Fluxes and Denitrification within Communities of Different Primary Producers
231
incubation of November, due probably to oxygen
deficiency in the uppermost sediment layers.
At station Smarlacca denitrification rates
were extremely low (between 3 and 25 Jlmol N
m- 2 h- 1 ) due to low nitrate concentrations in the
water column (Table). The Dn rates were
between 1 and 4 Fmo! N m- 2 h- 1 indicating that
nitrification was not important. Low rates of Dn
are probably due to the low oxygen penetration
into the sediments. At Station Smarlacca the root
system of Ruppia was not an efficient oxygen
transport system to the sediment, or chemical
oxidation due to reduced environment around
the rhizosphere outcompeted nitrifiers for oxygen released (Barbanti et al. 1997). Potential
nitrification rates have demonstrated that the
population of nitrifiers around the rhizosphere
of Ruppia is not significantly different from that
of the sediment far from the roots and nitrification does not seem to be an important process at
this site.
Discussion and Conclusion
At station Giralda nitrogen fluxes are influenced
by external inputs from the Po di Volano as well
as by water column transparency and organic
matter inputs. These factors also regulate the
structure and activities of benthic micro algal
and macrofaunal communities influencing Ncycle processes. Additionally, tidal changes in
water transparency, nutrient concentrations and
salinity resulted in dramatic changes in both
activities and fluxes over short time scales.
Microalgae efficiently take up the regenerated
NH/ (Fig. I), although high dark respiration
rates in the oxic layer (>2300 Fmol 02m-2h-1)
and high sulphate reduction rates at 4-6 em
depth (>700 Fmol 80/- m- 2 h-l, Giordani 1997)
maintained high ~H4 + concentrations in the sediment. Pore water concentrations up to 700 JlM
(data not shown) are generally found at 5 cm
depth. The availability of ammonium partially
explains the high coupled nitrification-denitrification rates (Dn) determined in March (Fig. 4).
No significant difference was found between Dn
values determined in light and dark incubations
meaning that the oxygen used by nitrifiers was
not derived from microalgae photosynthesis but
probably from burrow ventilation by benthic
macrofauna (Kristensen 1984). Indeed, oxygen
penetration in the sediment of station Giralda is
generally limited to the upper 3 mm (Bartoli
1996) whereas potential nitrification rates up to
12 JlM N g-l d,-l have been estimated at this site
at 2 cm depth (Castaldelli 1997) and active nitrifiers have been found down to 10 cm.
At station Gorino, DIN fluxes and denitrification rates were mostly controlled by macroalgal growth. In March, nitrate and ammonium
availability results in a high assimilation and
retention of DIN by the macro algal biomass; the
NO l - lost by the system via total denitrification
in the light incubation is just 5% of the NO lassimilated by Ulva (Figs. 2,4). In the summer
nitrate deficiency occurs in the water column
and Ulva growth collapses: the decomposition
of macro algal biomass causes ammonium
release and prolonged hypoxia (days to weeks)
in the water column (Table, Fig. 2). In August,
despite the absence of Ulva. denitrification of
nitrate from the water column is limited by its
low concentration whilst Dn is limited by the
effect of oxygen deficiency for nitrification due
to respiration processes. At this site the samplings of March and November were doubled
and bare sediments (without Ulva in the chambers) were incubated in parallel (data not
shown). Oxygen production and consumption
as well as DIN fluxes were dramatically affected
by the presence of the macroalgae. In the chambers with bare sediment, for example, net 02
fluxes in the light were similar in March and
November (-BOO pmol 02 m- 2 h- 1 ) and about 15
times smaller than those measured in chambers
with Uiva. Without macro algae light and dark
NH4 + fluxes were always from the sediment to
the water column (with values between 90 and
260 Fmol N m- 2 h- 1 ), the opposite of those measured in the chambers with Ulva where ammonium was assimilated. In the bare sediment furthermore NO;- fluxes were negligible. The Dw in
the chambers without Ulva was two times higher than for Ulva colonised sediments indicating
the competition for inorganic nitrogen between
denitrification and assimilation by primary producers.
At station Smarlacca nitrogen fluxes and
water column DIN concentrations were regulated throughout the year by N -assimilation and
storage by the seagrasses and their epiphytes,
which are the dominant N -cycling processes.
