Import DATA

Classeurs EXCEL

1992

1993

1994-1995

All

Abstract.

Between January 1990 and March 1995, the research project KERFIX undertook the first regular non-coastal multiyear acquisition of parameters related to the carbon cycle in the Southern Ocean at a time series station located at 50° 40'S - 68° 25' E, 60 miles south-west of Kerguelen Islands. We present here a general overview of the bacteriological data collected during this survey. Bacterioplankton biomass in Kerfix station was always significantly lower than in the direct vicinity of Kerguelen Islands. Bacterial biomass and production decreased from surface to deepest layers (1500m). In contrast, mean cell volume (0.1 µm3) and proportion of free living bacteria (80%) were relatively constant through the entire water column. The results suggest that a strong seasonal and inter-annual variability affects the total bacterial abundance, the mean cell volume, the culturable saprophytic bacterial communities and, to a lesser extent, the percentage of free living cells.

Introduction

Because the oceans may be an important sink for the rapidly increasing concentration of anthropogenic CO2, a central objective of major biological oceanographic programs is to quantify, model and predict, at global and annual scales, the flux of biogenic carbon to deep waters (Trembley et al., 1997). Most of the biological studies conducted off shipboard in the Southern Ocean were focused on regional variability aspects. The seasonal variability in plankton biomass is poorly documented due to scarcity of time series observations conducted over one or several years (Horne et al., 1969; Whitaker, 1982; Delille, 1990; Helbling et al., 1995; Moline and Prézelin, 1996). However, seasonal changes have to be understood if accurate carbon budgets are to be constructed (Platt et al., 1992, Priddle et al., 1992, Tréguer & Jacques, 1992). This is particularly true for the Austral Ocean with intense temporal variability, perhaps the most extreme seasonality observed anywhere in the world ocean (Karl, 1993).

Furthermore the majority of the available long term planktonic studies in the Antarctic zone are focused on phytoplankton. Few investigators have examined both bacteria and phytoplankton over an annual cycle (Gibson et al., 1990; Rivkin, 1991; Leakey et al., 1994; Delille et al., 1996). From 1990 to 1994, the research program Kerfix (as part of French contribution to JGOFS) proposed a time-series station located in the Indian sector of the Southern Ocean, Southwest off Kerguelen Islands. The main objectives of this project were to monitor the ocean/atmosphere CO2 fluxes and to understand the seasonal variability of associated elements at this location (Jeandel et al., in press). The purpose of the research presented here was to document, for the fisrt time in the northern part of the Southern Ocean, the seasonal and interannual changes in the biomass and abundance of the total bacterioplankton present on the water column of the station Kerfix.

Materials and methods

The Kerfix station is located in the Southern Ocean, 60 miles Southwest off Kerguelen Islands (50° 40'S, 68° 25'E, Figure 1) in the North of the POOZ (Permanently Open Ocean Zone). It was investigated during 3 years, from January 1992 to March 1995. Samples were collected monthly aboard the coastal oceanographic ship "La Curieuse" using Niskin bottles. Biological samples were collected at 24 depths in the entire water column (from 0 to 1500m).

 

Total bacteria were determined on formalin preserved samples by acridine orange direct count (AODC) with an Olympus epifluorescence microscope according to the method of Hobbie et al. (1977). A minimum of 3OO fluorescing cells with a clear outline and definite cell shape were counted as bacterial cells on 0.22 µm nuclepore filters in a minimum of ten randomly selected microscope fields. Free living and particle related bacteria were counted separately.

Cell volumes were estimated using an ocular micrometer. Cells were divided into two classes for calculation of bacterial volumes: spheres and rods (cylinders with hemispheres at each end).

Results

A clear decreasing gradient of bacterial abundance is generally obderved in the upper part of the water column (Figure 2).

 

The % of free-living bacteria showed only little vertical and seasonal variability (between 75 and 80 %)

 

A clear seasonal trend was recorded in the bacterial abundance (Figure 5). During early winter, the mean count concentration was low (< 0.2 mg m-3). It increased from September - October and reached more than 0.8 mg m-3 in November-December, in the upper 100 m. During summer, the vertical biomass distribution was closely related to the thickness of the upper mixed layer (UML) of the mean value between 50 m and 75 m The highest chl a values were found within the upper 100 m and a maximum concentration was reached in the sub-surface waters. In winter, the chl a biomass distribution was low and homogenous throughout the water column.

Discussion

A relatively high fraction of non-active cells and ghosts has recently been reported (Zweifel and Hagström, 1995; Gasol et al., 1995; Lovejoy et al., 1996). Thus, as previously suggested in the same area (Delille, 1990), the observed bacterial abundance does not necessarily reflect the bacterial activity. In any case the observed changes are too large (2 orders of magnitude) to be related uniquely to fluctuation of the proportion of active and inactive cells.

