464
A. Puddu et al.
G., personal communication) extend information on P availability to natural samples. Mean
orthophosphate concentrations in the northern
area in June 1996 (0.02±O.003 p.M PO/-) were
found to be significantly lower than in June 1997
(0.04±O.006 p.M PO/-). The very low inorganic
nutrient concentrations observed in June 1996,
possibly due to a "post bloom" situation (Lipizer
et al. 1997), may explain the increase in DOC values with a scarce uptake of DOC by bacteria in
this period. Unusual high bottom bacterial productions measured in the north in February
1997) correspond to inorganic P concentrations
> 0.10 pM PO/-. These levels have been very seldom measured during the project, strengthening
the important role of low phosphorus concentrations in controlling secondary production in our
system.
Compared to data for other marine environments, where eoc concentrations gave percentage contributions to DOC of 10 to 64% (Benner
et al. 1992; Guo et al. 1994,1995; Martin and Dai
1995), our results on colloidal organic matter
show a shift in the molecular weight distribution
toward the large size fraction. Since coe contributions to DOC were found to increase in riverine waters compared to seawaters (76% in the
Amazon river, Benner and Hedges 1993), this
shift is consistent with an important role of
freshwater inputs. The strong correlations of
both DOC and BCP values as a function of salinity along with estimates of a bacterial carbon
demand higher than primary production in low
productivity periods (Puddu et aL 1998) also
strongly support a key role of allochthonous
inputs in controlling the trophic dynamics in this
basin.
Setting up of strong vertical stratification
and eddy circulation, which characterizes the
northern Adriatic in the summer period,
increases freshwater residence time and favours
the accumulation onow degradable organic matter directly discharged by rivers. However, the
accumulation of organic matter in the refractory
pool may partially be counteracted by photoxidation processes which tend to increase the liability of aged DOM (Wetzel et al. 1995). In
spring-summer periods the in situ production of
DOC, mainly due to phytoplankton activity, is
also strongly enhanced and often phosphorus
becomes exhausted after micro algae blooms.
Under these conditions, the fraction of biogenic
carbon which is not immediately degraded may
undergo, with ageing, abiotic processes which
are strongly driven by interactions with terrestrial-derived DOC. The enrichment in organic
polymers and particles gives rise to high colloidal components and organo-mineral aggregates. Under these conditions, the probability
that organic macromolecules spontaneously
assemble to form micro gels and that secondary
annealing of assembled gels occurs is very high,
according to findings by Chin et al. (199B).
Furthermore, the shift we have found in the molecular weight distribution toward the large size
fraction favours DOM assembling, since the
probability of spontaneous assembly of organic
networks increases with the second power of the
polymerlength (Chin et al. 1998). Then, seasonal
increase in DOC concentrations along with its
high colloidal organic fraction and high molecular weight are necessary conditions which may
lead to the formation of mucillaginous aggregates.
Conclusions
Labile to refractory conversions of aged organic
matter and a restricted bacterial consumption of
DOC are two interrelated factors which concur,
along with refractory matter directly discharged
by rivers and escaping photoxidation processes,
to the observed seasonal DOC enrichment in the
North Adriatic. Grazing or viral infections may
also contribute in limiting bacterial activity, but
information on this matter is very scarce. While
we have environmental conditions (high COC
levels and high molecular weights) which are
apparently favourable to the formation of
mucilaginous aggregates in our system, their
occurrence shows wide interannual variations
(in size and weight) which are difficult to explain
only on a chemico-physical basis. However,
mucilaginous aggregates are not simply physicostructural components which add to the liquid
and particulate matrices; they behave as preferential microhabitats due to their strong scavenging properties which make them rich in nutrients compared to surrounding water. High primary and secondary productions have been
