2.76
(b) WS4 Sample no.
Sampling interval
Duration (days)
Flux rates (mg m- 2 dav") Total
I
2 3-20
03.03.89-21.03.89 21.03.89-08.04.89 08.04.89-26.02.90
18 18 18 (each)
g m "? (360 days)-I Annual estimate (g m -2)
30.4 102.9 n.d.
2.40 2.43
Opal 12.9 78.1 n.d. 1.64 1.66
Corg
CaC0 3
4.2 4.9 n.d.
3.7 1.6 n.d.
0.16 0.16
0.95 0.96
flux measured was the smallest yet observed in the World Ocean and showed extreme variability (Fischer et al., 1988). The variability of particle fluxes seemed to be due to spring thaw processes rather than to the occurrence of larger open seas. As observed in the Bransfield Strait, faecal pellet transport played an important role in the vertical transport of material. Most pellets,
603
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
TABLE 2 (continued) (c) PFI Sample no. I
2 3 4
Sampling interval
15.01.87-04.02.87 04.02.87-24.02.87 24.02.87-16.03.87 16.03.87-12.03.88
Duration (days) 20 20 20 361
g m- 2 (421 days)-' Annual estimate (g m
r
")
Flux rates (mg m -2 day-I) Total
Opal
Cerg
CaC0 3
268.2 192.9 229.0 84.3
79.4 42.6 95. 7 37.1
6.4 4.1 6.6 8.1
89.3 123.4 71.7 16.6
44.22 38.30
[ 7.74 15.30
3.33 2.86
11.68 10.10
n.d., not determined. Information on locations of mooring sites and deployment times is given in Fig. I and Table 1. The flux rates between 8 April 1989 and 26 February 1990 (samples 3-20, WS4) were almost zero. No particles could be identified in the sampling cups by eye. Thus, total flux rates were not determined.
which are of unknown origin, were about 0.5 mm in diameter and were slightly elongated. Cylindrical pellets produced by krill swarms were rarely found. The trap site west of the Maud Rise is in an area of a relatively small open ocean polynya located at the eastern margin of the Weddell Gyre. This Maud Rise polynya (65°S, 2 °E) was observed as a reduction of the sea-ice concentration during winter 1980 (Comiso and Gordon, 1987). The Maud Rise is a topographic high, reaching from about 5000 m to 2000 m below the sea surface. It is assumed that the formation of the Maud Rise open-ocean polynya is related to deep-reaching convection which introduces warm deep water into the surface layer, thus inhibiting sea-ice formation (Gordon, 1982; Gordon and Huber, 1984; Comiso and Gordon, 1987). This doming of warmer and saltier water and an early break-up and acceleration of the decay of the Weddell ice pack (Comiso and Gordon, 1987), should have an effect on the particle sedimentation. West of the Maud Rise (trap sites WS3 and WS4; Fig. 1), seasonal variability in the daily flux was smaller than at the Bransfield Strait and northern Weddell Sea sites. Total flux at WS3 ranged between 647 mg m -2 day" ', during austral summer months, when the mooring site was not ice covered, and 0.5 mg m -2 day-I, during austral winter months (Table 2a). Biogenic opal is dominant, accounting for about 70% of total flux. Most of the biogenic opal consists of diatom frustules and associated debris. In comparison with the other trap sites, carbonate content is high, sometimes reaching 45% (average about 8%). Preliminary microscopic examinations show that the planktonic foraminifera Neogloboquadrina pachyderma accounts for the entire calcium carbonate flux but other species such as Globigerinita glutinata and Turborotalita quinqueloba, adapted to warmer temperatures, are also present. These
604
G . WEFER AN D G. FISCHER
WS3/4 350 m,352 m 700 -r-----~~_:_-----.------------...,
WS3
a BOO -
~ 500~
v
~
400-
Ol
E ~ 300<;::
~
200-
r
100 -
...., o-LJ++-++++++f=1=l9=f=l-r,....\-+-I=f-i+++.... ..,-.,.,..,.,..,.,...,.,...,rr-r-r,,--j Sample No. I
sea-Ice coverage Qntenth)
El
15
10
sea-leeIree
5/10
kMN01lliill IMMI~ BD 7/10
9.5/10
to
10
20
sOB·lce".. 7110
B/l0
WS3
b
20
15
9.5110
8/10
WS4
60 ~
~
-0
C)l
50
E Ol
.§.
40
~
""c:
~
0 .~
30
20
~
0
10
0
I I
Somple NO.1
\6.1 4.3 21.4
15
10
9.6
15.\0 18.12 11188
10
20
30.8
3 .3 8.4
1889
15
20
26.2
1990
I
F ig. 2. Total ( a ) and organic car bon flux rates ( b ) west of Maud Rise (sites WS3 and WS4 ) . Locations of the sampling sites are shown in Fig. 1, deplo yment times are given in Table I, flux d at a are shown in Tabl e 2a and 2b.
605
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
TABLE 3 Annual flux rates (g m- 2 ) in the Southern Ocean (Atlantic sector) and percentages of total mass flux of individual biogenic components Trap site (depth) Bransfield Strait (494 m) 1983-1984 (693m) 1984-1985 (687 m) 1985-1986 Northern Weddell Sea (863 rn ) Maud Rise (4556m) 1987 (360 m) 1988-1989 (352 m ) 1989-1990 Polar Front (700m)
Total mass
Opal
%af total
Cors
%of total
Carbon
%of total
120 11.9 36.6
54.8 3.5 18.3
45.7 29.6 50.0
7.7 0.35 I.!
6.7 2.9 3.0
4.3 n.d. n.d.
3.6 n.d, n.d.
79.0
0.021
5.7
0.011
3.0
1.0 2.8 0.09
12.7 8.2 3.7
0.371
0.293
7.9 33.7 2.43
4.0 23.8 1.66
50.6 70.4 68.3
0.17 2.28 0.16
2.2 6.8 6.6
38.3
15.3
40.0
2.86
7.5
10.1
26.5
data confirm the assumption that productivity is increased in the open ocean polynya at the Maud Rise area. In comparison with the open ocean site WS1 in the northern Weddell Sea, the flux rates and the content of organic carbon are much higher at the Maud Rise site. Similar to the Bransfield Strait, a large interannual variability is observed (Fig. 2). Total mass flux in 1988-1989 was 33.7 g m- 2 year-I compared with 2.43 g m- 2 year- I in 1989-1990 (Table 2a and 2b). However, it has to be taken into account that we might have missed a period of higher sedimentation rates in February 1989. Because of the ship's schedule the sampling period was interrupted between 4 February and 3 March (Fig. 2). As is shown in Fig. 2, no peak in mass flux was found during January-February 1990, as observed for the years 1988 and 1989. Thus, one might conclude that the WS4 trap did not work properly. The sediment trap controller data and the fact that the sample collector was at zero position after recovery, give no indication of any malfunctions of the trap. Hence, we have to assume almost zero particle sedimentation during January-February 1990, which indicates great variability in particle flux between years. The trap site PF1 within the Polar Front zone (Fig. 1) is located in an area of strong water movement and strong seasonal variability. The total annual sedimentation at this site to a water depth of 700 m is 38.3 g m -2 (Table 2c). Biogenic opal contributes 40%, organic carbon 7.5% and carbonate 26.5%, respectively (Table 3). In comparison with the other sites, opal content is smaller and carbonate content is higher. Primary production and export flux estimates
Organic carbon fluxes give an approximate indication of the primary production in surface waters (Suess, 1980; Betzer et al., 1984; Pace et al., 1987).