In studies of carbon dynamics in aquatic microbial communities, the ability to convert bacterial abundance or bacterial biovolume to carbon is crucial in order to calculate bacterial biomass. A wide spectrum of conversion factors has been suggested. One of most used factor is 121 fgCµm-3 (Watson et al., 1977), this value had been recently confirmed by the work of Theil-Nielsen and Søndergaard (1997) who suggested a factor of 1O5 fgCµm3. Using X-ray microanalysis Fagerbakke et al., (1996) found an average conversion factor of 63 fgCµm-3 for native marine bacteria. However, higher values have been proposed. Bjørnsen (1986) found an average of 350 fgCµm-3, while Bratbak (1985) proposed a value of 560 fgCµm-3. In the Southern Ocean Bjørnsen and Kuparinen (1991) found a conversion factor of 400 fgCµm-3. Considering variability due to differences in bacterial species composition, which may depend on sampling time and location (Nagata and Watanabe, 1990; Kroer, 1994), and bacterial growth conditions (Nagata, 1986), it is not surprising to observe a wide spectrum of conversion factors. Obviously, the density of pelagic bacteria, which have to maintain their position in the water column, cannot differ greatly from 1. If the ratio carbon/dry matter of 50% is considered as universal, the only variable factor is the dry matter content. However, this factor cannot exceed 100%. Thus, the highest conversion factor cannot be very much larger than 500 fgCµm-3. Considering all these available data the extreme values of conversion factor will be in the range of 50 to 500 fgCµm-3.

As a consequence of the low correlation between carbon per cell and cell volume, a constant cell mass would seem to be a logical choice (Berger et al., 1995; Trousselier et al., 1997). However, cell mass is also subject to controversy. Lee and Fuhrman (1987) found a value of about 20 fgCcell-1 for small marine bacteria while Kroer (1994) found 117 fgCcell-1 for estuarine bacteria. Many other values has been proposed : 7-31 fgCcell-1 (Fagerbakke et al., 1996), 26 fgCcell-1 (Trousselier et al. 1997), 35 fgCcell-1 (Theil-Nielsen and Søndergaard, 1997), 82 fgCcell-1 (Bjørnsen, 1986). If, for some specific species, cell mass will be rather constant during cell volume decreases associated with starvation (Trousselier et al. 1997), for an assemblage of different species cell mass will remain dependent of cell volume (Gazol et al., 1995 ; Pernthaler et al., 1996 ; Theil-Nielsen and Søndergaard, 1997). Considering all these observations the extreme values of bacterial cell mass will be in the range of 20 to 120 fgCcell-1.

Results obtained by the two calculation modes are relatively different (Figures 5 and 7). Lowest values are of the same order of magnitude but the highest values are higher with the cell number calculation than with the cell volume one. The differences will be related with the discrepancy between bacterial abundance and cell volume. Largest cells are not necessarily observed during the maximal abundance periods. This discrepancy should also explain the differences observed in the general repartition of the bacterial biomass. Finally the data obtained by the two calculation modes agree relatively well. In any case, using the smallest values obtained, the integrated bacterial biomass in the upper 100 m will be higher than 200 mgCm-2. This value is of same order of magnitude than the phytoplankton biomass (38.8 to 852.1 mg C m-2 , with a mean value of 225.3 mg C m-2, Fiala et al. in press). Thus the bacterial biomass will at least be equivalent to that of phytoplankton in the surface layers. If integrated on the whole water column bacterial biomass will reach values higher than 1 g C m-2 which are obviously higher than autotrophic biomass. Estimates of phytoplankton primary production to community respiration ratios suggest that many parts of the ocean margins show a net heterotrophic activity in all but the coldest season (Smith & Mackenzie, 1987). Seasonal changes in growth rates and respiratory demands of aerobic heterotrophic bacteria, which dominate total community respiration, can induce changes from heterotrophy to autotrophy (Hopkinson, 1985; Griffith et al., 1990; Wiebe et al., 1993). Thus such "inverted pyramid" where heterotrophs exceed autotrophs is not surprising (see Gazol et al. 1997, for a review) and will be an indication of the oligotrophic characteristics of this part of the Southern Ocean.

Subantarctic bacterial biomass recorded during bacterial blooms in the Kerguelen coastal area are among the highest ever recorded (Delille, 1990; Delille et al., 1996). The present data were always smaller during this more offshore study. However, they remain relatively high when compared with some field data that are available for Antarctic Ocean (Hodson et al., 1981; Cota et al., 1990; Goeyens et al., 1991; Delille, 1992). They are consistent when compared to some values reported for Bransfield Strait (4 to 28 µg C l-1, Karl et al., 1991; 8 to 34 µg C l-1, Vosjan and Olanczuk-Neyman, 1991), southern Antarctic Pacific zone (9 to 82 µg C l-1, Sazhin, 1993) and Terre Adélie coastal area (1 to 30 µg C l-1, Delille, 1993).

Acknowledgements

This work was supported by CNRS-INSU (JGOFS-France program) and IFRTP (Insitut Français pour la Recherche et la Technologie Polaires). I thanks P. Guyot, G. Du Réau and J. Maison who collected samples during the 3 successive years of the study.

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Figure captions

 Location of the time-series station Kerfix (50° 40''S - 68° 25' E), 60 miles Southwest off Kerguelen Islands. The mean position of the Polar Front (PF) is indicated according to Park and Charriaud (in press).