Polar Biol (1996) 16:613 622

9 Springer-Verlag 1996

Carlos Pedros-Ali6 9 Juan I Calderon-Paz Nflria G u i x a 9 A n t o n i N a v a r r e t e 9 D o l o r s V a q u e

Microbial plankton across Drake Passage

Recei~'ed: 14 August 1995/Accepted: 15 January 1996

Abst~ract We determined biomass and activity of microbial plankton across the Polar Front (PF) in Drake Passage during January 1994. Temperature was around 0~ south and between 3 and 5~ north of the PF. Both biomass and activities of microorganisms were significantly lower in the Antarctic waters south of th:: PF than in the sub-Antarctic waters north of it. Thus. values of chlorophyll a, integrated between 0 and 200 m, reached 150 m g m -2 north, but only 25 mg m - a '.;outh of the PF. Likewise, bacteria varied between 101~ and 4 • 1013 cells m -2. However, the abundance of h~:terotrophic nanoflagellates was extremely low throughout Drake Passage (around 3 x 10 l~ cells m-2). Bacterial doubling times were long (mean of 25 days). Bactcrivory was estimated from the abundance of predators and prey and from temperature. The grazing impact on bacterioplankton biomass was insignificant (less that 0.05% per day) and low on bacterial heterotrophic production (15% per day). Neither biomass nor the activities of microorganisms were found to increase at the PF. The microbial food web was uncoupled and the bacteria did not seem to be controlled by predation.

Introdttction The Polar Front (PF) or Antarctic convergence is one of the major features of the Southern Ocean. It is a permanent circumpolar structure surrounding the Antarctic Ocean completely between 50 and 60 ~ South, wher,., cold Antarctic water slips below and mixes with warmer waters (Deacon 1982; Nowlin and Klinck

C. Ped :6s-Alio (IZ~) 9J.I. Calderdn-Paz 9N. Guixa 9A. Navarrete D. Vaq u~ Institut de Ci+ncies del Mar, CSIC, Passeig Joan de Borb6 s/n, E-0803~ Barcelona, Spain fax (3d3) 221-7340; e-mail [email protected]

1986). This front is considered to mark the transition from Antarctic to sub-Antarctic waters and to be the northern limit of the Antarctic Zone. Thus, different phytoplankton (Walsh 1971; Holm-Hansen et al. 1977), zooplankton and fish assemblages can be found at the two sides of the P F (Deacon 1982). Fronts are known to influence the chemistry and biology of the oceans. In particular, convergences are supposed to be places of decreased primary production and plankton activity. In the zones surrounding the PF, however, there is a general upwelling of nutrientrich deep circumpolar water that may cause local favourable conditions for phytoplankton. In addition, the north-south movements of this front and the formation of cold water rings cause the PF, and specially the Polar Frontal Zone (PFZ), to be quite variable. Thus, both increased and decreased phytoplankton biomass at the P F have been reported (Hohn-Hansen et al. 1977; Lutjeharms et al. 1985; Vincent 1988; E1-Sayed 1988). In fact, average monthly values from satelliteestimated chlorophyll in the Southern Ocean show that the largest variability occurs at the P F Z (Sullivan et al. 1993). Likewise, Hohn-Hansen et al. (1977) found that both the maximum and the minimum of integrated primary production in different transects of the Southern Indian and Pacific Oceans occurred at the PF. Despite being such a permanent and predominant physical structure, the ecological structure and function of microbial heterotrophic plankton across the PF remain unknown. Wiebe and Hendricks (1974) analysed bacterial plate counts across the front (at 150~ longitude) and found a very slight decrease in numbers in the PF with respect to both northern and southern waters.They also showed that the bacteria isolated on either side of the P F were different taxonomically and that many of the isolates from the Antarctic Zone were psychrophilic, whereas the isolates from the subAntarctic Zone were mesophilic (Wiebe and Hendricks 1974). Morita et al. (1977) measured substrate uptake kinetics along a similar transect (at 170~ longitude)

