456
A. Puddu et 81.
regions over all the Mediterranean sea and its
coastal waters are widely exploited for tourism.
In recent years this basin has been affected by
massive occurrences of mucilaginous aggregates
which have heavily affected fishing activity and
tourism and produced serious damage to the
ecosystem (Degobbis et al. 1995). Then, it is a
priority task to go deep into processes responsible for these phenomena. The hypotheses proposed to explain the origin of mucilage aggregates took into account a high number of variables, namely: trophic conditions of the northern
Adriatic, high NIP ratios, strong stratification
during the spring-summer period, limited grazing control of algal growth by zooplankton, suitable hydrodynamic conditions (calm sea, slow
currents and wind speeds), changes in the community structure with increased dominance of
diatoms, modifications in environmental conditions induced by P-substitutes in detergency
products, the presence of high C/P and slow-todegrade organic matter due to a faster cycle of P
compared to C, mucus excretion from bacterial
capsules (Degobbis et al. in press; Pettine et al.
1995). All these factors may indeed cooperate in
the formation of organo-mineral particles, of
various extents, which are difficult to quantify.
However, in order that aggregates become very
large and acquire a mucilaginous property particular conditions are needed. Particles have
either to become entrapped in a polysaccharide
matrix, generated by plankton and bacteria, or to
undergo surface modifications leading to the
production of sticking material which favours
coagulation-flocculation processes.
This paper presents data on dissolved organic carbon, colloidal organic carbon and bacterial
activity in the context of the PRISMA-2 research
project. subproject «Biogeochemical cycles",
whose goal was to evaluate biological and chemical phenomena affecting C cycle along the western coast of the northern Adriatic. Measurements
were performed in frontal regions defined by
sharp gradients between freshwater inputs
(mainly due to the Po discharge) and the waters
of the interior basin. Temporal and spatial variability of data and possible relationships among
these variables are discussed. Results of laboratory DOC degradation runs on samples, spiked
with inorganic and organic nutrients, are also
discussed to highlight possible limitations of bacterial activity which would favour DOC accumulation in the field.
Material and Methods
Water samples were collected with Niskin bottles
on a rosette with a Sea-Bird CTD system in two
frontal regions in the northern Adriatic basin
(Fig. 1) during four cruises on the R!V Urania
(June 1996, February and June 1997 and February
1998). In every area and in each cruise, sampling
was conducted in a grid of stations along 2-4
transects across the front at 4-6 depths. A bench
salinometer Autosal Guildline was used to measure the bottle salinities. The sampling generally
took 6 days per grid to be completed. Samples
were chosen in order to be representative of different water masses in the frontal region. The
total number of measurements was 401,232 and
30, respectively for DOC, BCP and COCo
For the analysis of dissolved organic carbon,
water samples were mtered aboard, immediately
after collection, through precombusted (4 hours
at 480 D C) Whatman GF/F glass fibre filters (0.7
pM nominal pore size) and transferred directly
in duplicate into 25 mL high density polyethylene (HDPE) bottles, previously treated with
HNO) 1.2 M at 50 n C for 1 h. The HDPE bottles
were quick-frozen in an aluminium block at
_ZODC. In the laboratory, filtered samples were
thawed, acidified to pH 2 with ultrapure HCI and
purged with N2 for about 10 minutes to remove
inorganic carbon. The DOC was assayed by high
temperature catalytic oxidation (HTCO) using a
Shimadzu TOC-SOOO A analyser and. on a limited
number of samples, it was fractionated in dissolved « 1 kDa) and colloidal (> 1 kDa) organic
carbon (Pettine et a!. 1999).
Bacterial production was measured on natural samples by the incorporation of 3H-thymidine in all cruises and 3H-Ieucine only in '97 and
'98 cruises. Triplicate 1.7 mL aliquots and one
killed control (TCA 5%) were spiked with 20 nM
(final concentration) radiotracer and incubated
for 1 h. from 10 to 11 am local time, at in situ
temperature. The extraction, with 5% ice-cold
TCA for thymidine and room temperature 5%
TCA for leucine and subsequent washing with
5% TCA and 80% ethanol, was performed by the
microcentrifugation method, according to Smith
and Azam (1992). To estimate the bacterial carbon production from rates of thymidine incorporated (BCPTdR), the most widely used conversion factor of 2 x 10 18 cells per mole thymidine
incorporated (Bell 1993) and a bacteria-to-carbon conversion factor of 20 fgC cell- l were used
A. Puddu et 81.
regions over all the Mediterranean sea and its
coastal waters are widely exploited for tourism.