Thus, losses of N via denitrification are small in
terms of the overall N-budget. The March and
November samplings were doubled and bare
231
incubation of November, due probably to oxygen
deficiency in the uppermost sediment layers.
At station Smarlacca denitrification rates
were extremely low (between 3 and 25 Jlmol N
m- 2 h- 1 ) due to low nitrate concentrations in the
water column (Table). The Dn rates were
between 1 and 4 Fmo! N m- 2 h- 1 indicating that
nitrification was not important. Low rates of Dn
are probably due to the low oxygen penetration
into the sediments. At Station Smarlacca the root
system of Ruppia was not an efficient oxygen
transport system to the sediment, or chemical
oxidation due to reduced environment around
the rhizosphere outcompeted nitrifiers for oxygen released (Barbanti et al. 1997). Potential
nitrification rates have demonstrated that the
population of nitrifiers around the rhizosphere
of Ruppia is not significantly different from that
of the sediment far from the roots and nitrification does not seem to be an important process at
this site.
Discussion and Conclusion
At station Giralda nitrogen fluxes are influenced
by external inputs from the Po di Volano as well
as by water column transparency and organic
matter inputs. These factors also regulate the
structure and activities of benthic micro algal
and macrofaunal communities influencing Ncycle processes. Additionally, tidal changes in
water transparency, nutrient concentrations and
salinity resulted in dramatic changes in both
activities and fluxes over short time scales.
Microalgae efficiently take up the regenerated
NH/ (Fig. I), although high dark respiration
rates in the oxic layer (>2300 Fmol 02m-2h-1)
and high sulphate reduction rates at 4-6 em
depth (>700 Fmol 80/- m- 2 h-l, Giordani 1997)
maintained high ~H4 + concentrations in the sediment. Pore water concentrations up to 700 JlM
(data not shown) are generally found at 5 cm
depth. The availability of ammonium partially
explains the high coupled nitrification-denitrification rates (Dn) determined in March (Fig. 4).
No significant difference was found between Dn
values determined in light and dark incubations
meaning that the oxygen used by nitrifiers was
not derived from microalgae photosynthesis but
probably from burrow ventilation by benthic
macrofauna (Kristensen 1984). Indeed, oxygen
penetration in the sediment of station Giralda is
generally limited to the upper 3 mm (Bartoli
1996) whereas potential nitrification rates up to
12 JlM N g-l d,-l have been estimated at this site
at 2 cm depth (Castaldelli 1997) and active nitrifiers have been found down to 10 cm.
At station Gorino, DIN fluxes and denitrification rates were mostly controlled by macroalgal growth. In March, nitrate and ammonium
availability results in a high assimilation and
retention of DIN by the macro algal biomass; the
NO l - lost by the system via total denitrification
in the light incubation is just 5% of the NO lassimilated by Ulva (Figs. 2,4). In the summer
nitrate deficiency occurs in the water column
and Ulva growth collapses: the decomposition
of macro algal biomass causes ammonium
release and prolonged hypoxia (days to weeks)
in the water column (Table, Fig. 2). In August,
despite the absence of Ulva. denitrification of
nitrate from the water column is limited by its
low concentration whilst Dn is limited by the
effect of oxygen deficiency for nitrification due
to respiration processes. At this site the samplings of March and November were doubled
and bare sediments (without Ulva in the chambers) were incubated in parallel (data not
shown). Oxygen production and consumption
as well as DIN fluxes were dramatically affected
by the presence of the macroalgae. In the chambers with bare sediment, for example, net 02
fluxes in the light were similar in March and
November (-BOO pmol 02 m- 2 h- 1 ) and about 15
times smaller than those measured in chambers
with Uiva. Without macro algae light and dark
NH4 + fluxes were always from the sediment to
the water column (with values between 90 and
260 Fmol N m- 2 h- 1 ), the opposite of those measured in the chambers with Ulva where ammonium was assimilated. In the bare sediment furthermore NO;- fluxes were negligible. The Dw in
the chambers without Ulva was two times higher than for Ulva colonised sediments indicating
the competition for inorganic nitrogen between
denitrification and assimilation by primary producers.
At station Smarlacca nitrogen fluxes and
water column DIN concentrations were regulated throughout the year by N -assimilation and
storage by the seagrasses and their epiphytes,
which are the dominant N -cycling processes.
Thus, losses of N via denitrification are small in
terms of the overall N-budget. The March and
November samplings were doubled and bare