Fluxes and Denitrification within Communities of Different Primary Producers
227
Materials and Methods
Fluxes of DIN (Dissolved Inorganic Nitrogen)
and 02 across the sediment-water interface and
coupled-uncoupled denitrification rates were
measured during light and dark incubations of
intact sediment cores. Distinct incubation systems were used for the different sites: relatively
small cores were used for micro algae-covered
sediments while larger cores and chambers were
used respectively for phanerogam and macroalgae dominated systems. All incubation methods
were tested and standardized before starting the
investigation. All sediment cores, whether containing microphytes, rooted macrophytes or
floating macroalgae were incubated the day after
the sampling using in situ water at in situ oxygen
concentration and temperature. Before starting
the incubations the cores were maintained submersed in large tanks without the top lid and
with the stirring system on to allow renewal of
the water inside the cores. With the microalgaecovered sediment, Plexiglas cores (i.d. 8 cm,
height 40 cm) were used for flux and denitrification measurements; a 4 em long Teflon-coated
magnetic stirring bar was suspended 6 em above
the sediment surface. The stirring was due to an
external rotating magnet. In the macroalgae-covered sediment flux and denitrification measurements were performed in Plexiglas chambers
(base 20xlO em, working height 40 em), stirring
was created by drawing water out through a
water dispersal unit placed at one side of the
chamber and pumping it back at the other side.
Undisturbed sediment with macroalgal mat
(when present) was obtained with a hand held
box corer pushed through the algal mat into the
sediment. At the site covered by rooted macrophytes Plexiglas cores (Ld. 20 em, height 40 cm)
were used; stirring of the water column was
obtained with a small aquarium pump placed
inside the core. In all the incubations water-sediment fluxes of ° 2 , NH4 + and NO 3 - were measured
as concentration changes in the water with time
according to Eq. (1)
Flux = (a "" V) / A
(1)
where:
flux is expressed in }lmol of the species of interest m- 2 h- 1 ;
a = slope of the linear regression of concentration versus time;
A = area of sediment surface in core/chamber
(m 2 );
V = volume of water in core/chamber (1).
Denitrification rates were measured by the
isotope pairing technique (Nielsen 1992) adding
15N0 3 - to the water column and then measuring
the produced labelled N 2 ; this method allows to
determine coupled (Dn) and uncoupled denitrification (Dw). Uncoupled denitrification (Dw) is
denitrification of nitrate diffusing to the anoxic
sediment from the water column while coupled
denitrification (Dn) removes nitrate produced
within the sediments via nitrification. Details of
the experimental setup and analytical methods
can be found in the protocol handbook of the
NICE project (Dalsgaard 1999).
Results
Primary Producer Biomass and Nutrient
Concentration in the Water Column
In 1997 sampling campaigns were carried out
approximately every 40 days at each site; here we
present results from three dates which represent
critical phases in the seasonal evolution of the
primary producer communities (Table). At station Giralda. chlorophyll a concentrations in the
upper few millimeters of the sediment were relatively constant and ranged between 24 and 48 mg
m- 2 ; the highest value was determined in summer. Ammonium and nitrate concentrations
were minimum in July (Table) while higher values were measured in March (32 pM of NH4 + and
50}lM ofN0 3 -) and November. At station Gorino
Ulva biomasses of 191±5 and 311±15 g m- 2
dw
were determined respectively in March and
November; no macro algae were present in
August due to the biomass collapse associated
with the dystrophic crisis. The collapse of Ulva
production is a phenomenon well described
which occurs regularly in the Gorino area
(Viaroli et al. 1999) and is associated with water
column hypoxia. N0 3 - deficiency and NH/
release from the sediment. Oxygen and DIN concentrations in the water column are in a good
agreement with this cycle, with an ammonium
peak of 35 }lM and a nitrate minimum of 11 pM
in August. In this month the water column was
hypoxic (110}lM 02' about 40% saturation) during the day and anoxic at night.
