OCEAN SEVEN, MILAN, ITALY). Calibration tests
were also performed monthly with the iodometric method of Winkler (APHA 1975). Soluble
reactive phosphorus (Valderrama 1977) and
nitrate (APHA 1975) were determined on water
subsamples obtained by mixing samples collected from both surface and near-bottom layers.
Macroalgal Biomass, Elemental Composition,
Growth Rates and Oxygen Metabolism
For each sampling date triplicate macro algal
samples were collected quantitatively with a benthic grab from an area of 2 to 10 m 2 of sediment
surface. We computed the areal biomass from the
dry weight and surface area sampled. In 1989
biomass values were not determined. In 1990 we
estimated mean biomass values using the data
reported by Piccoli and Godini (1994). In the laboratory Ulva thalli were cleaned to remove epiphytes, rinsed with tap water to remove salt. and
oven-dried at 70°C to determine the dry weight
(DW). Triplicate wet subsamples (approx. 100
mg) were extracted overnight with 90% acetone
and chlorophyll-a was determined according to
Jensen (1978). Dried subsamples ranging from 20
to 50 mg were analysed for carbon and nitrogen
content by elemental analysis (PERKIN ELMER
4200 CHNS).
In 1994 (Viaroli et a1. 1996) and 1997 net
growth rates (NGR) of Ulva were estimated by
means of incubations of Ulva thalli inside five
cylindrical enclosures positioned 20 cm above
the sediment surface. Enclosures (0=20 cm, volume = 10.5litres) were built using a plastic fencing net (mesh =: 8 mm) to allow free water
exchange. Here we present data of the experiments which were performed from 23 rd January
to 6 th February. 8 th to 17th April, and 27th May to
10 th June 1997. At the beginning of each experiment, each cage was filled with 20 g of wet Ulva
thalli. At the end of each incubation, the wet and
dry weight (DW) was determined. The NGR were
estimated considering an exponential growth
pattern (DeBoer et al. 1978). Since the mesh is
large enough to allow grazing and losses by thallus fragmentation. we consider the measured
rates as the net biomass increase resulting from
the equilibrium between growth and losses.
Along with the NGR measurements, photosynthetic rates were estimated by means of P/I
curves, recorded in the laboratory at selected
Trophic Conditions and Dystrophy in the Sacc:a di Garo Lagoon
469
temperatures. Macroalgal thalli were cut into
small disks (025 mm). Four cylindrical beakers
were then filled with 100 ml of fIltered lagoon
water with five disks added. Beakers were sealed
with floating lids, incubated under stirring at
increasing light intensities (I = 25, 50, 100, 200,
400 and 800 p.Em- 2 s- 1 ) for 30 minutes each and in
the dark for 3 hours. Dissolved oxygen (Winkler's
method, APHA 1975) and temperature were
determined at the beginning and at the end of
each incubation interval. Net oxygen production
was calculated as mgOzg-1h -1. Light was supplied
by a halogen cold white lamp (LAMPITALIA R7S
500 W, ITALY) regulated with a rheostat. Light
intensities were recorded with a quantum radiometric detector with cosine correction (HD 9021
RAD/PAR DELTA OHM, PADOVA, ITALY).
Sedimentary Iron, Phosphorus
and Reduced Sulphur
In the Sacca di Goro, benthic fluxes and features
of the surficial sediment have been analysed regularly since 1994 (Bartoli et aL 1996; Giordani et
al. 1996, 1997). Here we consider as an example,
two samplings conducted on 26 th August 1997,
during the summer hypoxia, and on 27th January
1998. On each date, five sediment cores (i.d. 5 cm,
length 30 cm) were collected at station 17 with a
hand corer. Cores were kept in an aerated water
bath until they were sliced (within 1 day). The
upper part of each core was sliced into three sections (0-2, 2-5 and 5-10 cm), here we consider
only the 0-2 cm section.
Iron pools were extracted immediately with
O.5M HC! and analysed by the ferrozine method
(LovIey and Phillips 1987) according to the procedures reported by Giordani et aL (1997).
The pore water P0 4 , the loosely bound P0 4
(exchangeable-P0 4 ), the PO. that is bound to Fe
and Al (Fe-PO 4) and the calcium bound PO 4
(Ca-PO) were determined according to
Ruttemberg (1992).
Reduced sulphur pools, namely acid volatile
sulphide (AVS) and chromium reducible sulphur
(eRS), were determined by means of two
sequential distillations under anoxic conditions
(Fossing and J0rgensen 1989). Free sulphide was
measured by the methylene blue method (Cline
1969).