614

and found low heterotrophic activity north of the PF, increased activity in the PF region and moderate to high activity south of the PF. Kogure et al. (1986)found a general decrease in bacterial numbers with increasing latitude (at 150~ longitude), although doubling times, calculated from thymidine uptake, were similar throughout the transect. More information is available about the PF in Drake Passage. The hydrologic structure and dynamics of the area have been extensively studied (Sievers and Emery 1978; Whitworth 1980; Nowlin and Klinck 1986; Sievers and Nowlin 1988). However, there are extremely few studies of the microbial plankton. Kriss (1973) determined viable counts of bacteria in a transect immediately east of the passage. He found between 10 and 100 plate-forming units (pfu) in the surface layers (0-150m) and very low numbers below (200 4000 m). In the early 1980s R.B. Hanson and co-workers (Hanson and Lowery 1983, 1985; Hanson et al. 1983a, 1983b) conducted an extensive bacterioplankton study across Drake Passage. They suggested that the PF supported increased bacterial biomass and activity, as measured by the FDC method (Hanson

Table 1 Average, standard deviation (STD) and maximal and minimal values for all the parameters determined in Drake Passage during January 1994 a Sample averages

Table 2 Bacterioplankton biomass and production estimates in Drake Passage a Only surface water values reported b Concentration of viruses also given: 0.07 0.49 x 106 m l - z c Concentration of viruses also given: 1.3 5.4 x 106 m l - 1 d Concentration of cyanobacteria also given: between 101 and 106 cells ml 1

et al. 1983a) or by the adenylate energy charge ratio (Hanson and Lowery 1985). More recently, Letelier and Karl (1989) reported bacterial and cyanobacterial numbers in surface waters of the passage. They found that cyanobacterial numbers increased from the colder southern waters (around 10 cells ml-1) to the warmer northern waters of the passage (around 103 cells ml-1). Smith et al. (1992) measured concentrations of bacteria and viruses in surface waters of the passage (see Table 2). Neither of these studies found significant increases or decreases in bacterial, cyanobacterial or viral cell numbers at the PF. However, these studies only analysed surface water. Finally, the RACER Project included a few stations on the southern side of the passage (Karl et al. 1991). No other studies have examined the microbiology of Drake Passage. During January 1994 the biomass and activity of microbial plankton were determined along a transect from Ushuaia to Livingston Island. Our purpose was to measure microbial plankton distribution and activities in Antarctic waters in order to answer two questions: (1) do the Polar Front or the Polar Frontal Zone sustain increased biomass and activity, as suggested by

Parameter

Average

STD

Maximum

Minimum

Measured ATP ( n g l - 1) Chlorophyll a ( g g l - 1) Bacteria (10 s cells ml-1) Bacterial cell volume (btm3) H N F (102 cells ml-1) P N F (102 cells ml 1) Leu (ggC m 3 d a y - l ) F D C (%) F D D C (%)

153.4 0.40 3.23 0.035 1.86 6.89 633.0 1.98 4.29

131.0 0.43 2.06 0.005 1.04 4.85 502.1 0.44 0.75

700.3 1.65 7.05 0.048 a 3.82 19.1 1733.9 2.88 5.65

13.2 0.01 0.49 0.025 a 0.36 0.33 7.2 1.14 2.71

Calculated Dt-Leu (days) Dt-FDC (days)

25.1 1.03

53.5 0.04

297.6 1.10

1.4 0.95

Estimated Grazing (cells cm 2 d a y - l ) %B %BHP

0.04 11.8

Time

1.96x 106

1.11 x 106 0.01 4.0

3.84x 106 0.05 15.8

Abundance x 105 cellsml 1

Production (ggC 1-1 d 1) FDC TdR/Leu/AdR 2.6 17.1

Oct 1980 (late winter)

4.0-10

Jan 1980 (summer) Jan 1994 (smnmer)

0.1-2.0 0.5-6.0

Aug 1991 (winter) a'b Jan 1991 (summer) "'c Feb 1987 (autumm) a'd

0.2-1.9 1.0-6.5 1.0-2.0

0.8

0.0002-0.054 0.007-1.7

0.9x 106 0.02 6.6

Reference Hanson et al. 1983a Hanson et al. 1983b Present study Smith et al. 1992 Smith et ai. 1992 Letelier and Karl 1989

615

Hanson and Lowery (1985), and (2) are there differences in the abundance and activity of microbial plankton north and south of the Polar Front?

