Discussion and Conclusion
This study offers, for the fIrst time, a picture of
bacterial and HN abundance in the whole
Adriatic Sea. Previous information on bacterial
distribution in the Adriatic Sea is very scarce and
representative only of specmc situations. Turk et
al. (1990) and Del Negro et al. (1995) reported
data on Gulf of Trieste, Fuks et al. (1991) and
Puddu et al. (1998) described seasonal and spatial distribution of bacterial biomass along the
northern Adriatic Sea while Krstulovic et al.
(1998) evaluated bacterial densities in the central
basin. Few data were available about southern
basins (Bregant et al. 1992; La Ferla et al. 1993;
Bregant et al. 1994). The HN distribution was
studied only in coastal areas of the northern
basins (Turk et al. 1990; Solic and Krstulovic
1995).
Regarding the current knowledge of the regulatory controls of bacterial development there
are only few studies along the Adriatic Sea (Turk
et al. 1990; Solie and Krstulovic 1995; Krstulovic
et al.1998) evidencing a strong top-down control
during the warmer period of the year particularly in eutrophic areas.
From our results, bacterial abundance
appeared signmcantly correlated with chlorophyll a in the northern basin in spring, summer
and autumn suggesting an important role exerted by resources derived from photosynthetic
production as reported for autotrophic systems
(Bird and Kalff 1984). The phytoplankton release
of dissolved organic carbon is considered as one
of the primary sources for bacterial growth. In
winter even if the availability of organic resource
decreases due to photolimitation affecting primary producers (Harding et al. 1999), bacterial
abundance was not controlled by nanoflagellate
grazing. The bacteriallHN abundance ratio
reached the maximum value due to HN abundance minimum. The temperature, in fact, seems
to affect protozoan community (Solie and
Krstulovic 1994).
In the central basin the variability of bacterial community could be explained by predatorprey dependence confrrming the observation of
Krstulovic et al. (1998) for the Kastela Bay. In
spring and summer, in fact, a positive correlation
with HN abundance was observed.
In the southern Adriatic basin HN influenced
the bacterial community in summer and winter.
In summer, autumn and winter also the organic
Bacterial Abundance in the Adriatic Sea
483
matter availability, deriving from phytoplankton.
played an important role in the regulation of
bacterial densities.
In the Strait of Otranto a positive correlation
between bacteria and HN were detected in summer and autumn while the chlorophyll a concentration was correlated with bacterial density in
spring, summer and autumn. A correlation
between bacterial and HN abundance was not
always established along the Adriatic basin; in
fact this has not always been noted in fIeld data
(Gasol and Vaque 1993).A lack of correlation can
be caused by different factors like utilization of
alternative preys by HN (Solie and Krstulovic
1995) as photosynthetic picoplankton and particularly cyanobacteria (Caron et al. 1991). The
abundance ratio was in accordance with 2-3
orders of magnitude as a general difference
between bacterial and HN abundance (Fenchel
1986; Sanders et al. 1992). In all the basins the
ratio showed the same trend: values increased
from spring to summer and then progressively
decreased. Only in the northern Adriatic the bacterial/HN abundance ratio reached the maximum value in winter.
In conclusion we can hypothesize that in the
Northern Adriatic Sea the microbial loop structure was clearly influenced by phytoplankton as
producer of DOM as prey for HN. In the central
and southern basins as well as in the Otranto
Strait HN abundance influenced bacterial community development in spring-summer, summer-winter and summer-autumn periods respectively. according to Solic and Krstulovic (1995). In
the southern basins the bacterial community
seemed also to be influenced by organic substrate
availability. In these areas the lack of nutrients
(Gacic et al.1999) could favour a trophic web consisting of small producer cells as picophytoplankton, which is more efficient in the utilization of
the resources in oligotrophic systems (Thingstad
and Sackshaug 1990) and grazed efficiently by
HN as alternative prey (Caron et al. 1991).
The general increase in the bacterialiHN
abundance ratio from eutrophic to oligotrophic
situations is consistent with the hypothesis of
Sanders et al. (1992) that bacteria should be more
strongly regulated by predation in eutrophic systems. On the other hand, the approach used in
this study did not consider factors of top-down
control, mainly viral lysis and grazing pressure
exerted by ciliates, tintinnids and meso zooplanktonic organisms.