606
G. WEFER AND G. FISCHER
The primary production and export flux estimates, using particle flux data of the sites analysed, are shown in Table 4. In Table 5 we calculated the primary production and carbon export flux at 100 m for the Southern Ocean using our estimates derived from sediment trap deployments and the geographical zones and areas of Treguer and van Bennekom (1991). Highest values ( 173 gem -2 of annual primary production and 30 g Crn "? of annual export production) were calculated for the coastal area (Bransfield Strait, trap site KG 1; 1983-1984). However, much lower primary production rates ( 10-32 gem - 2 year- 1 ) were recorded for the other 2 years at this site, resulting in an average value of about 90 gem -2 year- 1 (16 gem - 2 year- I of export production) for the 3 years (KGI-KG3). However, it must be emphasized that the use of the primary production equations are problematic when swarmers, e.g. krill, feed on phytoplankton (von Bodungen, 1989), as observed in the Bransfield Strait (von Bodungen et al., 1987). Hence, particle sedimentation is also dependent on the size and frequency of such swarmers (von Bodungen, 1989) and may not be related to surface productivity alone. This may be one reason for the large fluctuations of the organic carbon flux rates in the King George Basin. Another problem is that because of rapid sinking of the large krill strings, the efficiency of pelagic regeneration is reduced (von Bodungen, 1989). Thus, relatively more carbon than expected may sink through the water column. From this coastal region ofthe Antarctic Peninsula, numerous primary production data, mostly from the austral summer season, are available (BurkTABLE 4 Estimation of annual primary production and organic carbon export flux (water depth of 100 m) using the equations of Berger et al. (1988) and Martin et al. (1987) and organic carbon flux rates derived from sediment trap experiments Location
Bransfield Strait (KG I ) Bransfield Strait (KG2) Bransfield Strait (KG3) Northern Weddell Sea (WS 1) Maud Rise (WS2) Maud Rise (WS3) Maud Rise (WS4) Polar Front (PFI )
Water depth (m)
Annual flux (g C m- 2 )
Calculated flux at 100 m (gCm- 2 )
Calculated primary production (g C m- 2 )
494
7.7
30.32
173.4
693
0.35
1.84
10.13
687
1.1
5.75
31.66
0.133 4.42 6.84 0.47 15.19
0.71 12.31 39.84 2.74 83.42
863 4456 360 352 700
0.021 0.17 2.28 0.16 2.86
607
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
TABLE 5 Estimation of annual primary production and export flux for the entire Southern Ocean Area
PFZ POO and SIZ
Surface 106 km 2
3
30
PSIZ
4
Coastal
0.9
Total
37.9
Annual primary production (g C m- 1)
Annual C flux at 100 m (gC m- 1)
Range
Range
Average
3-40
21.5
0 10-170
0.47-6.84
3.6
0 90
2-30
C flux at 100m (g C 1011)
Average
15
83
Primary production (g C 1012 )
16
294
45
645
l08
0
0
8\
14
975
167
Geographical zones and areas are taken from Treguer and van Bennekom (1991 ). PFZ, Polar Front Zone; POO and SIZ, Permanent Open Ocean and Seasonal Sea-Ice Zone; PSIZ, Permanent Sea-Ice Zone; Coastal. Coastal and Shelf Zone. In the PSIZ, sedimentation of organic carbon is almost zero, as shown by the flux experiments. Nevertheless, data from U. Bathmann (personal communication, 1990) indicate some primary production in the surface water and under the sea ice.
holder and Mandelli, 1965: 0.04-0.646 g C m- 2 day-I; von Bodungen et aI., 1986: 0.23-1.66 g C m- 2 day " "). Nevertheless, these data are difficult to extrapolate because of uncertainties in the blooms' seasonal duration. According to findings obtained from sediment trap experiments (Wefer et aI., 1990), we use an average productivity duration of only 1.5 months for the Bransfield Strait area. This would result in primary production values between 15 and 42 gem - 2 year- I, which are somewhat lower than the value estimated above. A compilation of all available data by von Bodungen (1989) resulted in an estimation of total annual production of95-130 gem -2 yeac l for the Bransfield Strait area (von Bodungen, 1989). This value is somewhat higher than our average rate of about 90 gem - 2 year- J. At a coastal site in the southeastern Weddell Sea (KN 1, Kapp Norvegia; Bathmann et al., 1991), organic carbon flux at 250 m water depth was 1.9 g m -2 for the investigation period of 53.5 days (4 January-27 February 1988). Sedimentation of organic matter was dominated by zooplankton faeces. Assuming that this measured flux represents most of the annual sedimentation, an annual primary production rate of the order of 24 gem -2 and a carbon flux of 4.2 g m -2 to 100 m is estimated. We regard these values as the lowest possible estimate. They are comparable with values at the coastal sites in the Bransfield Strait (e.g. KG2 and KG3) and with those of the Maud Rise area (Permanent Open Ocean (POO) and Seasonal Sea-Ice Zone (SIZ); Tables 4 and 5). For the coastal zone of the Atlantic Sector, Hempel (1980) has given
608
G. WEFER AND G. FISCHER
an estimate of 50 gem -2 year- I, which is comparable with our estimates for the coastal areas. By means of seasonal nitrate depletion, von Bodungen et al. (1988) calculated a new production of26 and 33 g C m- 2 in January-February 1985 for the southeastern Weddell Sea. This value is much higher than the organic carbon flux to 100 m water depth calculated for the Kapp Norvegia site. On the other hand, the organic carbon flux for the investigation period was only 2.4 g m- 2 to a depth of 100 m (von Bodungen et al., 1988), which is in the range of the flux measured by Bathmann et al. (1991). One explanation for this discrepancy is that nitrate-based new production may be considerably higher than the organic carbon export at this site. One must also take into account that the trap collections by von Bodungen et al. (1988) may have underestimated vertical flux because of high current velocities at the Kapp Norvegia site which sometimes reach 40 em S-l (Bathmann et al., 1991). Similarly high primary production (83 g C m -2 year- I) and export flux rates (15.2 g C m- 2 year- I ) to those in the Bransfield Strait were determined for the Polar Front Zone (PF 1; Tables 2c and 4). Primary production measurements over longer periods of time are not yet available, but chlorophyll a peaks of up to 1.5 mg m -3 were observed at this site (Lutjeharms et al., 1985). The Polar Front area is known to be highly productive, as is also shown by elevated biogenic silica and organic carbon accumulation rates in the underlying sediments (DeMaster, 1981). Our annual estimate for the biogenic silica flux at the PF1 site to a depth of 700 m is around 15 g m -2 (Table 4). For the abyssal zone of the Weddell Sea, van Bennekom et al, (1988) estimated a production of 50 g Si0 2 m -2 year- 1• Direct measurements by Tsunogai et ai. (1986), covering the main production period, give a value of 36 g Si0 2 m- 2 year- I. Assuming that only a third to a half of the net surface silica production may be transported to the sediment (Nelson and Gordon, 1982; van Bennekom et al., 1988), our opal flux appears to be reasonable. On the other hand, in frontal systems, biogenic silica production may be enhanced (van Bennekom et al., 1988) and up to two-thirds of total produced Si0 2 may reach the sediment (van Bennekom et al., 1988). Thus, our silica flux value must be regarded as a low estimate. As an example for the POO and SIZ, we use the data from the deployments west of Maud Rise. Calculated annual primary production varies between 3 and 40 g C m - 2, and annual organic carbon flux at 100 m is between 0.5 and 6.8 g C m- 2 (Table 4). These data are within the range of primary production rates determined by Jennings et al. (1984). For the eastern Weddell Sea, Jennings et aI. (1984) used nutrient depletion for the calculation of average primary production, which they estimated at 0.22-0.42 gem -2 day-l in spring. To assume the lower value for a 90-day period would result in a minimum production of 20 g C m", which is the average annual primary production calculated from our trap data (Table 5). This value is also in agreement with
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERNOCEAN
609
average production estimates of 16 gem-2 year- 1 for the pelagic zone (HolmHansen et al. (1977». From SiOj- depletion in the mixed layer, van Bennekom et al. (1988) calculated a biogenic silica production of 85 g m -2 year- 1 for a site at 64 ° S, 5°E. This elevated Si0 2 production could be related to the presence of the Maud Rise polynya (van Bennekom et al., 1988). The biogenic silica fluxes derived from our trap data also indicate enhanced production near the Maud Rise (Table 3), although the values (1.7-24 g m -2 year- I) are smaller than those of van Bennekom et al. (1988). It is assumed that a third to a half of surface silica production is transported to the deeper waters. We have to assume extremely low primary production rates in the area with permanent sea-ice cover. The record from all deployments in the Weddell Sea (Fischer et al., 1988; Wefer et al., 1990, and Fig. 2) shows almost zero sedimentation during sea-ice coverage. Our assumption that primary production is extremely low in ice-covered areas is supported by sediment trap data from the Arctic Ocean. In the Greenland Basin (Honjo et al., 1987) and the Fram Strait (Hebbeln and Wefer, 1991), particle flux was also almost zero during closed sea-ice conditions. Nevertheless, it must be emphasized that there is some primary production in the surface waters of ice-covered areas (0. Bathmann, personal communication, 1990), but the export of carbon appears to be almost zero as indicated by the trap data. Therefore we used a primary production value of zero in our areal calculation (Table 5). Because of the large differences in primary production rates, the Weddell Sea was subdivided into a northern/central province with lower production (oceanic domain: 0.1-0.23 g C m- 2 day-I) and a southern/southeastern province characterized by elevated rates (coastal/shelf domain: 0.4-0.7 g C m- 2 day"; El-Sayed and Taguchi, 1981; von Brockel, 1985; von Bodungen et al., 1988). These findings, based on 14C uptake measurements, are in agreement with the results presented above and confirm the pattern of the productivity subsystems used for the Weddell Sea. The regional variability of primary production in the Southern Ocean is also reflected in the 210Pb inventories (Rutgers van der Loeff and Berger, 1991): the Polar Front and the Bransfield Strait area are characterized by high production rates and the northern Weddell Sea by very low rates. In Table 5, annual primary production rates for the Southern Ocean and the fluxes at 100 in are given. Using these data, annual average primary production is around 26 g C m- 2 and the average carbon flux to 100 m is 4.4 g C m - 2. This flux estimate to 100 m is 10 times lower than model-derived new production rates of Keir (1988), but is comparable with data from Bolin et al. (1983; 3.3 gem -2 year- I) and Broecker and Peng (1986; 9.9 g C m- 2 year- 1 ). We believe that the reason for the differences between our calculations and other productivity calculations is an overestimation of the duration of the
