462
A. Puddu et al.
glucose produced in both cases maximum values
in leucine incorporation rates with increasing
factors with respect to the control of 8.6 and 7.6.
It is worth noting that the only addition of glucose did not produce any significant stimulation
in the June 1998 sample.
Discussion
The DOC concentrations in northern Adriatic
waters show seasonal changes with the lowest
values in winter (74±2 pM for bottom samples)
comparable to DOC levels of surface oceanic
waters (70-80 }lM; Guo et aI. 1995). Our surface
DOC values in winter (92±5 }lM) correspond
to maxima values measured over a 12 month
period by Copin-Montegut and Avril (1993)
in the north western Mediterranean basin
between 0 and 50 m depths (92-100 }lM). Deep
Mediterranean waters (150-2000 m depth)
showed in this study values in the range 50 to 58
p.M. Our summer DOC concentrations are significantly higher than winter values, although
the increasing factors in the two investigated
years are different. Average surface values in June
1996 and 1991 are a factor of 2.0 and 1.3 higher
than those recorded in the respective following
February surveys. Thus, degradation processes
and water mass exchanges between the northern
and middle Adriatic are able to keep DOC concentrations at low levels despite the significant
external (50 10 4 tons year-I; Pettine et ala 1999)
and internal (170 10 4 t year l > Degobbis and
Gilmartin 1990; or 320 10 4 t year- 1 estimated
from recent PRISMA-1 results, not yet published) organic matter loads which affect this
basin. It may be noted that the summer of 1997
was again characterized by massive mucilage
occurrences. These did not reach the extension
recorded in 1988,1989 and 1991 years, but were
markedly more intensive compared to those of
1996 summer (Degobbis et al. in press). The
presence of mucilaginous materials may scavenge dissolved materials favouring their transfer
to the particulate phase. This might have contributed to the marked differences in concentrations between the two investigated summers.
That DOC concentrations show seasonal
changes also results from previous findings. In
the Gulf of Trieste DOC was found to vary in the
ranges 102-297 and 113-176 }lM in September
1994 and June 1995, respectively (Fonda Umani
et ala 1997), while in the same gulf Faganeli and
Herndl (1991) found distinct seasonal variations
ranging from 66-154}lM during the autumn and
winter to up to 500 }lM during the summer in
the Istrian coast. Finally, Vojvodic and Cosovic
(1996) found that DOC concentrations varied
approximately from 100-140 to 130-200 }lM in
winter and summer, respectively, over the period
1984-1993.
Average bacterial carbon production rates
represent, according to values reported by
Ducklow and Carlson (1992) for the euphotic
zones of major marine habitats. a variety of situations from estuaries (1.8-3.3 }lg C 1-1 h- 1 ) to the
ocean (0.1-0.2 }lg C 1-1 h- 1 ). The high range of
bacterial production rates measured in this area,
0.2-177 pM thymidine incorporated h- 1 , has a
parallel only in the estuarine environment of the
Chesapeake Bay (0.5-500 pM TdR h-l, Ducklow
and Shiah 1993). This wide variability reflects
relatively quick adaptations of microorganisms
to environmental changes in the water column
produced by biological processes and external
forcing. Bacterioplankton reacts primarily to
variations in the dissolved organic substrate
composition. The BCP variations observed are
consistent with increased DOC concentrations in
surface waters, which are more marked in summer than winter. and with a high bioavailability
of fresh biogenic organic matter resulting from
exudation or sloppy feeding compared to aged
products dominant in bottom waters. Previous
findings have shown a lack of synchrony in the
cycles of activity which characterizes heterotrophic bacteria and autotrophic phytoplankton in this area in different seasonal periods
(Puddu et al. 1998). Die! variations in the bacterioplankton activity also reflect short-term variations in enzymatic activities and diel periodicity
in bacterivorous flagellates which influence bacterial biomass (Riemann and S(.mdergaard 1984;
Karner and Rassoulzadegan 1995).
As shown in Fig. 6, where the behaviour of
bacterial productions are reported as a function
of DOC, BCP variations show in all cruises a tendency to increase with increasing DOC concentrations, but the slope of the linear regression in
June 1996 (0.098, P<0.01) is a factor of about 0.5
compared to those in June 1997 (0.202, P
BCP versus DOC slopes reflects differences in the
quality of available organic substrates and in the
nutrient optimal ratios which are required for
A. Puddu et al.
glucose produced in both cases maximum values
in leucine incorporation rates with increasing
factors with respect to the control of 8.6 and 7.6.