Materials and methods Study area and sampling Sevet~_stations were occupied along a transect across Drake Passage, between the waters off Cabo de Hornos (55~ 66~ and the South Shetland Islands (61~ 62~ from 2-4 January t994 (Fig. 1). The work was done aboard the BIO Hesperides, cruise ECOANTAR94. The hydrography was determined with a CTD E G & G model MkIIIC, supplemented with a fluorometer and a turbidity meter. Water samples were collected using a rosette sampler consisting of 24 Niskin bottles from a depth range of 5-200 m. Aliquots from the Niskin bottles were collected in 150-ml plastic bottles, while being screened through 200-~m Nitex mesh, and held in a container with surface water to keep the ambient temwrature of the samples until processed. The surface water temperature was different at each station. The difference between surface watel s and the deepest water sampled (200 m), however, was smaller than ~~ and, thus, all incubations were carried out at the surface ware1 temperature. The only exception was the 90-m sample from staticn 6, which had a temperature 2~ lower than the surface waters. In this case, activity measurements may be overestimates. All incuhtions were started and fixations completed within 30 rain of sampling.

Abunadance of bacteria and flagellates Water was dispensed into 125-ml plastic bottles and immediately fixed with glutaraldehyde (1% final concentration) and stored at 3~ in the dark. Aliquots of 15-20 ml (for bacteria) or 50 ml (for flagellates) were filtered through black-stained Nuclepore filters (0.2-gin pore diameter for bacteria and 0.8 b~m for flagellates) on board. Samples were stained with DAPI (4'-6-Diamidino-2-phenylindole dihydrochloride) (0.1 btg ml 1 final concentration) for 5 min before sucking the filters dry (Porter and Feig 1980). Filters were then mounted on microscope slides with non-fluorescent oil (R.P. Cargille Laboratory) and stored frozen until return to the laboratory in Barcelona. Filters were counted by epifluorescence microscopy with a Nikon Diaphot microscope. About 200 400 bacteria were counted per sample, while transects across the filters were used for nanoflagellates. Cell volumes of bacteria were determined using an image analysis system measuring at least 200 cells per sample. A Hamamatsu C2400-08 video camera was used to examine microscopic preparations. Images were captured using a personal computer with the software MIP (from Microm Espafia SA). The video ~mages were downloaded to a Macintosh con:puter and analysed using the shareware program NIH Image. The pixel size with this system was 0.067 ~tm. Image processing involved the following steps: Gauss 5 x 5 filter, Laplace 5 x 5 filter, manual thresholding, median filter and binarisation. Next, binarised images were compared to the original image, and detritus, aggregates of cells, and objects without smooth edges were eliminated. Finally, objects occupying less than 7 pixels (equivalent to a sphere with diameter less than 0.2 gin) were discarded. The remaining objects were measured and the volume calculated from area and perimeter measurements using the formula of Fry (1990). The system was calibrated with fluorescent latex beads and with natural bacterioplankton samples measured simultaneously by phase contrast microsopy and by epifluorescence (R. Massana, J.M. GasoI and C. Pedr6s-Ali6, unpublished work).

Activity of bacterioplankton

Frequency of dividing cells The frequency of dividing ceIts (FDC) and the frequency of dividing plus divided ceils (FDDC) of bacteria were determined, using the same filters, by counting at least 30 dividing cells per sample at a magnification of x 1250 (HagstrSm et al. 1979).

[;H]lteucine incorporatiou into cold TCA precipitate

Fig. 1 Station locations occupied in Drake Passage for BIO Hesperides cruise ECOANTAR94, 2 4 January 1994. The line labelled PF indic~ tes the mean position of the Polar Front (from Mackintosh 1946, ,'ited in Whitworth 1980)

The method of Kirchman (1993), with slight modifications, was used to estimate bacterial heterotrophic production. A radioactive leucine solution was added at a final concentration of 40 riM. This concentration was shown to be saturating on the basis of five concentration-dependent incorporation experiments conducted during the entire cruise (which included Bransfield Strait and the Weddell Sea in addition to Drake Passage, data not shown). The radioactive leucine solution was prepared as follows: 5 mCi (in 5 ml) of the commercial stock of L-[4,5-3H~leucine (153 Ci/mmol, Amersham) was diluted to 1-btM concentration with 0.2 gm filtered and autoclaved milliQ water. The resulting 32.68 ml was distributed into several 5-ml screw cap tubes and stored at 0-4~ until use. Immediately before use, this second stock was diluted ten times with 0.2 gm filtered, non-radioactive, 1-btM L-leucine. Two replicates of 20 ml and a killed control (4% final concentration of formaldehyde) were used for each determination. Throughout the manipuIations, the sample and incubation vials were kept in a container with surface water. Vials were incubated in the dark at in situ temperature (from 5~ at station 2 to 0~ at stations 5 7). Incubations lasted 150 rain,