610
G. WEFER AND G. FISCHER
growth period. To transform daily rates into annual rates, growth periods of 5 months were sometimes considered (e.g. Treguer and von Bennekom, 1991 ). Our flux data indicate a growth period of about 1 month and less (Bransfield Strait; see Wefer et al., 1990) to a maximum of 3 months (Maud Rise, Fig. 2). Although most of the trap positions are ice free for about 5-6 months, the period of elevated primary production appears to be shorter. The following physical and biological factors may be responsible for this (discussed by von Bodungen et al., 1988): (1) the transient nature ofthe shallowwater mixed layer in Antarctic waters, (2) horizontal advection processes, and (3) grazing by zooplankton. The values in Table 5 show the importance of relatively small areas with high productivities and fluxes for the whole Southern Ocean. Although the Coastal and the Polar Front Zones occupy only 10% of the Southern Ocean surface area, about 40% of organic carbon is synthesized there, according to our calculations. In contrast, large permanent sea-ice closed oceanic domains supply much smaller amounts of organic carbon to the sea-floor and are comparable with oligotrophic central gyres. The Maud Rise, an area with upwelling and entrainment of warm deep water and a recurring polynya, is characterized by intermediate annual primary production and flux rates. Summing the annual production rates from the various subsystems would result in a primary production in the order of 1X 10 15 g C ( 1X 109 tons) year- , for the entire Antarctic Ocean (Table 5). The organic carbon flux to 100 m water depth is about 0.17 X 109 tons year- '. These values are three to four times lower than productivity data of Koblents-Mishke et al. (1970) and Berger et al. (1987), obtained from 14C measurements and different algorithms. Unfortunately, no flux data are available from sites near the Filchner-Ronne or the Ross Shelf, where primary production and the export flux may be considerably higher, as indicated by satellite images. According to El-Sayed et al. (1983), primary production at the Ross Ice Shelf locally reached 1 g m- 2 day-I. Similarly high values were obtained by Wilson et al. (1986) in an iceedge bloom in the western Ross Sea. In addition, substantial deposits of diatom-rich sediments occur in this area (DeMaster, 1981), indicating high surface productivity. Thus, our calculated coastal primary production and the export flux may be underestimated. These calculations have to be improved with data from sediment trap experiments from those sites. CONCLUSIONS
(1) The Antarctic Ocean is divided into various subsystems, characterized by different primary production and organic carbon flux rates which were estimated by means of flux data from long-term sediment trap deployments. (2) The calculations show that primary productivity and export fluxes are
611
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
discontinuous in space and time. High flux rates were measured only in short episodes (1-3 months) during and after high production periods. (3) The flux data give indications that primary production and organic carbon flux are almost zero in the Permanent Sea-Ice Zone and during winter. The Coastal and Shelf Zone and the Polar Front Zone show highest annual production (83-170 g C m -2) and flux rates. Intermediate production rates, of the order of 20 g C m - 2 year- I, were obtained for the Maud Rise area. (4) The calculated average annual primary production in the Southern Ocean is around 26 g C m -2, and the average organic carbon flux rate is 4.4 g C m- 2 • This carbon flux estimate is about 10 times lower than a model-derived value for new production (42 g C m -2 year :") applied by Keir (1988), but is largely consistent with data obtained by Bolin et al. (1983) and Broecker and Peng (1986). (5) The total annual primary production for the entire Southern Ocean is estimated as 1X 109 tons, and total annual organic carbon flux is 0.17 x i 09 tons. The primary production value is three to four times lower than previously reported data for the Antarctic Ocean. ACKNOWLEDGEMENTS
We thank officers and crew of R/V "Polarstern" for their help during the fieldwork. We are further indebted to G. Ruhland and A. Wiilbers for deploying the mooring arrays equipped with the sediment traps. The investigations on particle flux in Antarctic waters were carried out in co-operation with R. Gersonde and D. Futterer from the Alfred Wegener Institute of Polar and Marine Research, Bremerhaven. W.H. Berger and U. Bathmann made valuable suggestions to improve the manuscript. This research was funded by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 261 at Bremen University, Contribution 25).
REFERENCES Bathmann, U., Fischer, G., MUller, P.J. and Gerder, D., 1991. Short-term variations in particulate matter sedimentation off Kapp Norvegia, Weddell Sea, Antarctica: relation to water mass advection, ice cover, plankton biomass and feeding activity. Polar Biol., 11: 185-195. Berger, W.H., Fischer, K., Lai, C. and Wu, G., 1987. Ocean productivity and organic carbon flux. 1. Overview and maps of primary production. University of California, San Diego, SIO (Scripps Institute of Oceanography) , Ref. 87-30. Berger, W.H., Smetacek, V.S. and Wefer, G., 1989. Ocean productivity and paleoproductivityan overview. In: W.H. Berger, V. Smetacek and G. Wefer (Editors), Productivity of the Ocean: Present and Past. Wiley, New York, pp. 1-34. Betzer, P.R., Showers, W.J., Laws, EA, Winn, C.D., DiTullio, G.R. and Kroopnick, P.M., 1984. Primary productivity and particle fluxes on a transect of the equator at 153 in the Pacific Ocean. Deep-Sea Res., 31: I-II. 0
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Bolin, B., Bjorkstrom, A., Holmen, K. and Moore, B., 1983. The simultaneous use of tracers for ocean circulation studies. Tellus, 35B: 206-236. Broecker, W.S. and Peng, T.-H., 1982. Tracers in the Sea. Eldgo Press, Palisades, NY, 690 pp. Broecker, W.S. and Peng, T.-H., 1986. Glacial to interglacial changes in the operation of the global carbon cycle. Radiocarbon, 28: 309-327. Burkholder, P.R. and Mandelli, E.F., 1965. Carbon assimilation of marine phytoplankton in Antarctica. Proc. Nat. Acad. Sci. U.S.A., 54: 437-444. Comiso, J.e. and Gordon, A.L., 1987. Recurring polynyas over the Cosmonaut Sea and the Maud Rise. J. Geophys, Res., 92 (C3): 2819-2833. DeMaster, D.J., 1981. The supply and accumulation of silica in the marine environment. Geochim. Cosmochim. Acta, 45: 1715-1732. El-Sayed, S.Z. and Taguchi, S., 1981. Primary production and standing crop of phytoplankton along the ice-edge in the Weddell Sea. Deep-Sea Res., 28: 1017-1032. El-Sayed, S.Z., Biggs, D.C. and Holm-Hansen, 0., 1983. Phytoplankton standing crop, primary productivity, and near-surface nitrogenous nutrient fields in the Ross Sea, Antarctica. DeepSea Res., 30: 871-886. Eppley, R.W. and Peterson, BJ., 1979. Particulate organic matter flux and planktonic new production in the deep ocean. Nature, 282: 677-680. Fischer, G., Fiitterer, D., Gersonde, R., Honjo, S., Ostermann, D. and Wefer, G., 1988. Seasonal variability of particle flux in the Weddell Sea and its relation to ice cover. Nature, 335: 426428. Gordon, A.L., 1982. Weddell deep water. J. Mar. Res., 40: 199-217. Gordon, A.L. and Huber, B.A., 1984. Thermohaline stratification below the Southern Ocean sea ice. l Geophys. Res., 89 (C I): 641-648. Hebbeln, D. and Wefer, G., 1991. Effects of ice coverage and ice-rafted material on sedimentation in the Fram Strait. Nature, 350: 409-411. Hempel, G., 1980. Antarctic. In: Fish Resources of the Ocean. FAO Fish. Tech. Rep., 97: 197203. Holm-Hansen, 0., El-Sayed, S.Z., Franceschini, G.S. and Cuhel, R.L., 1977. Primary production and factors controlling growth in the Southern Ocean. In: G. Llano (Editor), Adaptations within Antarctic Ecosystems. Gulf, Houston, TX, pp. II-50. Honjo, S., Manganini, SJ., Karowe, A. and Woodward, B.L., 1987. Particle fluxes, northeastern Nordic Seas 1983-1986. Nordic Seas Sedimentation Data File, Vol. I. Woods Hole Tech. Rep., 87-17: 1-84. Jacques, G., 1989. Primary production in the open Antarctic Ocean during the austral summer. A review. Vie Milieu, 39: 1-17. Jennings, lC., Gordon, L.I. and Nelson, D.M., 1984. Nutrient depletion indicates high primary productivity in the Weddell Sea. Nature, 309: 51-54. Keir, R.S., 1988. On the Late Pleistocene ocean geochemistry and circulation. Paleoceanography, 3: 413-445. Knox, F. and McElroy, B., 1984. Changes in atmospheric CO 2 : influence of the marine biota at high latitude. J. Geophys, Res., 89: 4629-4637. Koblents-Mishke, 0.1., Volkovinsky, V.V. and Kabanova, Yu.G., 1970. Plankton primary production of the World Ocean. In: W. Wooster (Editor), Scientific Exploration of the South Pacific. National Academy of Sciences, Washington DC, pp. 183-193. Lutjeharms, l.R.E., Walters, N.M. and Allanson, B.R., 1985. Oceanic frontal systems and biological enhancement. In: W.R. Siegfried, P.R. Condy and R.M. Laws (Editors), Antarctic Nutrient Cycles and Food Webs. Springer, Berlin, pp. 11-20. Martin, J.H., Knauer, G.A., Karl, n.M. and Broenkow, W.W., 1987. VERTEX: carbon cycling in the northeast Pacific. Deep-Sea Res., 34: 267-285.
