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M. Bartoli et al.
water fluxes of DIN through their assimilation
activity (Rysgaard et al. 1996) and nitrificationdenitrification processes within the sediment
(Caffrey and Kemp 1992; Risgaard-Petersen
and Jensen 1997). The roots of submerged
phanerogams can excrete organic carbon
(Sondergaard 1983) and transport ° 1 to the sediment (Sand-Jensen and Prahl 1982; Christensen
et al. 1994; Risgaard-Petersen and Jensen 1997).
This can create an oxic layer around the roots
resulting in optimal nitrification area (the availability of NH/ is higher in the deep sediment);
indirectly 02 transport can stimulate denitrification when the produced NO s - diffuses to the
anoxic sediment. This was clearly demonstrated
in freshwater phanerogams (Risgaard-Petersen
and Jensen 1997). However, aquatic plants also
assimilate and incorporate DIN from the leaves
and from the roots (Pedersen and Borum 1992)
and thereby compete with nitrifiers and denitrifiers for inorganic nitrogen. The net effect of
phanerogams on nitrification-denitrification
processes depends on the balance between DIN
uptake, DIN availability in the water column,
NH/ production in the sediment, 02 release and
the redox state of the sediment.
The role of primary producers and nitrifying-denitrifying bacteria on nitrogen transformations can be particularly strong in semienclosed bays and lagoons receiving high freshwater N-Ioads and where the water renewal is
slow. In these areas the retention of DIN in the
biomass and N -burial in the sediment as well as
the conversion of N0 3 - to N2 can quantitatively
attenuate the flux of nitrogen through the coastal
environment to the sea.
In this paper we consider oxygen and DIN
fluxes, and coupled-uncoupled denitrification
rates in three coastal areas of the North Adriatic
Sea during three critical phases of the seasonal
evolution of the benthic system. The investigated
areas were colonised respectively by benthic
diatoms (station Giralda), the macroalga Ulva
rigida (station Gorino) and the phanerogam
Ruppia cirrhosa (station Smarlacca).
Study Area
Station Giralda is located in the western part of
the Sacca di Goro (Po River Delta, Northern
Italy), dose to the freshwater inlet of the Po di
Volano Canal. The station is micro-tidal with
water depths ranging between 30 and 100 em
with a mean depth of approximately 50 cm. and,
although shallow, the water column at high tide
is stratified with freshwater overlaying the salt
water. The surficial sediments consist of a soft
mud colonised by benthic diatoms, salinity of
the bottom water varies between 0 and 25%0
while inorganic nitrogen concentrations range
between 5 and 200 11M. This area of the lagoon
receives an annual load of about 1,000 tons of N
of which 50% is NO.- and NH/ (Orlandi 1998).
Previous studies, have demonstrated a relationship between sediment oxygen demand, macrofaunal densities and suspended matter concentrations in the Po di Volano canal (Bartoli 1996;
Castaldelli 1998). Corophium, for example, may
attain densities of 10,000 indo m- 2 in early spring
(Ceccherelli V.U., personal communication), but
completely disappear during the summer.
Station Gorino is located in the eastern area
of the Sacca di Goro, where dense mats of the
floating macroalga Ulva rigida develop. The sediment is muddy sand and is disturbed due to the
intense harvesting of molluscs, in particular of
clams. During summer this station suffers severe
episodes of anoxia due to the decomposition of
huge amounts of Ulva. Long term monitoring of
this site shows consistent annual cycles, with
nitrates disappearing from the water column
during the entire growth season of Ulva and oxygen concentrations varying from supersaturated
in spring to zero during the summer dystrophic
crises (Viaroli et al. 1996a, b).
Station Smarlacca is a small basin (about 1.9
km2) in the Valli di Comacchio (North-Adriatic
Sea), a wide complex (about 100 km 2 ) of shallow
water impoundments connected to the Adriatic
Sea via a network of canals. Valle Smarlacca is
surrounded by embankments but receives fresh
water inputs from the adjacent Reno River
through man-regulated sluices. Salinity is relatively stable (20 to 22 %0) but can rise to 25-30 %0
due to evaporation in summer. The sediment is
colonised by a dense meadow of the aquatic
phanerogam Ruppia cirrhosa. These seagrass
meadows are often patchy and within the larger
seagrass meadows, areas of different sizes may be
devoid of plants due to disturbance events.
Therefore, it is possible to recognise a mosaic of
patches of bare sediments separated by seagrass
beds.