616 in accordance with the results of linearity experiments conducted during the cruise. For each station, six depths were chosen according to the vertical profiles of temperature, salinity and fluorescence taken by the CTD. After incubation, samples were killed with formaldehyde (4% final concentration). Fixed samples were filtered through cellulose acetate 0.45-btm filters (Kirchman 1992), rinsed twice with 5% ice-cold TCA and three times with 80% ethanol. Ethyl acetate (0.5 ml) was added to the filter to dissolve it and later a scintillation cocktail (5 ml) also. Radioactivity was counted on board using a Beckman scintillation counter that used the H number to calculate dpm.

Latitude (o South) 55

56

57

58

69

60

61

300

400

500

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0 20 40

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120 140 160

Other measurements and calcuiations Chlorophyll a fluorescence was determined in acetone extracts using a Turner Designs fluorometer (Parsons et al. 1984). ATP was measured with the Iuciferine-luciferase assay (Holm-Hansen 1969; Karl et al. 1991). Conversion factors from the literature were used in calculating microbial biomass: carbon/ATP 250/1 (Holm-Hansen 1969); carbon/ chlorophyll a 84/1 (Karl et al. 1991). The growth rate of bacteria (p.) was calculated from the FDC according to the equation In bt = 0.081 F D C - 3.73 (determined by Hanson et al. 1983a for these waters). Bacterial heterotrophic production (BHP) was calculated from leucine incorporation (Leu) according to the equation BHP = Leu 3.1 kgCmol Leu-1 (Simon and Azam 1988), and from this production, growth rate was calculated as bt = In (1 + BHP/B). Bacterial carbon content per cell was calculated as pgC/cell = 88.6 (cell volume gm3) ~ (Simon and Azam 1988). The average volume of heterotrophic nanoflagellates (HNF) was determined as 65.45 btm3 and carbon per cell was calculated as 0.22 pgCbtm -3 (Borsheim and Bratbak 1987). Grazing by protists was estimated using the empirical equations of Vaqu6 et al. (1994). These equations use abundance of bacteria, abundance of nanoflagellates and temperature as independent variables in a multiple linear regression to estimate total grazing (the dependent variable).

Results Hydrography and latitudinal distribution of integrated parameters Figure 2 shows the distributions of temperature and salinity across Drake Passage in the upper 200 m. The main feature is the Polar Front, which is indicated by the northernmost extent of the 2~ isotherm (Botnikov 1963, cited in Sievers and Emery 1978). In this region, the PF occurrs at subsurface waters, in this case at approximately 58~ The PF appears at the surface further south, around 59~ At lower latitudes, the upper water column was not sharply stratified and temperatures progressively decreased from 6 to 3~ moving southwards. South of the Polar Front, a very cold water mass (approximately - I~ was observed (Antarctic Surface Water). On the basis of these profiles, stations 1 and 2 are in typical sub-Antarctic waters, stations 5, 6 and 7 are in typical Antarctic waters and the remaining stations (3, 4) represent the transition across the PF. A clear difference between stations north and south of the Polar Front was apparent in most integrated

160

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Fig. 21, B Physical characteristics of the upper 200 m across Drake Passage in January 1994. Both latitude (upper scale) and distance from station 1 (lower scale) are indicated. A Temperature (~ and B salinity (%0)

parameters (Fig. 3, Table 3). Values of all microbiological parameters were significantly higher in subAntarctic than in Antarctic waters, while the transition stations showed intermediate values (Fig. 3A, B). In particular, chlorophyll was 10 times higher, ATP and bacterial abundance were double and leucine incorporation 3 times higher in sub-Antarctic than in Antarctic waters. Flagellate abundance followed a different pattern. Heterotrophic nanoflagellates (HNF) were found at concentrations between 200 and 300 individuals per milliliter throughout the transect. These values are relatively small even for oligotrophic oceans. Phototrophic nanoflagellates (PNF) were found at concentrations between 200 and 1000 individulas per milliliter. Integrated values of P N F increased towards the south (Fig. 41).