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Nelson, D.M. and Gordon, L.I., 1982. Production and pelagic dissolution of biogenic silica in the Southern Ocean. Geochim. Cosmochirn. Acta, 46: 491-50 I. Pace, M.L., Knauer, G.A., Karl, D.M. and Martin, J.H., 1987. Primary production, new production and vertical flux in the eastern Pacific Ocean. Nature, 325: 803-804. Rutgers van del' Loeff M.M. and Berger, G.W., 1991. Scavenging and particle flux: seasonal and regional variations in the Southern Ocean (Atlantic sector). Mar. Chem., 35: 553-567. Sarmiento, J.L. and Toggweiler, J.R., 1984. A new model for the role of the oceans in determining atmospheric pC0 2 • Nature, 308: 621-624. Siegenthaler, U. and Wenk, T, 1984. Rapid atmospheric CO 2 variations and ocean circulation. Nature, 308: 624-626. Smith, W.O., Jr. and Nelson, D.M., 1986. Importance of the ice edge: phytoplankton production in the Southern Ocean. BioScience, 36: 251-257. Suess, E., 1980. Particulate organic carbon flux in the oceans-surface productivity and oxygen utilization. Nature, 288: 260-263. Sundquist, E.T. and Broecker, W.S., 1985. The carbon cycle and atmospheric CO 2: natural variations Archean to Present. Geophys. Monogr. Am. Geophys. Union, 32, 627 pp. Treguer, P. and Van Bennekom, A.J" 1991. The annual production of biogenic silica in the Antarctic Ocean. Mar. Chern., 35: 477-487. Tsunogai, S., Shinichiso, N., Harada, K., Kurosaki, T, Watanabe, Y. and Maedaa, M., 1986. Large but variable particle flux in the Antarctic Ocean and its significance for the chemistry of Antarctic water. J. Oceano!. Soc. J pn., 42: 83-90. Van Bennekom, A.J., Berger, G.W., Van del' Gaast, S.J. and De Vries, R.TP., 1988. Primary productivity and the silica cycle in the Southern Ocean (Atlantic Sector). Palaeogeogr. Palaeoclirnatol. Palaeoecol., 67: 19-30. Von Bodungen, B., 1989. Pelagische Primarproduktion und vertikaJer Partikelfluss im Ozean. Habilitationsschrift, University of Kiel, 156 pp. Von Bodungen, B., Smetacek, V.S., Tilzer, M.M. and Zeitzschel, B., 1986. Primary production and sedimentation during spring in the Antarctic Peninsula region. Deep-Sea Res., 33: 177194. Von Bodungen, B., Fischer, G., Nothig, E.-M. and Wefer, G., 1987. Sedimentation of krill faeces during spring development of phytoplankton in the Bransfield Strait. In: E.T Degens, S. Honjo and E. Izdar (Editors), Particle Flux in the Ocean. Mitt. Geol-Palaontol. Inst. Univ. Hamburg, SCOPE UNEP Sonderband, 62: 243-257. Von Bodungen, B., Nothig, E.-M. and Sui, Q., 1988. New production of phytoplankton and sedimentation during summer 1985 in the south eastern Weddell Sea. Compo Biochem. Physio!., 90B: 475-487. Von Brockel, K., 1985. Primary production data from the south-eastern Weddell Sea. Polar Bio!., 4: 75-80. Wefer, G., Fischer, G., Fiitterer, D. and Gersonde, R., 1988. Seasonal particle flux in the Bransfield Strait, Antarctica. Deep-Sea Res., 35: 891-898. Wefer, G., Fischer, G., Filtterer, D., Gersonde, R., Honjo, S. and Ostermann, D., 1990. Particle sedimentation and productivity in Antarctic waters of the Atlantic Sector. In: U. Bleil and J. Thiede (Editors), Geological History of the Polar Oceans: Arctic versus Antarctic. Kluwer Academic, Dordrecht, pp. 363-379. Wilson, D.L., Smith, W.O., Jr., and Nelson, D.M., 1986. Phytoplankton bloom dynamics ofthe western Ross Sea ice edge-I. Primary productivity and species-specific production. DeepSea Res., 33: 1375-1387.
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Annual primary production and export flux in the Southern Ocean from sediment trap data
Gerold Wefer and Gerhard Fischer Fachbereich Geowissenschaften, Universitiii Bremen. Klagenfurter StrafJe. D-2800 Bremen 33. FRG (Received I September 1990; accepted 14 February 1991)
ABSTRACT Wefer, G. and Fischer, G., 1991. Annual primary production and export flux in the Southern Ocean from sediment trap data. Mar. Chem., 35: 597-613.
Since 1983 time-series traps have been deployed in the Atlantic sector of the Southern Ocean to measure the flux of organic carbon, biogenic silica and carbonate. The organic carbon flux data are used to calculate primary production rates and organic carbon fluxes at 100 m water depth. From these calculations, annual primary production rates range from about 170 g C m -2 in the coastal area (Bransfield Strait) to almost zero in the Permanent Sea-Ice Zone. High rates (of about 80 g C m- 2 vear:") were calculated for the Polar Front Zone and rather low values (about 20 g C m- 2 year-I) characterize the Maud Rise area. The estimated primary production for the entire Southern Ocean (south of 50° S), using various subsystems with characteristic carbon fluxes, is in the order of I X I09tons year- I: the organic carbon flux out of the photic layer is 0.17 X I09tons vearr '. Our calculation of the Southern Ocean total annual primary production is substantially lowerthan previously reported values.
