474
P. Viaroli et al.
umn which allowed only Ulva to grow, outcompeting other primary producers as well as denitrification processes (Bartoli et al. this volume).
Before the macro algal collapse. the overall Ulva
standing biomass accounted for 174 (133-221)
tons of nitrogen. This nitrogen bulk was stored
from late March to early June, which corresponded to an external total nitrogen loading of 540
tons of which 175 tons was comprised of dissolved inorganic N (P. Viaroli, unpublished data).
Therefore, most of the inorganic load was
retained in the macroalgal biomasses. However,
nitrogen accumulation within the Ulva was only
a temporary N-sink, due to its short life cycle. In
mid June, the macroalgal community shifted
from active production to rapid decomposition,
leading to a rapid nutrient loss and recycling.
The bloom collapse also had a significant
impact on dissolved oxygen, phosphorus and
sulphide concentrations (Viaroli et a11992; 1993;
Giordani et al. 1996; 1997). In 1997 we observed
an extended dystrophic crisis in mid-June and
several dystrophic episodes in July and August.
Moreover, a persistent hypoxia lasted until mid
September. Several studies have demonstrated
that the iron-iron monosulphide-pyrite system
buffers against free sulphides (Howarth and
Stewart 1992; Smolders et a1. 1995). However,
simple measurements of these variables are not
sufficient to estimate the true buffering capacity
of the sedimentary processes (Giordani et al.
1996; 1997). Additionally, the decomposition of
the accumulated biomasses may occur directly in
the anoxic bottom water, where the sulphide concentrations are independent of the potential
buffering capacity provided by sedimentary iron
pools. The buffering capacity of these ecosystems appears to depend mostly on the extent of
macro algal blooms, and seems to be influenced
by the timing and rates of biomass growth and
decomposition. Therefore. the abnormal growth
of ephemeral macro algae may lead to a high level
of intrasystem disturbance and instability, which
results in oxygen deficiencies that frequently
degenerate into dystrophic crisis.
Acknowledgements. This research has been jointly supported
by the EU projects ROBUST (contract ENV4-CT96-0218) and
NICE (contract MAS3-CT96-0048) of the EWISE network of
the Commission of the European Communities. We also
gratefully acknowledge S. Bencivelli, G. Grigatti and T.
Zappata (Consortium for the Sacca di Gom Management) for
field assistance and measurements.
References
APHA,AWWA, WPCF (1975) Standard methods for the examination of water and wastewaters. 14th edn. APHA, Washington
Atkinson MI, Smith SV(l983) C:N:P ratios of benthic marine
planm. Limnol Oceanogr 28: 568-574
Bartoli M, Cattadori M, Giordani G. Viaroli P {1996} Benthic oxygen respiration, ammonium and phosphorus regeneration
in surficial sediments of the Sacca eli Goro {Northern Italy}
and two French coastal lagoons: a comparative study.
Hydrobiologia 329: 143-159
Bartoli M, Castaldelli G, Nizzoli D, Gatti LG, Viaroli P (2000)
Benthic fluxes of oxygen. ammonium and nitrate and coupled and uncoupled denitrification rates in two eutrophic
coastal lagoons with different primary producer communities. (this volume)
Borum J (1996) Shallow waters and land/sea boundaries. In:
J0rgensen BB, Richardson K (eds) Eutrophication in coastal
marine ecosystems: coastal and estuarine studies. Am
Geophys Union 52: 179-203
Castel J, Caumette P, Herbert R (1996) Eutrophication gradients in
coastal lagoons as exemplified by the bassin d' Arcachon and
the Etang du Prevost. Hydrobiologia 329: ix-xxviii
Cline JD (1969) Spectrophotometric determination of hydrogen
sulphide in natural waters. Limnol Oceanogr 14: 454-459
DeBoer JA, Guigli HI, Israel TL, D'EIia CF (1978) Nutritional studies of two red algae. 1. Growth rate as a function of nitrogen
source and concentration. J Phycol14: 261-266
Flecht RL (1996) The ocurrence of "green tides" - a review. In:
Schramm W, Nienhuis PH (eds) Marine benthic vegetation:
recent changes and the effect of eutrophication. (Ecological
studies. 123), Springer, Berlin Heidelberg New York Tokyo,
pp 7-43
Fassing H, Jorgensen BB (1989) Measurement of bacterial sulphate reduction in sediment: evaluation of a single-step
chromium reduction method. Biogeochemistry 8: 205-222
Fujita RM. Wheeler PA, Zedler JB (1989) Assessment of macroalgal nitrogen limitation in a seasonal upwelling region. Mar
Bcol Prog Ser 53: 293-303
Giordani G, Cattadori M, Bartoli M, Viaroli P (1996) Sulphide
release from anoxic sediments in relation to iron availability
and organic matter recalcitrance and its effects on inorganic phosphorus recycling. Hydrobiologia 329: 211-222
Giordani G, Azzoni R, Bartoli M, Viaroli P (1997) Seasonal variations of sulphate reduction rates, sulphur pools and iron
availability in the sediment of a dystrophic lagoon (Sacca di
Goro, Italy). Water Air Soil Pollut 99: 363-371
Howarth RW, Stewart JWB (1992) The interactions of sulphur
with other element cycles in ecosystems. In: Howart RW,
Stewart JWE, Ivanov MU (eds) Sulphur cycling on the continents: wetlands, terrestrial ecosystems and associated water
bodies: SCOPE 33.1 Wlley and Sons, New York, pp 67-84.
Jensen A (1978) Chlorophylls and carotenoids. In: Hellebust lA,
Craigie IS (eds) Handbook of phycological methods.
Physiological and biochemical methods. Cambridge Univ
Press, Cambridge, pp 59-70.
Lapointe BE (1997) Nutrient thresholds for bottom-up control of
macroalgal blooms on coral reefs in Jamaica and southeast
Florida. Limnol Oceanogr 42: 1119-1131
Lavery PS, McComb AJ {1991} The nutritional eco-physiology of
Chaetomorpha linul and WVQ rigida in Peel Inlet, Western
Australia. Estuarine Coast Shelf Sci 33: 1-22
wvley DR, Phillips EJP (1987) Rapid assay for reducible ferric
P. Viaroli et al.
umn which allowed only Ulva to grow, outcompeting other primary producers as well as denitrification processes (Bartoli et al. this volume).
Before the macro algal collapse. the overall Ulva
standing biomass accounted for 174 (133-221)
tons of nitrogen. This nitrogen bulk was stored
from late March to early June, which corresponded to an external total nitrogen loading of 540
tons of which 175 tons was comprised of dissolved inorganic N (P. Viaroli, unpublished data).
Therefore, most of the inorganic load was
retained in the macroalgal biomasses. However,
nitrogen accumulation within the Ulva was only
a temporary N-sink, due to its short life cycle. In
mid June, the macroalgal community shifted
from active production to rapid decomposition,
leading to a rapid nutrient loss and recycling.
The bloom collapse also had a significant
impact on dissolved oxygen, phosphorus and
sulphide concentrations (Viaroli et a11992; 1993;
Giordani et al. 1996; 1997). In 1997 we observed
an extended dystrophic crisis in mid-June and
several dystrophic episodes in July and August.
Moreover, a persistent hypoxia lasted until mid
September. Several studies have demonstrated
that the iron-iron monosulphide-pyrite system
buffers against free sulphides (Howarth and
Stewart 1992; Smolders et a1. 1995). However,
simple measurements of these variables are not
sufficient to estimate the true buffering capacity
of the sedimentary processes (Giordani et al.
1996; 1997). Additionally, the decomposition of
the accumulated biomasses may occur directly in
the anoxic bottom water, where the sulphide concentrations are independent of the potential
buffering capacity provided by sedimentary iron
pools. The buffering capacity of these ecosystems appears to depend mostly on the extent of
macro algal blooms, and seems to be influenced
by the timing and rates of biomass growth and
decomposition. Therefore. the abnormal growth
of ephemeral macro algae may lead to a high level
of intrasystem disturbance and instability, which
results in oxygen deficiencies that frequently
degenerate into dystrophic crisis.