This study offers, for the fIrst time, a picture of
bacterial and HN abundance in the whole
Adriatic Sea. Previous information on bacterial
distribution in the Adriatic Sea is very scarce and
representative only of specmc situations. Turk et
al. (1990) and Del Negro et al. (1995) reported
data on Gulf of Trieste, Fuks et al. (1991) and
Puddu et al. (1998) described seasonal and spatial distribution of bacterial biomass along the
northern Adriatic Sea while Krstulovic et al.
(1998) evaluated bacterial densities in the central
basin. Few data were available about southern
basins (Bregant et al. 1992; La Ferla et al. 1993;
Bregant et al. 1994). The HN distribution was
studied only in coastal areas of the northern
basins (Turk et al. 1990; Solic and Krstulovic
1995).
Regarding the current knowledge of the regulatory controls of bacterial development there
are only few studies along the Adriatic Sea (Turk
et al. 1990; Solie and Krstulovic 1995; Krstulovic
et al.1998) evidencing a strong top-down control
during the warmer period of the year particularly in eutrophic areas.
From our results, bacterial abundance
appeared signmcantly correlated with chlorophyll a in the northern basin in spring, summer
and autumn suggesting an important role exerted by resources derived from photosynthetic
production as reported for autotrophic systems
(Bird and Kalff 1984). The phytoplankton release
of dissolved organic carbon is considered as one
of the primary sources for bacterial growth. In
winter even if the availability of organic resource
decreases due to photolimitation affecting primary producers (Harding et al. 1999), bacterial
abundance was not controlled by nanoflagellate
grazing. The bacteriallHN abundance ratio
reached the maximum value due to HN abundance minimum. The temperature, in fact, seems
to affect protozoan community (Solie and
Krstulovic 1994).
In the central basin the variability of bacterial community could be explained by predatorprey dependence confrrming the observation of
Krstulovic et al. (1998) for the Kastela Bay. In
spring and summer, in fact, a positive correlation
with HN abundance was observed.
In the southern Adriatic basin HN influenced
the bacterial community in summer and winter.
In summer, autumn and winter also the organic
Bacterial Abundance in the Adriatic Sea
483
matter availability, deriving from phytoplankton.
played an important role in the regulation of
bacterial densities.
In the Strait of Otranto a positive correlation
between bacteria and HN were detected in summer and autumn while the chlorophyll a concentration was correlated with bacterial density in
spring, summer and autumn. A correlation
between bacterial and HN abundance was not
always established along the Adriatic basin; in
fact this has not always been noted in fIeld data
(Gasol and Vaque 1993).A lack of correlation can
be caused by different factors like utilization of
alternative preys by HN (Solie and Krstulovic
1995) as photosynthetic picoplankton and particularly cyanobacteria (Caron et al. 1991). The
abundance ratio was in accordance with 2-3
orders of magnitude as a general difference
between bacterial and HN abundance (Fenchel
1986; Sanders et al. 1992). In all the basins the
ratio showed the same trend: values increased
from spring to summer and then progressively
decreased. Only in the northern Adriatic the bacterial/HN abundance ratio reached the maximum value in winter.
In conclusion we can hypothesize that in the
Northern Adriatic Sea the microbial loop structure was clearly influenced by phytoplankton as
producer of DOM as prey for HN. In the central
and southern basins as well as in the Otranto
Strait HN abundance influenced bacterial community development in spring-summer, summer-winter and summer-autumn periods respectively. according to Solic and Krstulovic (1995). In
the southern basins the bacterial community
seemed also to be influenced by organic substrate
availability. In these areas the lack of nutrients
(Gacic et al.1999) could favour a trophic web consisting of small producer cells as picophytoplankton, which is more efficient in the utilization of
the resources in oligotrophic systems (Thingstad
and Sackshaug 1990) and grazed efficiently by
HN as alternative prey (Caron et al. 1991).
The general increase in the bacterialiHN
abundance ratio from eutrophic to oligotrophic
situations is consistent with the hypothesis of
Sanders et al. (1992) that bacteria should be more
strongly regulated by predation in eutrophic systems. On the other hand, the approach used in
this study did not consider factors of top-down
control, mainly viral lysis and grazing pressure
exerted by ciliates, tintinnids and meso zooplanktonic organisms.