INTRODUCTION
The flux of organic particles out of the photic layer is a central element of the cycling of carbon and associated biogenic elements in the ocean (Eppley and Peterson, 1979; Suess, 1980; Broecker and Peng, 1982; Sundquist and Broecker, 1985; Berger et al., 1989). Sedimentation of organic carbon can play an important role in changing the CO 2 content of the atmosphere. Photosynthesis and the export of organic carbon from the photic zone as particulate organic carbon or dissolved organic carbon acts as a 'biological pump', which lowers the partial pressure of CO 2 in surface waters (e.g. Berger et al., 1989). The atmospheric CO 2 content orients itself mainly on the pC0 2 of the surface waters of the global ocean. The Southern Ocean (south of 50 S) occupies a position of special interest in this context. In this area, primary production is generally not restricted to 0
0304-4203/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
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the availability of nutrients. Therefore, the Southern Ocean has a high potential to extract CO 2 from the atmosphere if primary production and organic flux out of the photic zone increase (Knox and McElroy, 1984; Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984). To estimate the role of the Southern Ocean in the global carbon cycle today, primary production rates and the carbon flux out of the photic zone (export production) have to be known. Because of logistical problems, direct measurements of annual primary production over the whole year are difficult or almost impossible. Depending on the methods and the times of measurements, the published results show a high spatial and temporal variability of primary production (El-Sayed and Taguchi, 1981; Jennings et al., 1984; Smith and Nelson, 1986; Jacques, 1989) and organic carbon fluxes in the Southern Ocean (Wefer et al., 1990). First estimates of primary production and organic carbon sinking out of the euphotic zone are possible using annual organic carbon flux rates derived from sediment trap deployments. Since 1983 we have been deciphering the _10·
10·
20·
40·
,
50·
\ \
\ \ \
60·
70· 80·
Fig. 1. Locations of the mooring sites and minimal (February 1985; stippled line) and maximal (September 1985. dashed line) sea-ice coverage. The trap sites are typical for the following subsystems (zones): KGI-KG3: Coastal Zone; WSl: Permanent Sea-Ice Zone (PSIZ); WS3 and WS4: Permanent Open Ocean and Seasonal Sea-ice Zone (POO and SIZ): PFl: Polar Front Zone CPFZ).
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PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
TABLE 1 Locations and details of trap deployments Location
Mooring name
Bransfield Strai t 62' 15.4'S. 57'31.7'W 62'20.1 '5. 57'28.3'W 62°22.0'5. 57 Northern Weddell Sea 62°26.5'5. 34°45.5'W Maud Rise 64° 55.0'S. 02 0 3 0 . 0 ' W 64° 54.1 'So02' 33.8' W 64° 55.5'5.02 0 3 5 . 5 ' W Polar Front 50 0 0 9 . 0 ' S , 0 5 ' 4 6 . 4 ' E Q49.9'W
Water depth (m)
Trap depth
KGI
1952
KG2 KG3
1650 1992
494 1588 693 687
4.12.84-13.11.85 26.11.85-07.05.86
W51
3880
863
25.0 J.85-J 9.03.86
WS2 WS3 WS4
5000 5053 5044
4456 360 352
20.01.87-20.11.87 16.01.88-04.02.89 03.03.89-26.02.90
PFI
3750
700
15.01.87-12.03.88
Deployment time
(m) 1.12.83-25.11.84
seasonal sedimentation in the Southern Ocean with time-series sediment traps deployed in the area of the Bransfield Strait (KG1, KG2 and KG3), the northern Weddell Sea (WSl), west of Maud Rise (WS2, WS3 and WS4) and in the Polar Front (PFl) area (Fig. 1, Table 1). The flux data in part were reported elsewhere (Fischer et al., 1988; Wefer et al., 1988, 1990). Here we report seasonal flux data from two deployments west ofthe Maud Rise (WS3 and WS4) and annual flux rates from the Polar Front area (PF1). With these data, we calculated primary production and export carbon fluxes (water depth of 100 m) using empirical relationships between primary production and organic flux rates (Suess, 1980; Betzer et al., 1984; Pace et al., 1987; Berger et al., 1987) and the organic carbon flux-water depth relationship of Martin et al. (1987). The result is that annual primary production varies between 0.7 g C m -2 in the open ocean area and more than 170 g C m -2 in the coastal zone. The organic carbon flux at 100 m ranges from almost zero in the Northern Weddell Sea to 30 g C m -2 year- I in the Bransfield Strait. Total primary production and export production for the entire Southern Ocean were estimated using different Antarctic subsystems (Treguer and van Bennekom, 1991) with characteristic carbon fluxes derived from our trap experiments. The total primary production for the entire Southern Ocean is about 1X 109 tons year- I, and the organic carbon flux to 100 m water depth (export production) is less than one-fifth. FIELDWORK AND LABORATORY ANALYSIS
Table 1 shows the locations and deployment times of the sediment trap experiments. Before deployment, HgCl 2 was added to the seawater of the col-
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lecting bottles to minimize bacterial degradation; some dissolved HgCh was still present after recovery. On recovery, a very small amount of the trap material was removed for the preparation of scanning electron microscopy (SEM) slides and for inspection on board using a transmission light microscope. Subsequently, the trap material was wet-sieved with a 1 mm screen to remove larger swimmers and split into aliquots by a rotary liquid splitter. Generally we used a quarter-split from each sample for this study. The remainder was retained for detailed analysis of the organic chemistry and the micropalaeontological constituents. We obtained total fluxes from quarter-splits (smaller than I mm) which were desalted by means of repeated short washings, freeze-dried at 30°C and weighed. A portion of this material was decalcified with 2 N HCI, dried and measured in a Heraeus CHN Analyser for organic carbon. No filtration step was involved. Carbonate contents were measured with the CHN Analyser using samples which had not been treated with HCI before analysis. Carbonate is calculated as follows: (ClOtal-Corg) X 8.33.
Biogenic opal was determined using a modified sequential leaching technique (DeMaster, 1981; MUller and Schneider, personal communication, 1990). The sample was extracted with 1 M NaOH at 85°C under stirring in a stainless steel vessel. The increase in dissolved silica is simultaneously recorded by continuous flow analysis (Molybdate Blue method). A typical extraction curve for sediment trap particles is characterized by a rapid initial silica increase, as a result of the dissolution of opal particles, followed by a much slower increase representing the dissolution of silicate minerals (e.g. clay minerals). The y-intercept value obtained by linear extrapolation to an extraction time of zero gives the opal content of the sample. To calculate primary production from annual organic carbon flux data we used the equation of Berger et al. (1988):
J= 17XPP /z+ PP /100
(1)
which becomes
PP=J/ (17 /z+ 1/100)
(2)
where J is the flux at any depth, PP is the primary production (in g C m- 2 year- I) and z is the water depth (in metres). It must be emphasized that this equation and also other carbon flux-primary production relationships originate largely from the extrapolations of short-time productivity and particulate organic carbon (POC) flux measurements to annual averages (von Bodungen, 1989). The productivity-POC flux equations further assume that the proportions of new production of primary production (fratio) are relatively constant in all regions of the World Ocean. This is not true, as was demon-
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
601
strated by Eppley and Peterson ( 1979). Especially for the pulsed productivity in high latitudes a nonlinearity between the export flux and primary productivity can be expected (Berger et al., 1987). In the southeastern Weddell Sea, highly variable POC and particulate organic nitrogen (PON) flux rates may be due to changing mixing depths and, hence, to a seasonally variable .fratio (von Bodungen, 1989). Nevertheless, the average POC flux measured may correspond to the average primary productivity. To calculate the organic carbon flux out of the euphotic zone (below 100 m), the equation of Martin et al. (1987) is applied: F= FIQO X (zjlOO)b
(3)
which becomes F lQo= F j ( zj 100 )b
(4)
where b (log-log slope) is -0.858 (Martin et al., 1987), Fis the flux at any depth, F lOo is the flux at 100 m and z is the depth in metres. RESULTS AND DISCUSSION
Annual sedimentation patterns Data on deployments and annual flux rates (per m") of total mass, organic carbon, carbonate and biogenic silica for the two Maud Rise deployments (WS3 and WS4) and the annual flux rates for the Polar Front site (PF1) are given in Table 2a-2c. The seasonal patterns of total mass and organic carbon sedimentation for the two Maud Rise sites are shown in Fig. 2. Because of a malfunction of the trap at the Polar Front site, only annual flux rates are available. The annual patterns of the Bransfield Strait and Northern Weddell Sea sites have been given by Fischer et al. (1988) and Wefer et al. (1988, 1990). The main results are briefly reported here. In the Bransfield Strait (trap sites KG1-KG3; Fig. 1) particle flux is restricted to a surprisingly short period in austral summer (Wefer et al., 1988, 1990). During austral summer (December and January), total flux can be more than 1.5 g m -2 day-I, whereas during all other months flux was about 10-100 times lower. The flux of the most productive months or weeks can reach 97% of the total. Under conditions of high export production, faecal pellet transport is dominant. It is assumed that krill swarms feed on phytoplankton blooms and incorporate the small particles into their pellets. These pellets settle to the traps within days. For the other months of the year, downward transport mainly by settling of discrete small particles is assumed. The 3 year record also shows a large interannual variability. Total annual fluxes varied between 12 and 120 g m -2 (Table 3; Wefer et al., 1990). At the northern Weddell Sea site (trap site WS1; Fig. 1), the annual particle
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G. WEFER AND G. FISCHER
TABLE 2 Flux rates of total mass and individual biogenic components for traps WS3 (a), WS4 (b) and PF1 (c) (a) WS3 Sample no. I
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Sampling interval
16.01.88-01.02.88 01.02.88-17.02.88 17.02.88-04.03.88 04.03.88-20.03.88 20.03.88-05.04.88 05.04.88-21.04.88 21.04.88-06.05.88 06.05.88-23.05.88 23.05.88-09.06.88 09.06.88-11.07.88 11.07.88-12.08.88 12.08.88-13.09.88 13.09.88-15.10.88 15.10.88-31.10.88 31.10.88-16.11.88 16.11.88-02.12.88 02.12.88-18.12.88 18.12.88-03.01.89 03.01.89-19.01.89 19.01.89-04.02.89
Duration (days) 16 16 16 16 16 16 16 16 16 32 32 32 32 16 16 16 16 16 16 16
g m _2 (384 days)-I Annual estimate (g m -2)
Flux rates (mg m -2 day") Total
Opal
Corg
CaC0 3
96.5 424.0 647.5 332.5 202.3 121.0 36.8 113.5 12.8 15.9 13.3 5.6 4.4 15.3
5.1 30.0 41.8 21.9 9.4 5.5 2.4 5.8 1.0
43.8 24.2 53.5 15.6 15.4 9.1 2.1 7.3 1.9 1.4 1.6 0.5 0.9 0.0
0.5 1.8 93.0 17.0 15.8
38.8 298.9 472.6 215.5 171.1 89.3 22.8 97.2 7.8 8.8 7.7 3.0 1.4 0.5 n.d. n.d. n.d. 76.9 15.5 10.9
35.51 33.70
24.95 23.75
1.3
1.7
1.2 0.8 0.7 6.1 n.d. n.d. n.d. 4.8 3.0 1.4 2.36 2.28
n.d,
n.d. n.d. 0.4 1.2 0.3 2.94 2.76
(b) WS4 Sample no.