In recent years this basin has been affected by
massive occurrences of mucilaginous aggregates
which have heavily affected fishing activity and
tourism and produced serious damage to the
ecosystem (Degobbis et al. 1995). Then, it is a
priority task to go deep into processes responsible for these phenomena. The hypotheses proposed to explain the origin of mucilage aggregates took into account a high number of variables, namely: trophic conditions of the northern
Adriatic, high NIP ratios, strong stratification
during the spring-summer period, limited grazing control of algal growth by zooplankton, suitable hydrodynamic conditions (calm sea, slow
currents and wind speeds), changes in the community structure with increased dominance of
diatoms, modifications in environmental conditions induced by P-substitutes in detergency
products, the presence of high C/P and slow-todegrade organic matter due to a faster cycle of P
compared to C, mucus excretion from bacterial
capsules (Degobbis et al. in press; Pettine et al.
1995). All these factors may indeed cooperate in
the formation of organo-mineral particles, of
various extents, which are difficult to quantify.
However, in order that aggregates become very
large and acquire a mucilaginous property particular conditions are needed. Particles have
either to become entrapped in a polysaccharide
matrix, generated by plankton and bacteria, or to
undergo surface modifications leading to the
production of sticking material which favours
coagulation-flocculation processes.
This paper presents data on dissolved organic carbon, colloidal organic carbon and bacterial
activity in the context of the PRISMA-2 research
project. subproject «Biogeochemical cycles",
whose goal was to evaluate biological and chemical phenomena affecting C cycle along the western coast of the northern Adriatic. Measurements
were performed in frontal regions defined by
sharp gradients between freshwater inputs
(mainly due to the Po discharge) and the waters
of the interior basin. Temporal and spatial variability of data and possible relationships among
these variables are discussed. Results of laboratory DOC degradation runs on samples, spiked
with inorganic and organic nutrients, are also
discussed to highlight possible limitations of bacterial activity which would favour DOC accumulation in the field.
Material and Methods
Water samples were collected with Niskin bottles
on a rosette with a Sea-Bird CTD system in two
frontal regions in the northern Adriatic basin
(Fig. 1) during four cruises on the R!V Urania
(June 1996, February and June 1997 and February
1998). In every area and in each cruise, sampling
was conducted in a grid of stations along 2-4
transects across the front at 4-6 depths. A bench
salinometer Autosal Guildline was used to measure the bottle salinities. The sampling generally
took 6 days per grid to be completed. Samples
were chosen in order to be representative of different water masses in the frontal region. The
total number of measurements was 401,232 and
30, respectively for DOC, BCP and COCo
For the analysis of dissolved organic carbon,
water samples were mtered aboard, immediately
after collection, through precombusted (4 hours
at 480 D C) Whatman GF/F glass fibre filters (0.7
pM nominal pore size) and transferred directly
in duplicate into 25 mL high density polyethylene (HDPE) bottles, previously treated with
HNO) 1.2 M at 50 n C for 1 h. The HDPE bottles
were quick-frozen in an aluminium block at
_ZODC. In the laboratory, filtered samples were
thawed, acidified to pH 2 with ultrapure HCI and
purged with N2 for about 10 minutes to remove
inorganic carbon. The DOC was assayed by high
temperature catalytic oxidation (HTCO) using a
Shimadzu TOC-SOOO A analyser and. on a limited
number of samples, it was fractionated in dissolved « 1 kDa) and colloidal (> 1 kDa) organic
carbon (Pettine et a!. 1999).
Bacterial production was measured on natural samples by the incorporation of 3H-thymidine in all cruises and 3H-Ieucine only in '97 and
'98 cruises. Triplicate 1.7 mL aliquots and one
killed control (TCA 5%) were spiked with 20 nM
(final concentration) radiotracer and incubated
for 1 h. from 10 to 11 am local time, at in situ
temperature. The extraction, with 5% ice-cold
TCA for thymidine and room temperature 5%
TCA for leucine and subsequent washing with
5% TCA and 80% ethanol, was performed by the
microcentrifugation method, according to Smith
and Azam (1992). To estimate the bacterial carbon production from rates of thymidine incorporated (BCPTdR), the most widely used conversion factor of 2 x 10 18 cells per mole thymidine
incorporated (Bell 1993) and a bacteria-to-carbon conversion factor of 20 fgC cell- l were used