Latitudinal and vertical distribution of microbial populations The vertical distributions of chlorophyll a and ATP across Drake Passage are shown in Fig. 5. North of the PF, chlorophyll reached higher concentrations in the upper 100-m water column than south of the PF.

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Table .3 Mean values for biomass and activity of the main microbial components north (stations 1 and 2) and south (stations 5, 6 and 7) of the Ptdar Front. Standard error in parentheses. All means were signit)cantly different (P < 0.001) with a t-test Varia:,le

Stations 1 and 2 north of PF

Stations 5-7 south of PF

76.87 (17.28) 89.27 (5.99)

30.42 (3.42) 8.77 (1.54)

Bacte:'ia (mgC m - 3) BHP mgC m -3 day -1) Bacteria turnover time (days)

5.84 (0.66)

2.72 (0.49)

H N F (mgC m 3) Grazirg (mgC m -3 day 1)

ATP I:ngC m-3) ~ ChIorophyll a (mgC

m-3)a

1.39 (0.13)

0.45 (0.08)

1.72 (2.07)

27.47 (8.27)

2.70 (0.43) 0.14 (0.04)

1.59 (0.38) 0.08 (0.03)

" Carb)n biomass determined from ATP or from chlorophyll a. See Methods for conversion factors

VerticaIly, the distribution was more uniform in Antarctic: waters, whereas north of the PF chlorophyll a showed a deep maximum between 60 and 100 m. PN F were a component of the phytoplankton under two very different conditions (Fig. 6A). They were present in the upper 60 m north of the P F and formed a deeper maximum (at 100 m) immediately south of the PF. Obviously, P N F were a small component of the phytcplankton since their distribution coincided with zones of low chlorophyll a concentration (Fig. 5A). Figure 6B shows the distribution of HNF. Values were

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Fig. 4 A Distribution of integrated values of heterotrophic (HNF) and phototrophic (PNF) nanoflagellates across Drake Passage. B Distribution of integrated values of estimated total grazing on bacteria. Total grazing has been estimated with equation 1 (continuous line) and equation 2 (discontinuous line) from Vaqu~ et al. (1994). C Impact of predation on the bacterioplankton as percentage of the bacterial biomass and production (B) across Drake Passage

extremely low (30-300 individuals ml-1) throughout the transect (Table 1) and the distribution followed a pattern similar to that of P N F (Fig. 6). Bacterial abundance changed by an order of magnitude (Table 1), both latitudinally and vertically. Values ranged from 0.5 to 6 • l0 s cells ml-1 (Fig. 7A). North of the PF concentrations were higher than south of it and presented a distribution very similar to that of chlorophyll a (Fig. 7A). South of the PF, bacteria showed a maximum of abundance between 50 and 100 m. Abundance at this maximum was 2 - 4 times the abundance at surface waters.

Distribution of bacterioplankton activity, production and growth rates Leucine incorporation was relatively high north of the P F and fairly low at the PF (Fig. 7B). However, slightly higher values were observed at station 7 in the upper

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waters (40 80 m). Samples for station I were lost due to bad weather conditions. Using literature conversion factors (see Methods), we calculated bacterial growth rates (Fig. 8A) and doubling times (Tables 1, 3) from our abundance and leucine incorporation data. Doubling times varied between 1.4 and 300 days (average 25 days, Table 1). The fastest growth was detected at 30 m in stations 6 and 7 at the interface between the very cold Antarctic Surface Water and slightly warmer surface waters. However, the zones of slowest growth were found at the Antarctic Surface Water tongue at intermediate depths and at the surface in stations 4 to 6. North of the PF growth rates were relatively high throughout the water column. FDC (Fig. 8B, Table 1) and F D D C (Table 1) values were always extremely low, ranging from 1,1 to 2.9 and from 2.7 to 5.7 respectively. There was an apparent trend for slightly higher FDC values at the surface and at the deepest waters sampled. F D D C also showed the trend for slightly higher values in surface waters. Values seemed to be slightly lower north of the Polar Front, Differences between all samples, however, were not significant. Growth rates calculated from FDC values ranged between 0.65 and 0.72 d - l , while rates calculated from Leu incorporation ranged between 0.002 and 0.502 d -1 (Table 1). Values calculated from

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Fig. 6A, B Vertical d i s t r i b u t i o n of P N F (A) a n d H N F (B), b o t h in individuais per milliliter, a c r o s s D r a k e P a s s a g e