Sampling interval
Duration (days)
Flux rates (mg m- 2 dav") Total
I
2 3-20
03.03.89-21.03.89 21.03.89-08.04.89 08.04.89-26.02.90
18 18 18 (each)
g m "? (360 days)-I Annual estimate (g m -2)
30.4 102.9 n.d.
2.40 2.43
Opal 12.9 78.1 n.d. 1.64 1.66
Corg
CaC0 3
4.2 4.9 n.d.
3.7 1.6 n.d.
0.16 0.16
0.95 0.96
flux measured was the smallest yet observed in the World Ocean and showed extreme variability (Fischer et al., 1988). The variability of particle fluxes seemed to be due to spring thaw processes rather than to the occurrence of larger open seas. As observed in the Bransfield Strait, faecal pellet transport played an important role in the vertical transport of material. Most pellets,
603
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
TABLE 2 (continued) (c) PFI Sample no. I
2 3 4
Sampling interval
15.01.87-04.02.87 04.02.87-24.02.87 24.02.87-16.03.87 16.03.87-12.03.88
Duration (days) 20 20 20 361
g m- 2 (421 days)-' Annual estimate (g m
r
")
Flux rates (mg m -2 day-I) Total
Opal
Cerg
CaC0 3
268.2 192.9 229.0 84.3
79.4 42.6 95. 7 37.1
6.4 4.1 6.6 8.1
89.3 123.4 71.7 16.6
44.22 38.30
[ 7.74 15.30
3.33 2.86
11.68 10.10
n.d., not determined. Information on locations of mooring sites and deployment times is given in Fig. I and Table 1. The flux rates between 8 April 1989 and 26 February 1990 (samples 3-20, WS4) were almost zero. No particles could be identified in the sampling cups by eye. Thus, total flux rates were not determined.
which are of unknown origin, were about 0.5 mm in diameter and were slightly elongated. Cylindrical pellets produced by krill swarms were rarely found. The trap site west of the Maud Rise is in an area of a relatively small open ocean polynya located at the eastern margin of the Weddell Gyre. This Maud Rise polynya (65°S, 2 °E) was observed as a reduction of the sea-ice concentration during winter 1980 (Comiso and Gordon, 1987). The Maud Rise is a topographic high, reaching from about 5000 m to 2000 m below the sea surface. It is assumed that the formation of the Maud Rise open-ocean polynya is related to deep-reaching convection which introduces warm deep water into the surface layer, thus inhibiting sea-ice formation (Gordon, 1982; Gordon and Huber, 1984; Comiso and Gordon, 1987). This doming of warmer and saltier water and an early break-up and acceleration of the decay of the Weddell ice pack (Comiso and Gordon, 1987), should have an effect on the particle sedimentation. West of the Maud Rise (trap sites WS3 and WS4; Fig. 1), seasonal variability in the daily flux was smaller than at the Bransfield Strait and northern Weddell Sea sites. Total flux at WS3 ranged between 647 mg m -2 day" ', during austral summer months, when the mooring site was not ice covered, and 0.5 mg m -2 day-I, during austral winter months (Table 2a). Biogenic opal is dominant, accounting for about 70% of total flux. Most of the biogenic opal consists of diatom frustules and associated debris. In comparison with the other trap sites, carbonate content is high, sometimes reaching 45% (average about 8%). Preliminary microscopic examinations show that the planktonic foraminifera Neogloboquadrina pachyderma accounts for the entire calcium carbonate flux but other species such as Globigerinita glutinata and Turborotalita quinqueloba, adapted to warmer temperatures, are also present. These
604
G . WEFER AN D G. FISCHER
WS3/4 350 m,352 m 700 -r-----~~_:_-----.------------...,
WS3
a BOO -
~ 500~
v
~
400-
Ol
E ~ 300<;::
~
200-
r
100 -
...., o-LJ++-++++++f=1=l9=f=l-r,....\-+-I=f-i+++.... ..,-.,.,..,.,..,.,...,.,...,rr-r-r,,--j Sample No. I
sea-Ice coverage Qntenth)
El
15
10
sea-leeIree
5/10
kMN01lliill IMMI~ BD 7/10
9.5/10
to
10
20
sOB·lce".. 7110
B/l0
WS3
b
20
15
9.5110
8/10
WS4
60 ~
~
-0
C)l
50
E Ol
.§.
40
~
""c:
~
0 .~
30
20
~
0
10
0
I I
Somple NO.1
\6.1 4.3 21.4
15
10
9.6
15.\0 18.12 11188
10
20
30.8
3 .3 8.4
1889
15
20
26.2
1990
I
F ig. 2. Total ( a ) and organic car bon flux rates ( b ) west of Maud Rise (sites WS3 and WS4 ) . Locations of the sampling sites are shown in Fig. 1, deplo yment times are given in Table I, flux d at a are shown in Tabl e 2a and 2b.
605
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
TABLE 3 Annual flux rates (g m- 2 ) in the Southern Ocean (Atlantic sector) and percentages of total mass flux of individual biogenic components Trap site (depth) Bransfield Strait (494 m) 1983-1984 (693m) 1984-1985 (687 m) 1985-1986 Northern Weddell Sea (863 rn ) Maud Rise (4556m) 1987 (360 m) 1988-1989 (352 m ) 1989-1990 Polar Front (700m)
Total mass
Opal
%af total
Cors
%of total
Carbon
%of total
120 11.9 36.6
54.8 3.5 18.3
45.7 29.6 50.0
7.7 0.35 I.!
6.7 2.9 3.0
4.3 n.d. n.d.
3.6 n.d, n.d.