Acknowledgements. This research has been jointly supported
by the EU projects ROBUST (contract ENV4-CT96-0218) and
NICE (contract MAS3-CT96-0048) of the EWISE network of
the Commission of the European Communities. We also
gratefully acknowledge S. Bencivelli, G. Grigatti and T.
Zappata (Consortium for the Sacca di Gom Management) for
field assistance and measurements.
References
APHA,AWWA, WPCF (1975) Standard methods for the examination of water and wastewaters. 14th edn. APHA, Washington
Atkinson MI, Smith SV(l983) C:N:P ratios of benthic marine
planm. Limnol Oceanogr 28: 568-574
Bartoli M, Cattadori M, Giordani G. Viaroli P {1996} Benthic oxygen respiration, ammonium and phosphorus regeneration
in surficial sediments of the Sacca eli Goro {Northern Italy}
and two French coastal lagoons: a comparative study.
Hydrobiologia 329: 143-159
Bartoli M, Castaldelli G, Nizzoli D, Gatti LG, Viaroli P (2000)
Benthic fluxes of oxygen. ammonium and nitrate and coupled and uncoupled denitrification rates in two eutrophic
coastal lagoons with different primary producer communities. (this volume)
Borum J (1996) Shallow waters and land/sea boundaries. In:
J0rgensen BB, Richardson K (eds) Eutrophication in coastal
marine ecosystems: coastal and estuarine studies. Am
Geophys Union 52: 179-203
Castel J, Caumette P, Herbert R (1996) Eutrophication gradients in
coastal lagoons as exemplified by the bassin d' Arcachon and
the Etang du Prevost. Hydrobiologia 329: ix-xxviii
Cline JD (1969) Spectrophotometric determination of hydrogen
sulphide in natural waters. Limnol Oceanogr 14: 454-459
DeBoer JA, Guigli HI, Israel TL, D'EIia CF (1978) Nutritional studies of two red algae. 1. Growth rate as a function of nitrogen
source and concentration. J Phycol14: 261-266
Flecht RL (1996) The ocurrence of "green tides" - a review. In:
Schramm W, Nienhuis PH (eds) Marine benthic vegetation:
recent changes and the effect of eutrophication. (Ecological
studies. 123), Springer, Berlin Heidelberg New York Tokyo,
pp 7-43
Fassing H, Jorgensen BB (1989) Measurement of bacterial sulphate reduction in sediment: evaluation of a single-step
chromium reduction method. Biogeochemistry 8: 205-222
Fujita RM. Wheeler PA, Zedler JB (1989) Assessment of macroalgal nitrogen limitation in a seasonal upwelling region. Mar
Bcol Prog Ser 53: 293-303
Giordani G, Cattadori M, Bartoli M, Viaroli P (1996) Sulphide
release from anoxic sediments in relation to iron availability
and organic matter recalcitrance and its effects on inorganic phosphorus recycling. Hydrobiologia 329: 211-222
Giordani G, Azzoni R, Bartoli M, Viaroli P (1997) Seasonal variations of sulphate reduction rates, sulphur pools and iron
availability in the sediment of a dystrophic lagoon (Sacca di
Goro, Italy). Water Air Soil Pollut 99: 363-371
Howarth RW, Stewart JWB (1992) The interactions of sulphur
with other element cycles in ecosystems. In: Howart RW,
Stewart JWE, Ivanov MU (eds) Sulphur cycling on the continents: wetlands, terrestrial ecosystems and associated water
bodies: SCOPE 33.1 Wlley and Sons, New York, pp 67-84.
Jensen A (1978) Chlorophylls and carotenoids. In: Hellebust lA,
Craigie IS (eds) Handbook of phycological methods.
Physiological and biochemical methods. Cambridge Univ
Press, Cambridge, pp 59-70.
Lapointe BE (1997) Nutrient thresholds for bottom-up control of
macroalgal blooms on coral reefs in Jamaica and southeast
Florida. Limnol Oceanogr 42: 1119-1131
Lavery PS, McComb AJ {1991} The nutritional eco-physiology of
Chaetomorpha linul and WVQ rigida in Peel Inlet, Western
Australia. Estuarine Coast Shelf Sci 33: 1-22
wvley DR, Phillips EJP (1987) Rapid assay for reducible ferric