M. Bartoli et al.
water fluxes of DIN through their assimilation
activity (Rysgaard et al. 1996) and nitrificationdenitrification processes within the sediment
(Caffrey and Kemp 1992; Risgaard-Petersen
and Jensen 1997). The roots of submerged
phanerogams can excrete organic carbon
(Sondergaard 1983) and transport ° 1 to the sediment (Sand-Jensen and Prahl 1982; Christensen
et al. 1994; Risgaard-Petersen and Jensen 1997).
This can create an oxic layer around the roots
resulting in optimal nitrification area (the availability of NH/ is higher in the deep sediment);
indirectly 02 transport can stimulate denitrification when the produced NO s - diffuses to the
anoxic sediment. This was clearly demonstrated
in freshwater phanerogams (Risgaard-Petersen
and Jensen 1997). However, aquatic plants also
assimilate and incorporate DIN from the leaves
and from the roots (Pedersen and Borum 1992)
and thereby compete with nitrifiers and denitrifiers for inorganic nitrogen. The net effect of
phanerogams on nitrification-denitrification
processes depends on the balance between DIN
uptake, DIN availability in the water column,
NH/ production in the sediment, 02 release and
the redox state of the sediment.
The role of primary producers and nitrifying-denitrifying bacteria on nitrogen transformations can be particularly strong in semienclosed bays and lagoons receiving high freshwater N-Ioads and where the water renewal is
slow. In these areas the retention of DIN in the
biomass and N -burial in the sediment as well as
the conversion of N0 3 - to N2 can quantitatively
attenuate the flux of nitrogen through the coastal
environment to the sea.
In this paper we consider oxygen and DIN
fluxes, and coupled-uncoupled denitrification
rates in three coastal areas of the North Adriatic
Sea during three critical phases of the seasonal
evolution of the benthic system. The investigated
areas were colonised respectively by benthic
diatoms (station Giralda), the macroalga Ulva
rigida (station Gorino) and the phanerogam
Ruppia cirrhosa (station Smarlacca).
Study Area
Station Giralda is located in the western part of
the Sacca di Goro (Po River Delta, Northern
Italy), dose to the freshwater inlet of the Po di
Volano Canal. The station is micro-tidal with
water depths ranging between 30 and 100 em
with a mean depth of approximately 50 cm. and,
although shallow, the water column at high tide
is stratified with freshwater overlaying the salt
water. The surficial sediments consist of a soft
mud colonised by benthic diatoms, salinity of
the bottom water varies between 0 and 25%0
while inorganic nitrogen concentrations range
between 5 and 200 11M. This area of the lagoon
receives an annual load of about 1,000 tons of N
of which 50% is NO.- and NH/ (Orlandi 1998).
Previous studies, have demonstrated a relationship between sediment oxygen demand, macrofaunal densities and suspended matter concentrations in the Po di Volano canal (Bartoli 1996;
Castaldelli 1998). Corophium, for example, may
attain densities of 10,000 indo m- 2 in early spring
(Ceccherelli V.U., personal communication), but
completely disappear during the summer.
Station Gorino is located in the eastern area
of the Sacca di Goro, where dense mats of the
floating macroalga Ulva rigida develop. The sediment is muddy sand and is disturbed due to the
intense harvesting of molluscs, in particular of
clams. During summer this station suffers severe
episodes of anoxia due to the decomposition of
huge amounts of Ulva. Long term monitoring of
this site shows consistent annual cycles, with
nitrates disappearing from the water column
during the entire growth season of Ulva and oxygen concentrations varying from supersaturated
in spring to zero during the summer dystrophic
crises (Viaroli et al. 1996a, b).
Station Smarlacca is a small basin (about 1.9
km2) in the Valli di Comacchio (North-Adriatic
Sea), a wide complex (about 100 km 2 ) of shallow
water impoundments connected to the Adriatic
Sea via a network of canals. Valle Smarlacca is
surrounded by embankments but receives fresh
water inputs from the adjacent Reno River
through man-regulated sluices. Salinity is relatively stable (20 to 22 %0) but can rise to 25-30 %0
due to evaporation in summer. The sediment is
colonised by a dense meadow of the aquatic
phanerogam Ruppia cirrhosa. These seagrass
meadows are often patchy and within the larger
seagrass meadows, areas of different sizes may be
devoid of plants due to disturbance events.
Therefore, it is possible to recognise a mosaic of
patches of bare sediments separated by seagrass
beds.