79.0
0.021
5.7
0.011
3.0
1.0 2.8 0.09
12.7 8.2 3.7
0.371
0.293
7.9 33.7 2.43
4.0 23.8 1.66
50.6 70.4 68.3
0.17 2.28 0.16
2.2 6.8 6.6
38.3
15.3
40.0
2.86
7.5
10.1
26.5
data confirm the assumption that productivity is increased in the open ocean polynya at the Maud Rise area. In comparison with the open ocean site WS1 in the northern Weddell Sea, the flux rates and the content of organic carbon are much higher at the Maud Rise site. Similar to the Bransfield Strait, a large interannual variability is observed (Fig. 2). Total mass flux in 1988-1989 was 33.7 g m- 2 year-I compared with 2.43 g m- 2 year- I in 1989-1990 (Table 2a and 2b). However, it has to be taken into account that we might have missed a period of higher sedimentation rates in February 1989. Because of the ship's schedule the sampling period was interrupted between 4 February and 3 March (Fig. 2). As is shown in Fig. 2, no peak in mass flux was found during January-February 1990, as observed for the years 1988 and 1989. Thus, one might conclude that the WS4 trap did not work properly. The sediment trap controller data and the fact that the sample collector was at zero position after recovery, give no indication of any malfunctions of the trap. Hence, we have to assume almost zero particle sedimentation during January-February 1990, which indicates great variability in particle flux between years. The trap site PF1 within the Polar Front zone (Fig. 1) is located in an area of strong water movement and strong seasonal variability. The total annual sedimentation at this site to a water depth of 700 m is 38.3 g m -2 (Table 2c). Biogenic opal contributes 40%, organic carbon 7.5% and carbonate 26.5%, respectively (Table 3). In comparison with the other sites, opal content is smaller and carbonate content is higher. Primary production and export flux estimates
Organic carbon fluxes give an approximate indication of the primary production in surface waters (Suess, 1980; Betzer et al., 1984; Pace et al., 1987).
606
G. WEFER AND G. FISCHER
The primary production and export flux estimates, using particle flux data of the sites analysed, are shown in Table 4. In Table 5 we calculated the primary production and carbon export flux at 100 m for the Southern Ocean using our estimates derived from sediment trap deployments and the geographical zones and areas of Treguer and van Bennekom (1991). Highest values ( 173 gem -2 of annual primary production and 30 g Crn "? of annual export production) were calculated for the coastal area (Bransfield Strait, trap site KG 1; 1983-1984). However, much lower primary production rates ( 10-32 gem - 2 year- 1 ) were recorded for the other 2 years at this site, resulting in an average value of about 90 gem -2 year- 1 (16 gem - 2 year- I of export production) for the 3 years (KGI-KG3). However, it must be emphasized that the use of the primary production equations are problematic when swarmers, e.g. krill, feed on phytoplankton (von Bodungen, 1989), as observed in the Bransfield Strait (von Bodungen et al., 1987). Hence, particle sedimentation is also dependent on the size and frequency of such swarmers (von Bodungen, 1989) and may not be related to surface productivity alone. This may be one reason for the large fluctuations of the organic carbon flux rates in the King George Basin. Another problem is that because of rapid sinking of the large krill strings, the efficiency of pelagic regeneration is reduced (von Bodungen, 1989). Thus, relatively more carbon than expected may sink through the water column. From this coastal region ofthe Antarctic Peninsula, numerous primary production data, mostly from the austral summer season, are available (BurkTABLE 4 Estimation of annual primary production and organic carbon export flux (water depth of 100 m) using the equations of Berger et al. (1988) and Martin et al. (1987) and organic carbon flux rates derived from sediment trap experiments Location
Bransfield Strait (KG I ) Bransfield Strait (KG2) Bransfield Strait (KG3) Northern Weddell Sea (WS 1) Maud Rise (WS2) Maud Rise (WS3) Maud Rise (WS4) Polar Front (PFI )
Water depth (m)
Annual flux (g C m- 2 )
Calculated flux at 100 m (gCm- 2 )
Calculated primary production (g C m- 2 )
494
7.7
30.32
173.4
693
0.35
1.84
10.13
687
1.1
5.75
31.66
0.133 4.42 6.84 0.47 15.19
0.71 12.31 39.84 2.74 83.42
863 4456 360 352 700
0.021 0.17 2.28 0.16 2.86
607
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
TABLE 5 Estimation of annual primary production and export flux for the entire Southern Ocean Area
PFZ POO and SIZ
Surface 106 km 2
3
30
PSIZ
4
Coastal
0.9
Total
37.9
Annual primary production (g C m- 1)
Annual C flux at 100 m (gC m- 1)
Range
Range
Average
3-40
21.5
0 10-170
0.47-6.84
3.6
0 90
2-30
C flux at 100m (g C 1011)
Average
15
83
Primary production (g C 1012 )
16
294
45
645
l08
0
0
8\
14
975
167
Geographical zones and areas are taken from Treguer and van Bennekom (1991 ). PFZ, Polar Front Zone; POO and SIZ, Permanent Open Ocean and Seasonal Sea-Ice Zone; PSIZ, Permanent Sea-Ice Zone; Coastal. Coastal and Shelf Zone. In the PSIZ, sedimentation of organic carbon is almost zero, as shown by the flux experiments. Nevertheless, data from U. Bathmann (personal communication, 1990) indicate some primary production in the surface water and under the sea ice.
holder and Mandelli, 1965: 0.04-0.646 g C m- 2 day-I; von Bodungen et aI., 1986: 0.23-1.66 g C m- 2 day " "). Nevertheless, these data are difficult to extrapolate because of uncertainties in the blooms' seasonal duration. According to findings obtained from sediment trap experiments (Wefer et aI., 1990), we use an average productivity duration of only 1.5 months for the Bransfield Strait area. This would result in primary production values between 15 and 42 gem - 2 year- I, which are somewhat lower than the value estimated above. A compilation of all available data by von Bodungen (1989) resulted in an estimation of total annual production of95-130 gem -2 yeac l for the Bransfield Strait area (von Bodungen, 1989). This value is somewhat higher than our average rate of about 90 gem - 2 year- J. At a coastal site in the southeastern Weddell Sea (KN 1, Kapp Norvegia; Bathmann et al., 1991), organic carbon flux at 250 m water depth was 1.9 g m -2 for the investigation period of 53.5 days (4 January-27 February 1988). Sedimentation of organic matter was dominated by zooplankton faeces. Assuming that this measured flux represents most of the annual sedimentation, an annual primary production rate of the order of 24 gem -2 and a carbon flux of 4.2 g m -2 to 100 m is estimated. We regard these values as the lowest possible estimate. They are comparable with values at the coastal sites in the Bransfield Strait (e.g. KG2 and KG3) and with those of the Maud Rise area (Permanent Open Ocean (POO) and Seasonal Sea-Ice Zone (SIZ); Tables 4 and 5). For the coastal zone of the Atlantic Sector, Hempel (1980) has given
608
G. WEFER AND G. FISCHER
an estimate of 50 gem -2 year- I, which is comparable with our estimates for the coastal areas. By means of seasonal nitrate depletion, von Bodungen et al. (1988) calculated a new production of26 and 33 g C m- 2 in January-February 1985 for the southeastern Weddell Sea. This value is much higher than the organic carbon flux to 100 m water depth calculated for the Kapp Norvegia site. On the other hand, the organic carbon flux for the investigation period was only 2.4 g m- 2 to a depth of 100 m (von Bodungen et al., 1988), which is in the range of the flux measured by Bathmann et al. (1991). One explanation for this discrepancy is that nitrate-based new production may be considerably higher than the organic carbon export at this site. One must also take into account that the trap collections by von Bodungen et al. (1988) may have underestimated vertical flux because of high current velocities at the Kapp Norvegia site which sometimes reach 40 em S-l (Bathmann et al., 1991). Similarly high primary production (83 g C m -2 year- I) and export flux rates (15.2 g C m- 2 year- I ) to those in the Bransfield Strait were determined for the Polar Front Zone (PF 1; Tables 2c and 4). Primary production measurements over longer periods of time are not yet available, but chlorophyll a peaks of up to 1.5 mg m -3 were observed at this site (Lutjeharms et al., 1985). The Polar Front area is known to be highly productive, as is also shown by elevated biogenic silica and organic carbon accumulation rates in the underlying sediments (DeMaster, 1981). Our annual estimate for the biogenic silica flux at the PF1 site to a depth of 700 m is around 15 g m -2 (Table 4). For the abyssal zone of the Weddell Sea, van Bennekom et al, (1988) estimated a production of 50 g Si0 2 m -2 year- 1• Direct measurements by Tsunogai et ai. (1986), covering the main production period, give a value of 36 g Si0 2 m- 2 year- I. Assuming that only a third to a half of the net surface silica production may be transported to the sediment (Nelson and Gordon, 1982; van Bennekom et al., 1988), our opal flux appears to be reasonable. On the other hand, in frontal systems, biogenic silica production may be enhanced (van Bennekom et al., 1988) and up to two-thirds of total produced Si0 2 may reach the sediment (van Bennekom et al., 1988). Thus, our silica flux value must be regarded as a low estimate. As an example for the POO and SIZ, we use the data from the deployments west of Maud Rise. Calculated annual primary production varies between 3 and 40 g C m - 2, and annual organic carbon flux at 100 m is between 0.5 and 6.8 g C m- 2 (Table 4). These data are within the range of primary production rates determined by Jennings et al. (1984). For the eastern Weddell Sea, Jennings et aI. (1984) used nutrient depletion for the calculation of average primary production, which they estimated at 0.22-0.42 gem -2 day-l in spring. To assume the lower value for a 90-day period would result in a minimum production of 20 g C m", which is the average annual primary production calculated from our trap data (Table 5). This value is also in agreement with