It is worth noting that the only addition of glucose did not produce any significant stimulation
in the June 1998 sample.
Discussion
The DOC concentrations in northern Adriatic
waters show seasonal changes with the lowest
values in winter (74±2 pM for bottom samples)
comparable to DOC levels of surface oceanic
waters (70-80 }lM; Guo et aI. 1995). Our surface
DOC values in winter (92±5 }lM) correspond
to maxima values measured over a 12 month
period by Copin-Montegut and Avril (1993)
in the north western Mediterranean basin
between 0 and 50 m depths (92-100 }lM). Deep
Mediterranean waters (150-2000 m depth)
showed in this study values in the range 50 to 58
p.M. Our summer DOC concentrations are significantly higher than winter values, although
the increasing factors in the two investigated
years are different. Average surface values in June
1996 and 1991 are a factor of 2.0 and 1.3 higher
than those recorded in the respective following
February surveys. Thus, degradation processes
and water mass exchanges between the northern
and middle Adriatic are able to keep DOC concentrations at low levels despite the significant
external (50 10 4 tons year-I; Pettine et ala 1999)
and internal (170 10 4 t year l > Degobbis and
Gilmartin 1990; or 320 10 4 t year- 1 estimated
from recent PRISMA-1 results, not yet published) organic matter loads which affect this
basin. It may be noted that the summer of 1997
was again characterized by massive mucilage
occurrences. These did not reach the extension
recorded in 1988,1989 and 1991 years, but were
markedly more intensive compared to those of
1996 summer (Degobbis et al. in press). The
presence of mucilaginous materials may scavenge dissolved materials favouring their transfer
to the particulate phase. This might have contributed to the marked differences in concentrations between the two investigated summers.
That DOC concentrations show seasonal
changes also results from previous findings. In
the Gulf of Trieste DOC was found to vary in the
ranges 102-297 and 113-176 }lM in September
1994 and June 1995, respectively (Fonda Umani
et ala 1997), while in the same gulf Faganeli and
Herndl (1991) found distinct seasonal variations
ranging from 66-154}lM during the autumn and
winter to up to 500 }lM during the summer in
the Istrian coast. Finally, Vojvodic and Cosovic
(1996) found that DOC concentrations varied
approximately from 100-140 to 130-200 }lM in
winter and summer, respectively, over the period
1984-1993.
Average bacterial carbon production rates
represent, according to values reported by
Ducklow and Carlson (1992) for the euphotic
zones of major marine habitats. a variety of situations from estuaries (1.8-3.3 }lg C 1-1 h- 1 ) to the
ocean (0.1-0.2 }lg C 1-1 h- 1 ). The high range of
bacterial production rates measured in this area,
0.2-177 pM thymidine incorporated h- 1 , has a
parallel only in the estuarine environment of the
Chesapeake Bay (0.5-500 pM TdR h-l, Ducklow
and Shiah 1993). This wide variability reflects
relatively quick adaptations of microorganisms
to environmental changes in the water column
produced by biological processes and external
forcing. Bacterioplankton reacts primarily to
variations in the dissolved organic substrate
composition. The BCP variations observed are
consistent with increased DOC concentrations in
surface waters, which are more marked in summer than winter. and with a high bioavailability
of fresh biogenic organic matter resulting from
exudation or sloppy feeding compared to aged
products dominant in bottom waters. Previous
findings have shown a lack of synchrony in the
cycles of activity which characterizes heterotrophic bacteria and autotrophic phytoplankton in this area in different seasonal periods
(Puddu et al. 1998). Die! variations in the bacterioplankton activity also reflect short-term variations in enzymatic activities and diel periodicity
in bacterivorous flagellates which influence bacterial biomass (Riemann and S(.mdergaard 1984;
Karner and Rassoulzadegan 1995).
As shown in Fig. 6, where the behaviour of
bacterial productions are reported as a function
of DOC, BCP variations show in all cruises a tendency to increase with increasing DOC concentrations, but the slope of the linear regression in
June 1996 (0.098, P<0.01) is a factor of about 0.5
compared to those in June 1997 (0.202, P
BCP versus DOC slopes reflects differences in the
quality of available organic substrates and in the
nutrient optimal ratios which are required for