FDC and from Leu were not significantly correlated when the Pearson correlation coefficient was calculated. Estimation of total grazing on bacterioplankton An estimate of the grazing impact on bacteria was obtained using the empirical equations of Vaqu6 et al. (1994) with our data on bacterial and H N F abundance and temperature (Vaqu6 et al.'s equations 1 and 2). Equation 1 takes bacterial abundance into account (in addition to H N F abundance and temperature) while equation 2 does not. The results obtained with the two regression equations were not significantly different. Figure 4B shows integrated values of total grazing. Following the general trend in almost all measured parameters, total grazing was 2 times higher north of the PF than south of it. The percentage of bacterial biomass (between 0.02 and 0.05%) and production (between 7 and i6%) represented by this estimated grazing, by contrast, did not change with latitude (Fig. 4C), The percentage of the bacterial biomass grazed daily was always less than 0.05% (Table l). The percentage of bacterial production grazed varied between 10 and 15%, which is consistent with

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~.~.,B Vertical distribution of heterotrophic bacterial abundance in cells ml ~ (A) and leucine incorporation in ggC m -3 d-1 (B) acloss Drake Passage Fig.

the !generally long doubling times calculated from leucine incorporation (Table 1).

Discu~;sion Bact::rioplankton production in Drake Passage There are three sets of values of bacterioplankton production in Drake Passage (Table 2), two different cruises by Hanson and co-workers and this study. First, Hanz~on et al. (1983a) determined F D C in a transect across the passage. Second, Hanson et al. (1983b) determined adenine and thymidine uptake at several stations in the sub-Antarctic and Polar Frontal zones of the passage. Finally, we determined leucine incorporation and F D C along a transect. Our F D C values showed very little variation, while the leucine values ranged 2 orders of magnitude. It has been claimed that bacteria from deep waters may show a hit!her F D C than surface bacteria due to longer divisian times under very slow growth conditions (Biddanda and Benner 1994). This would be consistent with the tendency for slightly higher F D C values at the deepest waters found in our study. The possible

Fig. 8A, B Vertical distribution of growth rates in days- 1 (A) and FDC values in % (B) across Drake Passage

presence of a large fraction of inactive bacteria would also tend to mask changes in the F D C between regions of different (but slow) bacterial growth. These differences, however, could be easier to detect with radioactive methods. F D C values respond to increases in growth rates in cultures (Hanson et al. 1983a, Pedr6sAli6 and Newell 1989) and show good correlation with growth where large differences in activity are concerned, but the method seems too insensitive to detect fine differences in growth rates of slow-growing assemblages such as those present in the oligotrophic areas of the ocean. Our F D C values were considerably lower than those of Hanson et al. (1983a). These differences may be a result of seasonal changes, since Hanson et al.'s study was conducted in the austral spring, when the ice edge was receding, while the present study was conducted during the austral summer, when the spring phytoplankton bloom had probably already declined. Our bacterial production values were about an order of magnitude higher than the summer rates found by Hanson et al. (1983b) in central Drake Passage (Table 2). This difference may be a consequence of several factors. First, most of the stations studied by Hanson et al. (1983b) were in the P F area, which

620

showed the lowest values in our study. Second, the different techniques used could be responsible for the different values. Third, our study was conducted in early January 1994, while theirs spanned the whole month of January 1980. Besides year-to-year variations, perhaps the decay of the annual spring phytoplankton bloom (Karl 1993) resulted in lower bacterial activities as the month went by, producing lower overall values in the study by Hanson et al. (1983b). The range of values calculated from leucine uptake in the present study spans the same range of values as those of most studies done with thymidine in Antarctic waters (Furhman and Azam 1980; Azam et al. 1981; Kottmeier et al. 1987; Karl 1993). Bacterial activity varied by several orders of magnitude among the three studies, probably as a consequence of seasonal differences. The three different sets of production values for Drake Passage are well correlated with their respective bacterial abundances (Table 2). Thus, both abundance and activity were lowest in January 1980, intermediate in January 1994, and highest in October 1980. In our study, bacteria were growing very slowly, with a mean doubling time of 25 days (Table 1).