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERNOCEAN
609
average production estimates of 16 gem-2 year- 1 for the pelagic zone (HolmHansen et al. (1977». From SiOj- depletion in the mixed layer, van Bennekom et al. (1988) calculated a biogenic silica production of 85 g m -2 year- 1 for a site at 64 ° S, 5°E. This elevated Si0 2 production could be related to the presence of the Maud Rise polynya (van Bennekom et al., 1988). The biogenic silica fluxes derived from our trap data also indicate enhanced production near the Maud Rise (Table 3), although the values (1.7-24 g m -2 year- I) are smaller than those of van Bennekom et al. (1988). It is assumed that a third to a half of surface silica production is transported to the deeper waters. We have to assume extremely low primary production rates in the area with permanent sea-ice cover. The record from all deployments in the Weddell Sea (Fischer et al., 1988; Wefer et al., 1990, and Fig. 2) shows almost zero sedimentation during sea-ice coverage. Our assumption that primary production is extremely low in ice-covered areas is supported by sediment trap data from the Arctic Ocean. In the Greenland Basin (Honjo et al., 1987) and the Fram Strait (Hebbeln and Wefer, 1991), particle flux was also almost zero during closed sea-ice conditions. Nevertheless, it must be emphasized that there is some primary production in the surface waters of ice-covered areas (0. Bathmann, personal communication, 1990), but the export of carbon appears to be almost zero as indicated by the trap data. Therefore we used a primary production value of zero in our areal calculation (Table 5). Because of the large differences in primary production rates, the Weddell Sea was subdivided into a northern/central province with lower production (oceanic domain: 0.1-0.23 g C m- 2 day-I) and a southern/southeastern province characterized by elevated rates (coastal/shelf domain: 0.4-0.7 g C m- 2 day"; El-Sayed and Taguchi, 1981; von Brockel, 1985; von Bodungen et al., 1988). These findings, based on 14C uptake measurements, are in agreement with the results presented above and confirm the pattern of the productivity subsystems used for the Weddell Sea. The regional variability of primary production in the Southern Ocean is also reflected in the 210Pb inventories (Rutgers van der Loeff and Berger, 1991): the Polar Front and the Bransfield Strait area are characterized by high production rates and the northern Weddell Sea by very low rates. In Table 5, annual primary production rates for the Southern Ocean and the fluxes at 100 in are given. Using these data, annual average primary production is around 26 g C m- 2 and the average carbon flux to 100 m is 4.4 g C m - 2. This flux estimate to 100 m is 10 times lower than model-derived new production rates of Keir (1988), but is comparable with data from Bolin et al. (1983; 3.3 gem -2 year- I) and Broecker and Peng (1986; 9.9 g C m- 2 year- 1 ). We believe that the reason for the differences between our calculations and other productivity calculations is an overestimation of the duration of the
610
G. WEFER AND G. FISCHER
growth period. To transform daily rates into annual rates, growth periods of 5 months were sometimes considered (e.g. Treguer and von Bennekom, 1991 ). Our flux data indicate a growth period of about 1 month and less (Bransfield Strait; see Wefer et al., 1990) to a maximum of 3 months (Maud Rise, Fig. 2). Although most of the trap positions are ice free for about 5-6 months, the period of elevated primary production appears to be shorter. The following physical and biological factors may be responsible for this (discussed by von Bodungen et al., 1988): (1) the transient nature ofthe shallowwater mixed layer in Antarctic waters, (2) horizontal advection processes, and (3) grazing by zooplankton. The values in Table 5 show the importance of relatively small areas with high productivities and fluxes for the whole Southern Ocean. Although the Coastal and the Polar Front Zones occupy only 10% of the Southern Ocean surface area, about 40% of organic carbon is synthesized there, according to our calculations. In contrast, large permanent sea-ice closed oceanic domains supply much smaller amounts of organic carbon to the sea-floor and are comparable with oligotrophic central gyres. The Maud Rise, an area with upwelling and entrainment of warm deep water and a recurring polynya, is characterized by intermediate annual primary production and flux rates. Summing the annual production rates from the various subsystems would result in a primary production in the order of 1X 10 15 g C ( 1X 109 tons) year- , for the entire Antarctic Ocean (Table 5). The organic carbon flux to 100 m water depth is about 0.17 X 109 tons year- '. These values are three to four times lower than productivity data of Koblents-Mishke et al. (1970) and Berger et al. (1987), obtained from 14C measurements and different algorithms. Unfortunately, no flux data are available from sites near the Filchner-Ronne or the Ross Shelf, where primary production and the export flux may be considerably higher, as indicated by satellite images. According to El-Sayed et al. (1983), primary production at the Ross Ice Shelf locally reached 1 g m- 2 day-I. Similarly high values were obtained by Wilson et al. (1986) in an iceedge bloom in the western Ross Sea. In addition, substantial deposits of diatom-rich sediments occur in this area (DeMaster, 1981), indicating high surface productivity. Thus, our calculated coastal primary production and the export flux may be underestimated. These calculations have to be improved with data from sediment trap experiments from those sites. CONCLUSIONS
(1) The Antarctic Ocean is divided into various subsystems, characterized by different primary production and organic carbon flux rates which were estimated by means of flux data from long-term sediment trap deployments. (2) The calculations show that primary productivity and export fluxes are
611
PRIMARY PRODUCTION AND EXPORT FLUX IN SOUTHERN OCEAN
discontinuous in space and time. High flux rates were measured only in short episodes (1-3 months) during and after high production periods. (3) The flux data give indications that primary production and organic carbon flux are almost zero in the Permanent Sea-Ice Zone and during winter. The Coastal and Shelf Zone and the Polar Front Zone show highest annual production (83-170 g C m -2) and flux rates. Intermediate production rates, of the order of 20 g C m - 2 year- I, were obtained for the Maud Rise area. (4) The calculated average annual primary production in the Southern Ocean is around 26 g C m -2, and the average organic carbon flux rate is 4.4 g C m- 2 • This carbon flux estimate is about 10 times lower than a model-derived value for new production (42 g C m -2 year :") applied by Keir (1988), but is largely consistent with data obtained by Bolin et al. (1983) and Broecker and Peng (1986). (5) The total annual primary production for the entire Southern Ocean is estimated as 1X 109 tons, and total annual organic carbon flux is 0.17 x i 09 tons. The primary production value is three to four times lower than previously reported data for the Antarctic Ocean. ACKNOWLEDGEMENTS
We thank officers and crew of R/V "Polarstern" for their help during the fieldwork. We are further indebted to G. Ruhland and A. Wiilbers for deploying the mooring arrays equipped with the sediment traps. The investigations on particle flux in Antarctic waters were carried out in co-operation with R. Gersonde and D. Futterer from the Alfred Wegener Institute of Polar and Marine Research, Bremerhaven. W.H. Berger and U. Bathmann made valuable suggestions to improve the manuscript. This research was funded by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 261 at Bremen University, Contribution 25).
REFERENCES Bathmann, U., Fischer, G., MUller, P.J. and Gerder, D., 1991. Short-term variations in particulate matter sedimentation off Kapp Norvegia, Weddell Sea, Antarctica: relation to water mass advection, ice cover, plankton biomass and feeding activity. Polar Biol., 11: 185-195. Berger, W.H., Fischer, K., Lai, C. and Wu, G., 1987. Ocean productivity and organic carbon flux. 1. Overview and maps of primary production. University of California, San Diego, SIO (Scripps Institute of Oceanography) , Ref. 87-30. Berger, W.H., Smetacek, V.S. and Wefer, G., 1989. Ocean productivity and paleoproductivityan overview. In: W.H. Berger, V. Smetacek and G. Wefer (Editors), Productivity of the Ocean: Present and Past. Wiley, New York, pp. 1-34. Betzer, P.R., Showers, W.J., Laws, EA, Winn, C.D., DiTullio, G.R. and Kroopnick, P.M., 1984. Primary productivity and particle fluxes on a transect of the equator at 153 in the Pacific Ocean. Deep-Sea Res., 31: I-II. 0
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