measured inside these aggregates in previous
studies (Muller et al. 1994) and biological
processes may profoundly influence the fate of
aggregates. A high residence time in the water
column and low wind-induced mixing are
A. Puddu et al.
G., personal communication) extend information on P availability to natural samples. Mean
orthophosphate concentrations in the northern
area in June 1996 (0.02±O.003 p.M PO/-) were
found to be significantly lower than in June 1997
(0.04±O.006 p.M PO/-). The very low inorganic
nutrient concentrations observed in June 1996,
possibly due to a "post bloom" situation (Lipizer
et al. 1997), may explain the increase in DOC values with a scarce uptake of DOC by bacteria in
this period. Unusual high bottom bacterial productions measured in the north in February
1997) correspond to inorganic P concentrations
> 0.10 pM PO/-. These levels have been very seldom measured during the project, strengthening
the important role of low phosphorus concentrations in controlling secondary production in our
system.
Compared to data for other marine environments, where eoc concentrations gave percentage contributions to DOC of 10 to 64% (Benner
et al. 1992; Guo et al. 1994,1995; Martin and Dai
1995), our results on colloidal organic matter
show a shift in the molecular weight distribution
toward the large size fraction. Since coe contributions to DOC were found to increase in riverine waters compared to seawaters (76% in the
Amazon river, Benner and Hedges 1993), this
shift is consistent with an important role of
freshwater inputs. The strong correlations of
both DOC and BCP values as a function of salinity along with estimates of a bacterial carbon
demand higher than primary production in low
productivity periods (Puddu et aL 1998) also
strongly support a key role of allochthonous
inputs in controlling the trophic dynamics in this
basin.
Setting up of strong vertical stratification
and eddy circulation, which characterizes the
northern Adriatic in the summer period,
increases freshwater residence time and favours
the accumulation onow degradable organic matter directly discharged by rivers. However, the
accumulation of organic matter in the refractory
pool may partially be counteracted by photoxidation processes which tend to increase the liability of aged DOM (Wetzel et al. 1995). In
spring-summer periods the in situ production of
DOC, mainly due to phytoplankton activity, is
also strongly enhanced and often phosphorus
becomes exhausted after micro algae blooms.
Under these conditions, the fraction of biogenic
carbon which is not immediately degraded may
undergo, with ageing, abiotic processes which
are strongly driven by interactions with terrestrial-derived DOC. The enrichment in organic
polymers and particles gives rise to high colloidal components and organo-mineral aggregates. Under these conditions, the probability
that organic macromolecules spontaneously
assemble to form micro gels and that secondary
annealing of assembled gels occurs is very high,
according to findings by Chin et al. (199B).
Furthermore, the shift we have found in the molecular weight distribution toward the large size
fraction favours DOM assembling, since the
probability of spontaneous assembly of organic
networks increases with the second power of the
polymerlength (Chin et al. 1998). Then, seasonal
increase in DOC concentrations along with its
high colloidal organic fraction and high molecular weight are necessary conditions which may
lead to the formation of mucillaginous aggregates.
Conclusions
Labile to refractory conversions of aged organic
matter and a restricted bacterial consumption of
DOC are two interrelated factors which concur,
along with refractory matter directly discharged
by rivers and escaping photoxidation processes,
to the observed seasonal DOC enrichment in the
North Adriatic. Grazing or viral infections may
also contribute in limiting bacterial activity, but
information on this matter is very scarce. While
we have environmental conditions (high COC
levels and high molecular weights) which are
apparently favourable to the formation of
mucilaginous aggregates in our system, their
occurrence shows wide interannual variations
(in size and weight) which are difficult to explain
only on a chemico-physical basis. However,
mucilaginous aggregates are not simply physicostructural components which add to the liquid
and particulate matrices; they behave as preferential microhabitats due to their strong scavenging properties which make them rich in nutrients compared to surrounding water. High primary and secondary productions have been
measured inside these aggregates in previous
studies (Muller et al. 1994) and biological
processes may profoundly influence the fate of
aggregates. A high residence time in the water
column and low wind-induced mixing are