Estimates of grazing on bacteria The impact of protists grazing on bacterioplankton was estimated through empirical relationships. The data base used to derive such relationships did not include studies from extremely cold waters such as those of Drake Passage and, therefore, the estimates have to be considered with caution. However, the little information available on grazing by psychrophilic protists (Choi and Peters 1992) and the few data obtained from cold waters (Putt et al. 1991; Bird and Karl 1990; D. Vaqu6, N. Guixa, J.I. Calder6n-Paz and C. PedrdsAli6, unpublished work) are consistent with low grazing impact by protists at low temperatures. The very low numbers of heterotrophic nanoflagellates found (between 40 and 400 cells per ml) also argue for very low grazing impact. Across Drake Passage, the estimated impact of grazing was negligible on the biomass (0.04%) and very small on the production of bacterioplankton (12%). This reinforces the impression of a bacterial assemblage controlled by resource limitation (bottom-up) and/or temperature, rather than by predation (top-down). In early January, therefore, the microbial loop seemed uncoupled and grazing was very low. Changes in the microbial food web across the P F In January 1994 water masses were displaced northwards with respect to the situation found by Hanson et al. (1983a). These authors found the P F at around

59~ and relatively close to the ice edge. We, however, found the PF at approximately 1~ further north, and the whole passage was completely ice free. Hanson and Lowery (1985) found elevated biomass at the Polar Frontal Zone and higher F D C values at the Polar Front (Hanson et al. 1983a). But their cruise study was carried out in October. Thus, the increased activity found by these authors was probably related to the proximity of the ice edge, which at that time was retreating. It is now established that receding ice edges produce a trailing phytoplankton bloom followed by increased bacterial activities (Sullivan et al. 1990). During our visit the ice edge was south of Drake Passage and the P F was far from the shore, and therefore, there was no increase in activity at the front. The decreased biomass and activity of bacterioplankton in the frontal zone, in fact, are consistent with the general model of impoverished waters at convergences. Chlorophyll a showed a similar decrease at the PF. This lowered chlorophyll concentration at the PF seems to be a permanent feature of the Southern Ocean. Sullivan et al. (1993) plotted mean pigment concentrations from CZCS satellite data averaged between November 1978 and June 1986. Their global image (Sullivan et al. 1993) shows a circumpolar ring of lowered pigment values that essentially coincides with the mean position of the PF. Neither bacteria nor flagellates showed an increase at the PF. ATP concentrations showed a slight decrease at the front (Figs. 3A, 5B). There was no increase either in microbial plankton biomass or activities at the PF. Rather, in accordance with the fact that it is a convergence, it had reduced plankton biomass and activities. Stations 1 and 2 have been chosen as representative of the waters north of the PF, and stations 5, 6 and 7 as representative of the waters south of the PF (Table 3). The main difference found was that microbial biomass calculated from ATP, phytoplankton biomass calculated from chlorophyll and bacterial biomass were substantially greater north than south of the PF. The ratio of chlorophyll to ATP was also very different. While chlorophyll-calculated biomass accounted for most of the ATP biomass north of the PF, it accounted for only 40% south of the PF. Bacterial biomass was 11% and 17% of chlorophyll biomass north and south of the PF respectively. The biomass of HNF, finally, was low in both areas. The calculated turnover times for bacteria were 1.7 and 27.5 days, north and south of the PF respectively. Likewise, estimated grazing was twice as low south of the PF. Thus the community south of the P F was not only less abundant, but it was also less active than that from the sub-Antarctic waters. In summary, no enhancement of either biomass or activities of the microbial food web was found at the PF. Both biomass and activities were, in general, similar to those in other oligotrophic areas of the world. In particular the abundance and activities of H N F were

621

insignificant. Finally, both biomass and activities were significantly lower in Antarctic waters south of the PF, t h a t in sub-Antarctic waters north of it. This difference corr::lates with temperature, which is around 0~ to the south and between 3 and 5~ to the north of the PF. Whether these are permanent features of the two communities or seasonal characteristics, however, cannot be resolved without seasonal studies. Ackn~wledgements This research was supported by grant ANT930997 l'rom the Spanish National Research Program on Antarctica, CICYT. We thank scientists of the ECOANTAR-94 program and the crew of the BIO Hesp6rides for their help and cooperation. Phys cal data were gathered by Timothy Granata, Mario Manrique~:, Damifi Gdmis, Oswaldo Ldpez, M" Pilaf Rojas and Joaqnim Sospedra, and chlorophyll a data by Marta Estrada, Elisa Berdalet, Laura Arin and Roser Ventosa.

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