influence possible sur les taux de croissance et rythmes des communautés bactériennes dans chacune de ces
masses d’eau est discutée.
Mots clés: Taux de croissance, bactéries, variations journalières, in situ, fronts.
INTRODUCTION
There have been only a few reports of diel changes in the growth of planktonic bacteria.
Diurnal variation of bacterial biovolumes and cell numbers (Krambeck, 1978 & 1984;
Krambeck et ai, 1981) and bacterial uptake of glucose and algal exudates (Straskrabova
& Fuksa, 1982) have been found in freshwater environments. Meyer-Reil et al., (1979)
examined one water body in the Baltic Sea and found fastest glucose turnover times in the
evening. Lochte (1985), however, found no distinct diel patterns in the heterotrophic
turnover of glucose in a water body in the Irish Sea marked with a drogue. Both authors
point out the difficulty of being sure that the same water body is sampled.
Rieman et ai, (1984) found diel changes in bacterial growth by directly monitoring water
samples enclosed in bottles. However, enclosure in bottles can result in nutrient limitation, the degree of which varies with the water sample (Turley & Lochte, 1985).
By enclosing natural bacterial communities in dialysis bags and incubating them in situ
we have attempted to overcome both the problem of following the same water body and
that of nutient limitation in ‘closed’ glass enclosures. Assimilable dissolved organic
carbon diffuses through the dialysis membrane to allow exponential growth of the
bacterial community. This represents a simple and reproducible method of studying the in
situ growth and production rates of marine planktonic bacteria (Turley & Lochte 1985).
The western Irish Sea front (Fig. 1 a) is a shallow sea tidal mixing front. The waters to the
northwest become stratified during spring and summer due to increased solar heating
while the waters to the southeast remain tidally mixed throughout the year. The front is
the gradient interface between the stratified and mixed water masses. The distinct water
masses (Fig. lb) associated with the front during spring and summer can therefore be
sampled repeatedly.
Since aspects of the physical, chemical and biological oceanography of the experimental
site have been intensively studied over the last years (Fogg et ai, 1985 a & b ; Lochte, 1985,
Turley, 1985 ; Scrope-Howe& Jones, 1985 ; Egan & Floodgate, 1985 ; Kassab et al, 1985)
it may be possible to relate any changes in bacterial growth to the environment.
In this paper we present some of the data published by Turley & Lochte (1986) in
combination with other data collected in the study site both during the course of the above
experiment and at other periods. This highlights the complexity of the environment the
bacterial community is exposed to, and the variety, the spatial and temporal variability in
the sources of potential substrates available for microheterotrophs.
METHODS
Sea water samples were taken aseptically well above (4 m) and below (60) the thermocline
in the vicinity of the front (station 5. Fig. la) on 6 July 1982. A subsample was filtered
under sterile conditions through 3 µm pore size Nuclepore polycarbonate filters to
remove grazers. One litre dialysis bags were filled with the filtered and unfiltered subsamples and incubated in situ for 48 h. At intervals samples were removed for enumeration of
bacteria and determination of their cell volume and shape by an epifluorescent direct
count technique (Daley & Hobbie, 1975 ; Hobbie et ai, 1977). At the end of the experiment sections of the dialysis tubing were examined by SEM. No colonization or pitting of
110
masses d’eau est discutée.
Mots clés: Taux de croissance, bactéries, variations journalières, in situ, fronts.
INTRODUCTION
There have been only a few reports of diel changes in the growth of planktonic bacteria.
Diurnal variation of bacterial biovolumes and cell numbers (Krambeck, 1978 & 1984;
Krambeck et ai, 1981) and bacterial uptake of glucose and algal exudates (Straskrabova
& Fuksa, 1982) have been found in freshwater environments. Meyer-Reil et al., (1979)
examined one water body in the Baltic Sea and found fastest glucose turnover times in the
evening. Lochte (1985), however, found no distinct diel patterns in the heterotrophic
turnover of glucose in a water body in the Irish Sea marked with a drogue. Both authors
point out the difficulty of being sure that the same water body is sampled.
Rieman et ai, (1984) found diel changes in bacterial growth by directly monitoring water
samples enclosed in bottles. However, enclosure in bottles can result in nutrient limitation, the degree of which varies with the water sample (Turley & Lochte, 1985).
By enclosing natural bacterial communities in dialysis bags and incubating them in situ
we have attempted to overcome both the problem of following the same water body and
that of nutient limitation in ‘closed’ glass enclosures. Assimilable dissolved organic
carbon diffuses through the dialysis membrane to allow exponential growth of the
bacterial community. This represents a simple and reproducible method of studying the in
situ growth and production rates of marine planktonic bacteria (Turley & Lochte 1985).
The western Irish Sea front (Fig. 1 a) is a shallow sea tidal mixing front. The waters to the
northwest become stratified during spring and summer due to increased solar heating
while the waters to the southeast remain tidally mixed throughout the year. The front is
the gradient interface between the stratified and mixed water masses. The distinct water
masses (Fig. lb) associated with the front during spring and summer can therefore be
sampled repeatedly.
Since aspects of the physical, chemical and biological oceanography of the experimental
site have been intensively studied over the last years (Fogg et ai, 1985 a & b ; Lochte, 1985,
Turley, 1985 ; Scrope-Howe& Jones, 1985 ; Egan & Floodgate, 1985 ; Kassab et al, 1985)
it may be possible to relate any changes in bacterial growth to the environment.
In this paper we present some of the data published by Turley & Lochte (1986) in
combination with other data collected in the study site both during the course of the above
experiment and at other periods. This highlights the complexity of the environment the
bacterial community is exposed to, and the variety, the spatial and temporal variability in
the sources of potential substrates available for microheterotrophs.
METHODS
Sea water samples were taken aseptically well above (4 m) and below (60) the thermocline
in the vicinity of the front (station 5. Fig. la) on 6 July 1982. A subsample was filtered
under sterile conditions through 3 µm pore size Nuclepore polycarbonate filters to
remove grazers. One litre dialysis bags were filled with the filtered and unfiltered subsamples and incubated in situ for 48 h. At intervals samples were removed for enumeration of
bacteria and determination of their cell volume and shape by an epifluorescent direct
count technique (Daley & Hobbie, 1975 ; Hobbie et ai, 1977). At the end of the experiment sections of the dialysis tubing were examined by SEM. No colonization or pitting of
110