227
Materials and Methods
Fluxes of DIN (Dissolved Inorganic Nitrogen)
and 02 across the sediment-water interface and
coupled-uncoupled denitrification rates were
measured during light and dark incubations of
intact sediment cores. Distinct incubation systems were used for the different sites: relatively
small cores were used for micro algae-covered
sediments while larger cores and chambers were
used respectively for phanerogam and macroalgae dominated systems. All incubation methods
were tested and standardized before starting the
investigation. All sediment cores, whether containing microphytes, rooted macrophytes or
floating macroalgae were incubated the day after
the sampling using in situ water at in situ oxygen
concentration and temperature. Before starting
the incubations the cores were maintained submersed in large tanks without the top lid and
with the stirring system on to allow renewal of
the water inside the cores. With the microalgaecovered sediment, Plexiglas cores (i.d. 8 cm,
height 40 cm) were used for flux and denitrification measurements; a 4 em long Teflon-coated
magnetic stirring bar was suspended 6 em above
the sediment surface. The stirring was due to an
external rotating magnet. In the macroalgae-covered sediment flux and denitrification measurements were performed in Plexiglas chambers
(base 20xlO em, working height 40 em), stirring
was created by drawing water out through a
water dispersal unit placed at one side of the
chamber and pumping it back at the other side.
Undisturbed sediment with macroalgal mat
(when present) was obtained with a hand held
box corer pushed through the algal mat into the
sediment. At the site covered by rooted macrophytes Plexiglas cores (Ld. 20 em, height 40 cm)
were used; stirring of the water column was
obtained with a small aquarium pump placed
inside the core. In all the incubations water-sediment fluxes of ° 2 , NH4 + and NO 3 - were measured
as concentration changes in the water with time
according to Eq. (1)
Flux = (a "" V) / A
(1)
where:
flux is expressed in }lmol of the species of interest m- 2 h- 1 ;
a = slope of the linear regression of concentration versus time;
A = area of sediment surface in core/chamber
(m 2 );
V = volume of water in core/chamber (1).
Denitrification rates were measured by the
isotope pairing technique (Nielsen 1992) adding
15N0 3 - to the water column and then measuring
the produced labelled N 2 ; this method allows to
determine coupled (Dn) and uncoupled denitrification (Dw). Uncoupled denitrification (Dw) is
denitrification of nitrate diffusing to the anoxic
sediment from the water column while coupled
denitrification (Dn) removes nitrate produced
within the sediments via nitrification. Details of
the experimental setup and analytical methods
can be found in the protocol handbook of the
NICE project (Dalsgaard 1999).
Results
Primary Producer Biomass and Nutrient
Concentration in the Water Column
In 1997 sampling campaigns were carried out
approximately every 40 days at each site; here we
present results from three dates which represent
critical phases in the seasonal evolution of the
primary producer communities (Table). At station Giralda. chlorophyll a concentrations in the
upper few millimeters of the sediment were relatively constant and ranged between 24 and 48 mg
m- 2 ; the highest value was determined in summer. Ammonium and nitrate concentrations
were minimum in July (Table) while higher values were measured in March (32 pM of NH4 + and
50}lM ofN0 3 -) and November. At station Gorino
Ulva biomasses of 191±5 and 311±15 g m- 2
dw
were determined respectively in March and
November; no macro algae were present in
August due to the biomass collapse associated
with the dystrophic crisis. The collapse of Ulva
production is a phenomenon well described
which occurs regularly in the Gorino area
(Viaroli et al. 1999) and is associated with water
column hypoxia. N0 3 - deficiency and NH/
release from the sediment. Oxygen and DIN concentrations in the water column are in a good
agreement with this cycle, with an ammonium
peak of 35 }lM and a nitrate minimum of 11 pM
in August. In this month the water column was
hypoxic (110}lM 02' about 40% saturation) during the day and anoxic at night.