Sulphate reduction rates (SRR) were determined separately in five sediment cores follow-
were also performed monthly with the iodometric method of Winkler (APHA 1975). Soluble
reactive phosphorus (Valderrama 1977) and
nitrate (APHA 1975) were determined on water
subsamples obtained by mixing samples collected from both surface and near-bottom layers.
Macroalgal Biomass, Elemental Composition,
Growth Rates and Oxygen Metabolism
For each sampling date triplicate macro algal
samples were collected quantitatively with a benthic grab from an area of 2 to 10 m 2 of sediment
surface. We computed the areal biomass from the
dry weight and surface area sampled. In 1989
biomass values were not determined. In 1990 we
estimated mean biomass values using the data
reported by Piccoli and Godini (1994). In the laboratory Ulva thalli were cleaned to remove epiphytes, rinsed with tap water to remove salt. and
oven-dried at 70°C to determine the dry weight
(DW). Triplicate wet subsamples (approx. 100
mg) were extracted overnight with 90% acetone
and chlorophyll-a was determined according to
Jensen (1978). Dried subsamples ranging from 20
to 50 mg were analysed for carbon and nitrogen
content by elemental analysis (PERKIN ELMER
4200 CHNS).
In 1994 (Viaroli et a1. 1996) and 1997 net
growth rates (NGR) of Ulva were estimated by
means of incubations of Ulva thalli inside five
cylindrical enclosures positioned 20 cm above
the sediment surface. Enclosures (0=20 cm, volume = 10.5litres) were built using a plastic fencing net (mesh =: 8 mm) to allow free water
exchange. Here we present data of the experiments which were performed from 23 rd January
to 6 th February. 8 th to 17th April, and 27th May to
10 th June 1997. At the beginning of each experiment, each cage was filled with 20 g of wet Ulva
thalli. At the end of each incubation, the wet and
dry weight (DW) was determined. The NGR were
estimated considering an exponential growth
pattern (DeBoer et al. 1978). Since the mesh is
large enough to allow grazing and losses by thallus fragmentation. we consider the measured
rates as the net biomass increase resulting from
the equilibrium between growth and losses.
Along with the NGR measurements, photosynthetic rates were estimated by means of P/I
curves, recorded in the laboratory at selected
Trophic Conditions and Dystrophy in the Sacc:a di Garo Lagoon
469
temperatures. Macroalgal thalli were cut into
small disks (025 mm). Four cylindrical beakers
were then filled with 100 ml of fIltered lagoon
water with five disks added. Beakers were sealed
with floating lids, incubated under stirring at
increasing light intensities (I = 25, 50, 100, 200,
400 and 800 p.Em- 2 s- 1 ) for 30 minutes each and in
the dark for 3 hours. Dissolved oxygen (Winkler's
method, APHA 1975) and temperature were
determined at the beginning and at the end of
each incubation interval. Net oxygen production
was calculated as mgOzg-1h -1. Light was supplied
by a halogen cold white lamp (LAMPITALIA R7S
500 W, ITALY) regulated with a rheostat. Light
intensities were recorded with a quantum radiometric detector with cosine correction (HD 9021
RAD/PAR DELTA OHM, PADOVA, ITALY).
Sedimentary Iron, Phosphorus
and Reduced Sulphur
In the Sacca di Goro, benthic fluxes and features
of the surficial sediment have been analysed regularly since 1994 (Bartoli et aL 1996; Giordani et
al. 1996, 1997). Here we consider as an example,
two samplings conducted on 26 th August 1997,
during the summer hypoxia, and on 27th January
1998. On each date, five sediment cores (i.d. 5 cm,
length 30 cm) were collected at station 17 with a
hand corer. Cores were kept in an aerated water
bath until they were sliced (within 1 day). The
upper part of each core was sliced into three sections (0-2, 2-5 and 5-10 cm), here we consider
only the 0-2 cm section.
Iron pools were extracted immediately with
O.5M HC! and analysed by the ferrozine method
(LovIey and Phillips 1987) according to the procedures reported by Giordani et aL (1997).
The pore water P0 4 , the loosely bound P0 4
(exchangeable-P0 4 ), the PO. that is bound to Fe
and Al (Fe-PO 4) and the calcium bound PO 4
(Ca-PO) were determined according to
Ruttemberg (1992).
Reduced sulphur pools, namely acid volatile
sulphide (AVS) and chromium reducible sulphur
(eRS), were determined by means of two
sequential distillations under anoxic conditions
(Fossing and J0rgensen 1989). Free sulphide was
measured by the methylene blue method (Cline
1969).
Sulphate reduction rates (SRR) were determined separately in five sediment cores follow-